explanation blue bibcodes open ADS page with paths to full text
Author name code: ludwig
ADS astronomy entries on 2022-09-14
author:"Ludwig, Hans-Guenter"
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Title: The Gaia-ESO Public Spectroscopic Survey: Motivation,
implementation, GIRAFFE data processing, analysis, and final data
products
Authors: Gilmore, G.; Randich, S.; Worley, C. C.; Hourihane, A.;
Gonneau, A.; Sacco, G. G.; Lewis, J. R.; Magrini, L.; Francois, P.;
Jeffries, R. D.; Koposov, S. E.; Bragaglia, A.; Alfaro, E. J.; Allende
Prieto, C.; Blomme, R.; Korn, A. J.; Lanzafame, A. C.; Pancino, E.;
Recio-Blanco, A.; Smiljanic, R.; Van Eck, S.; Zwitter, T.; Bensby, T.;
Flaccomio, E.; Irwin, M. J.; Franciosini, E.; Morbidelli, L.; Damiani,
F.; Bonito, R.; Friel, E. D.; Vink, J. S.; Prisinzano, L.; Abbas,
U.; Hatzidimitriou, D.; Held, E. V.; Jordi, C.; Paunzen, E.; Spagna,
A.; Jackson, R. J.; Maiz Apellaniz, J.; Asplund, M.; Bonifacio, P.;
Feltzing, S.; Binney, J.; Drew, J.; Ferguson, A. M. N.; Micela, G.;
Negueruela, I.; Prusti, T.; Rix, H. -W.; Vallenari, A.; Bergemann,
M.; Casey, A. R.; de Laverny, P.; Frasca, A.; Hill, V.; Lind, K.;
Sbordone, L.; Sousa, S. G.; Adibekyan, V.; Caffau, E.; Daflon, S.;
Feuillet, D. K.; Gebran, M.; Gonzalez Hernandez, J. I.; Guiglion,
G.; Herrero, A.; Lobel, A.; Merle, T.; Mikolaitis, S.; Montes, D.;
Morel, T.; Ruchti, G.; Soubiran, C.; Tabernero, H. M.; Tautvaisiene,
G.; Traven, G.; Valentini, M.; Van der Swaelmen, M.; Villanova, S.;
Viscasillas Vazquez, C.; Bayo, A.; Biazzo, K.; Carraro, G.; Edvardsson,
B.; Heiter, U.; Jofre, P.; Marconi, G.; Martayan, C.; Masseron, T.;
Monaco, L.; Walton, N. A.; Zaggia, S.; Aguirre Borsen-Koch, V.; Alves,
J.; Balaguer-Nunez, L.; Barklem, P. S.; Barrado, D.; Bellazzini, M.;
Berlanas, S. R.; Binks, A. S.; Bressan, A.; Capuzzo-Dolcetta, R.;
Casagrande, L.; Casamiquela, L.; Collins, R. S.; D'Orazi, V.; Dantas,
M. L. L.; Debattista, V. P.; Delgado-Mena, E.; Di Marcantonio, P.;
Drazdauskas, A.; Evans, N. W.; Famaey, B.; Franchini, M.; Fremat, Y.;
Fu, X.; Geisler, D.; Gerhard, O.; Gonzalez Solares, E. A.; Grebel,
E. K.; Gutierrez Albarran, M. L.; Jimenez-Esteban, F.; Jonsson, H.;
Khachaturyants, T.; Kordopatis, G.; Kos, J.; Lagarde, N.; Ludwig,
H. -G.; Mahy, L.; Mapelli, M.; Marfil, E.; Martell, S. L.; Messina,
S.; Miglio, A.; Minchev, I.; Moitinho, A.; Montalban, J.; Monteiro,
M. J. P. F. G.; Morossi, C.; Mowlavi, N.; Mucciarelli, A.; Murphy,
D. N. A.; Nardetto, N.; Ortolani, S.; Paletou, F.; Palous, J.;
Pickering, J. C.; Quirrenbach, A.; Re Fiorentin, P.; Read, J. I.;
Romano, D.; Ryde, N.; Sanna, N.; Santos, W.; Seabroke, G. M.; Spina,
L.; Steinmetz, M.; Stonkute, E.; Sutorius, E.; Thevenin, F.; Tosi,
M.; Tsantaki, M.; Wright, N.; Wyse, R. F. G.; Zoccali, M.; Zorec,
J.; Zucker, D. B.
2022arXiv220805432G Altcode:
The Gaia-ESO Public Spectroscopic Survey is an ambitious project
designed to obtain astrophysical parameters and elemental abundances
for 100,000 stars, including large representative samples of the
stellar populations in the Galaxy, and a well-defined sample of 60
(plus 20 archive) open clusters. We provide internally consistent
results calibrated on benchmark stars and star clusters, extending
across a very wide range of abundances and ages. This provides a
legacy data set of intrinsic value, and equally a large wide-ranging
dataset that is of value for homogenisation of other and future
stellar surveys and Gaia's astrophysical parameters. This article
provides an overview of the survey methodology, the scientific aims,
and the implementation, including a description of the data processing
for the GIRAFFE spectra. A companion paper (arXiv:2206.02901)
introduces the survey results. Gaia-ESO aspires to quantify both
random and systematic contributions to measurement uncertainties. Thus
all available spectroscopic analysis techniques are utilised, each
spectrum being analysed by up to several different analysis pipelines,
with considerable effort being made to homogenise and calibrate the
resulting parameters. We describe here the sequence of activities up to
delivery of processed data products to the ESO Science Archive Facility
for open use. The Gaia-ESO Survey obtained 202,000 spectra of 115,000
stars using 340 allocated VLT nights between December 2011 and January
2018 from GIRAFFE and UVES. The full consistently reduced final data set
of spectra was released through the ESO Science Archive Facility in late
2020, with the full astrophysical parameters sets following in 2022.
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Title: A revisit of the standard composition of the Sun
Authors: Bautista, Manuel; Magg, Ekaterina; Bergemann, Maria;
Serenelli, Aldo; Plez, Bertrand; Gerber, Geffrey; Ludwig, Hans-Gunter;
Basu, Sarbani; Ferguson, Jason; Carvajal Gallego, Helena; Gamrath,
Sebaastien; Palmeri, Patrick; Quinet, Pascal
2022AAS...24035011B Altcode:
The chemical composition of the Sun is requested in the context of
various studies in astrophysics, among them in the calculation of the
standard solar models (SSMs), which describe the evolution of the Sun
from the pre-main-sequence to its present age. <P />In this work,
we re-analysis of the solar chemical abundances and corresponding
SSMs. We employ new high-quality solar observational data collected
with the IAG facility, state-of-the-art non-equilibrium modelling,
new oscillator strengths, and different atmospheric models, including
the MARCS model, but also averages based on Stagger and CO5BOLD 3D
radiation-hydrodynamics simulations of stellar convection. We perform
new calculations of oscillator strengths for transitions in O I and
N I. For O I — the critical element for the interior models —
calculations are carried out using several independent methods. We
find unprecedented agreement between the new estimates of transition
probabilities, thus supporting our revised solar oxygen abundance. We
also provide new estimates of the noble gas Ne abundance. <P />We
compare our photospheric abundances with the previous estimates. We
discuss the consistency of our measurements with meteoritic values,
taking into account systematic and correlated errors. Finally, we
provide revised chemical abundances, leading to a new value of the
solar photospheric present-day metallicity Z=X = 0:0225, and employ
them in the calculations of the SSM. We find that the puzzling mismatch
between the helioseismic constraints on the solar interior structure
and the model is resolved with the new chemical composition.
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Title: Interpolation of spectra from 3D model atmospheres
Authors: Bertran de Lis, S.; Prieto, Allende; Ludwig, H. -G.;
Koesterke, L.
2022A&A...661A..76B Altcode: 2022arXiv220212086B
Context. The use of 3D hydrodynamical simulations of stellar surface
convection for model atmospheres is computationally expensive. Although
these models have been available for quite some time, their use is
limited because of the lack of extensive grids of simulations and
associated spectra. <BR /> Aims: Our goal is to provide a method to
interpolate spectra that can be applied to both 1D and 3D models,
and implement it in a code available to the community. This tool will
enable the routine use of 3D model atmospheres in the analysis of
stellar spectra. <BR /> Methods: We have developed a code that makes
use of radial basis functions to interpolate the spectra included in
the CIFIST grid of 84 three-dimensional model atmospheres. Spectral
synthesis on the hydrodynamical simulations was previously performed
with the code ASSϵT. <BR /> Results: We make a tool for the
interpolation of 3D spectra available to the community. The code
provides interpolated spectra and interpolation errors for a given
wavelength interval, and a combination of effective temperature,
surface gravity, and metallicity. In addition, it optionally provides
graphical representations of the RMS and mean ratio between 1D and
3D spectra, and maps of the errors in the interpolated spectra across
the parameter space.
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Title: Observational constraints on the origin of the
elements. IV. Standard composition of the Sun
Authors: Magg, Ekaterina; Bergemann, Maria; Serenelli, Aldo; Bautista,
Manuel; Plez, Bertrand; Heiter, Ulrike; Gerber, Jeffrey M.; Ludwig,
Hans-Günter; Basu, Sarbani; Ferguson, Jason W.; Gallego, Helena
Carvajal; Gamrath, Sébastien; Palmeri, Patrick; Quinet, Pascal
2022A&A...661A.140M Altcode: 2022arXiv220302255M
Context. The chemical composition of the Sun is required in the context
of various studies in astrophysics, among them in the calculation
of standard solar models (SSMs) used to describe the evolution of
the Sun from the pre-main-sequence to its present age. <BR /> Aims:
In this work, we provide a critical re-analysis of the solar chemical
abundances and corresponding SSMs. <BR /> Methods: For the photospheric
values, we employed new high-quality solar observational data collected
with the IAG facility, state-of-the art non-equilibrium modelling,
new oscillator strengths, and different atmospheric models, including
the MARCS model, along with averages based on Stagger and CO5BOLD 3D
radiation-hydrodynamics simulations of stellar convection. We performed
new calculations of oscillator strengths for transitions in O I and N
I. For O I, which is a critical element with regard to the interior
models, calculations were carried out using several independent
methods. We investigated our results in comparison with the previous
estimates. <BR /> Results: We find an unprecedented agreement between
the new estimates of transition probabilities, thus supporting our
revised solar oxygen abundance value. We also provide new estimates of
the noble gas Ne abundance. In addition, we discuss the consistency
of our photospheric measurements with meteoritic values, taking into
account the systematic and correlated errors. Finally, we provide
revised chemical abundances, leading to a new value proposed for the
solar photospheric present-day metallicity of Z/X = 0.0225, which we
then employed in SSM calculations. We find that the puzzling mismatch
between the helioseismic constraints on the solar interior structure
and the model can be resolved thanks to this new chemical composition.
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Title: The Gaia RVS benchmark stars. I. Chemical inventory of the
first sample of evolved stars and its Rb NLTE investigation
Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; François,
P.; Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.;
Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.;
Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite,
F.; Panuzzo, P.; Royer, F.; Thévenin, F.; Ludwig, H. -G.; Marchal,
O.; Plum, G.
2021A&A...651A..20C Altcode:
Context. The Radial Velocity Spectrometer (RVS) on board the Gaia
satellite is not provided with a wavelength calibration lamp. It uses
its observations of stars with known radial velocity to derive the
dispersion relation. To derive an accurate radial velocity calibration,
a precise knowledge of the line spread function (LSF) of the RVS is
necessary. Good-quality ground-based observations in the wavelength
range of the RVS are highly desired to determine the LSF. <BR /> Aims:
Several radial velocity standard stars are available to the Gaia
community. The highest possible number of calibrators will surely
allow us to improve the accuracy of the radial velocity. Because
the LSF may vary across the focal plane of the RVS, a large number
of high-quality spectra for the LSF calibration may allow us to
better sample the properties of the focal plane. <BR /> Methods:
We selected a sample of stars to be observed with UVES at the Very
Large Telescope, in a setting including the wavelength range of RVS,
that are bright enough to allow obtaining high-quality spectra in a
short time. We also selected stars that lack chemical investigation in
order to increase the sample of bright, close by stars with a complete
chemical inventory. <BR /> Results: We here present the chemical
analysis of the first sample of 80 evolved stars. The quality of the
spectra is very good, therefore we were able to derive abundances for
20 elements. The metallicity range spanned by the sample is about 1
dex, from slightly metal-poor to solar metallicity. We derived the
Rb abundance for all stars and investigated departures from local
thermodynamical equilibrium (NLTE) in the formation of its lines. <BR />
Conclusions: The sample of spectra is of good quality, which is useful
for a Gaia radial velocity calibration. The Rb NLTE effects in this
stellar parameters range are small but sometimes non-negligible,
especially for spectra of this good quality. <P />Tables B.3
and C.1 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A20">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A20</A>
<P />Based on observations made with UVES at VLT 104.D.0325.
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Title: ALMA small-scale features in the quiet Sun and active regions
Authors: Brajša, R.; Skokić, I.; Sudar, D.; Benz, A. O.; Krucker,
S.; Ludwig, H. -G.; Saar, S. H.; Selhorst, C. L.
2021A&A...651A...6B Altcode: 2021arXiv210503644B
<BR /> Aims: The main aim of the present analysis is to decipher (i)
the small-scale bright features in solar images of the quiet Sun and
active regions obtained with the Atacama Large Millimeter/submillimeter
Array (ALMA) and (ii) the ALMA correspondence of various known
chromospheric structures visible in the Hα images of the Sun. <BR
/> Methods: Small-scale ALMA bright features in the quiet Sun region
were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz)
and in an active region using interferometric ALMA measurements (3
mm, 100 GHz). With the single-dish observations, a full-disk solar
image is produced, while interferometric measurements enable the
high-resolution reconstruction of part of the solar disk, including
the active region. The selected quiet Sun and active regions are
compared with the Hα (core and wing sum), EUV, and soft X-ray images
and with the magnetograms. <BR /> Results: In the quiet Sun region,
enhanced emission seen in the ALMA is almost always associated with a
strong line-of-sight magnetic field. Four coronal bright points were
identified, while other small-scale ALMA bright features are most likely
associated with magnetic network elements and plages. In the active
region, in 14 small-scale ALMA bright features randomly selected and
compared with other images, we found five good candidates for coronal
bright points, two for plages, and five for fibrils. Two unclear cases
remain: a fibril or a jet, and a coronal bright point or a plage. A
comparison of the Hα core image and the 3 mm ALMA image of the analyzed
active region showed that the sunspot appears dark in both images
(with a local ALMA radiation enhancement in sunspot umbra), the four
plage areas are bright in both images and dark small Hα filaments are
clearly recognized as dark structures of the same shape also in ALMA.
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Title: TOPoS. VI. The metal-weak tail of the metallicity distribution
functions of the Milky Way and the Gaia-Sausage-Enceladus structure
Authors: Bonifacio, P.; Monaco, L.; Salvadori, S.; Caffau, E.; Spite,
M.; Sbordone, L.; Spite, F.; Ludwig, H. -G.; Di Matteo, P.; Haywood,
M.; François, P.; Koch-Hansen, A. J.; Christlieb, N.; Zaggia, S.
2021A&A...651A..79B Altcode: 2021arXiv210508360B
Context. The goal of the Turn-Off Primordial Stars survey (TOPoS)
project is to find and analyse turn-off (TO) stars of extremely low
metallicity. To select the targets for spectroscopic follow-up at high
spectral resolution, we relied on low-resolution spectra from the
Sloan Digital Sky Survey (SDSS). <BR /> Aims: In this paper, we use
the metallicity estimates we obtained from our analysis of the SDSS
spectra to construct the metallicity distribution function (MDF) of the
Milky Way, with special emphasis on its metal-weak tail. The goal is
to provide the underlying distribution out of which the TOPoS sample
was extracted. <BR /> Methods: We made use of SDSS photometry, Gaia
photometry, and distance estimates derived from the Gaia parallaxes to
derive a metallicity estimate for a large sample of over 24 million TO
stars. This sample was used to derive the metallicity bias of the sample
for which SDSS spectra are available. <BR /> Results: We determined that
the spectroscopic sample is strongly biased in favour of metal-poor
stars, as intended. A comparison with the unbiased photometric
sample allows us to correct for the selection bias. We selected a
sub-sample of stars with reliable parallaxes for which we combined
the SDSS radial velocities with Gaia proper motions and parallaxes to
compute actions and orbital parameters in the Galactic potential. This
allowed us to characterise the stars dynamically, and in particular
to select a sub-sample that belongs to the Gaia-Sausage-Enceladus
(GSE) accretion event. We are thus also able to provide the MDF of
GSE. <BR /> Conclusions: The metal-weak tail derived in our study is
very similar to that derived in the H3 survey and in the Hamburg/ESO
Survey. This allows us to average the three MDFs and provide an
error bar for each metallicity bin. Inasmuch as the GSE structure is
representative of the progenitor galaxy that collided with the Milky
Way, that galaxy appears to be strongly deficient in metal-poor
stars compared to the Milky Way, suggesting that the metal-weak
tail of the latter has been largely formed by accretion of low-mass
galaxies rather than massive galaxies, such as the GSE progenitor. <P
/>Spectroscopic and photometric metallicities derived and discussed
in this paper as well as orbital actions computed and discussed in
this paper are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A79">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A79</A>
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Title: VizieR Online Data Catalog: TO stars metallicity estimate
(Bonifacio+, 2021)
Authors: Bonifacio, P.; Monaco, L.; Salvadori, S.; Caffau, E.; Spite,
M.; Sbordone, L.; Spite, F.; Ludwig, H. -G.; Di Matteo, P.; Haywood,
M.; Francois, P.; Koch-Hansen, A. J.; Christlieb, N. C.; Zaggia, S.
2021yCat..36510079B Altcode:
We made use of SDSS photometry, Gaia photometry, and distance estimates
derived from the Gaia parallaxes to derive a metallicity estimate for
a large sample of over 24 million TO stars. This sample was used to
derive the metallicity bias of the sample for which SDSS spectra are
available. <P />(3 data files).
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Title: Spatially resolved spectroscopy across stellar surfaces. IV. F,
G, and K-stars: Synthetic 3D spectra at hyper-high resolution
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd
2021A&A...649A..16D Altcode: 2021arXiv210303880D
Context. High-precision stellar analyses require hydrodynamic 3D
modeling. Such models predict changes across stellar disks of spectral
line shapes, asymmetries, and wavelength shifts. For testing models in
stars other than the Sun, spatially resolved observations are feasible
from differential spectroscopy during exoplanet transits, retrieving
spectra of those stellar surface segments that successively become
hidden behind the transiting planet, as demonstrated in Papers I, II,
and III. <BR /> Aims: Synthetic high-resolution spectra over extended
spectral regions are now available from 3D models. Similar to other ab
initio simulations in astrophysics, these data contain patterns that
have not been specifically modeled but may be revealed after analyses
to be analogous to those of a large volume of observations. <BR />
Methods: From five 3D models spanning T<SUB>eff</SUB> = 3964-6726 K
(spectral types ~K8 V-F3 V), synthetic spectra at hyper-high resolution
(λ/Δλ >1 000 000) were analyzed. Selected Fe I and Fe II lines at
various positions across stellar disks were searched for characteristic
patterns between different types of lines in the same star and for
similar lines between different stars. <BR /> Results: Spectral-line
patterns are identified for representative photospheric lines of
different strengths, excitation potentials, and ionization levels,
thereby encoding the hydrodynamic 3D structure. Line profiles and
bisectors are shown for various stars at different positions across
stellar disks. Absolute convective wavelength shifts are obtained
as differences to 1D models, where such shifts do not occur. <BR />
Conclusions: Observable relationships for line properties are retrieved
from realistically complex synthetic spectra. Such patterns may also
test very detailed 3D modeling, including non-LTE effects. While present
results are obtained at hyper-high spectral resolution, the subsequent
Paper V examines their practical observability at realistically lower
resolutions, and in the presence of noise.
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Title: Spatially resolved spectroscopy across stellar
surfaces. V. Observational prospects: toward Earth-like exoplanet
detection
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd
2021A&A...649A..17D Altcode: 2021arXiv210304996D
Context. High-precision stellar analyses require hydrodynamic 3D
modeling. Testing such models is feasible by retrieving spectral line
shapes across stellar disks, using differential spectroscopy during
exoplanet transits. Observations were presented in Papers I, II, and
III, while Paper IV explored synthetic data at hyper-high spectral
resolution for different classes of stars, identifying characteristic
patterns for Fe I and Fe II lines. <BR /> Aims: Anticipating future
observations, the observability of patterns among photospheric lines
of different strength, excitation potential and ionization level are
examined from synthetic spectra, as observed at ordinary spectral
resolutions and at different levels of noise. Time variability in 3D
atmospheres induces changes in spectral-line parameters, some of which
are correlated. An adequate calibration could identify proxies for
the jitter in apparent radial velocity to enable adjustments to actual
stellar radial motion. <BR /> Methods: We used spectral-line patterns
identified in synthetic spectra at hyper-high resolution in Paper IV
from 3D models spanning T<SUB>eff</SUB> = 3964-6726 K (spectral types
~K8 V-F3 V) to simulate practically observable signals at different
stellar disk positions at various lower spectral resolutions, down
to λ/Δλ = 75 000. We also examined the center-to-limb temporal
variability. <BR /> Results: Recovery of spatially resolved line
profiles with fitted widths and depths is shown for various noise
levels, with gradual degradation at successively lower spectral
resolutions. Signals during exoplanet transit are simulated. In
addition to Rossiter-McLaughlin type signatures in apparent radial
velocity, analogous effects are shown for line depths and widths. In
a solar model, temporal variability in line profiles and apparent
radial velocity shows correlations between jittering in apparent
radial velocity and fluctuations in line depth. <BR /> Conclusions:
Spatially resolved spectroscopy using exoplanet transits is feasible
for main-sequence stars. Overall line parameters of width, depth and
wavelength position can be retrieved already with moderate efforts,
but a very good signal-to-noise ratio is required to reveal the more
subtle signatures between subgroups of spectral lines, where finer
details of atmospheric structure are encoded. Fluctuations in line depth
correlate with those in wavelength, and because both can be measured
from the ground, searches for low-mass exoplanets should explore these
to adjust apparent radial velocities to actual stellar motion.
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Title: VizieR Online Data Catalog: Gaia RVS benchmark
stars. I. (Caffau+, 2021)
Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; Francois, P.;
Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.;
Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.;
Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite,
F.; Panuzzo, P.; Royer, F.; Thevenin, F.; Ludwig, H. -G.; Marchal,
O.; Plum, G.
2021yCat..36510020C Altcode:
For this project on the UVES spectrograph, we selected the setting
437+760. The choices on the setting were that (i) the 760 range
completely covers the RVS range without any gaps, and (ii) the 437 range
is the reddest setting that can be coupled with the 760 setting. For
metal-rich stars (the majority of our targets), observations in
blue settings provide very crowded spectra, and a higher S/N can be
achieved in this selected setting than in bluer settings such as the
390 setting. <P />We chose the highest UVES resolution (slit 0.4"
in the blue arm and 0.3" in the red arm). For all observations, the
DIC2 437+760 setting was used. For the stars brighter than V magnitude
8.5, an observing block comprises ten observations of 77.5s to avoid
detector saturation. For the stars fainter than 8.5, five exposures of
202 s allow avoiding detector saturation. In this program, 90 stars
have been observed, 80 of which are evolved stars and are analysed
here. The 10 unevolved stars will be analysed with stars of similar
stellar parameters that are observed or are scheduled to be observed
for the following two ESO periods (P105 and P106). <P />(2 data files).
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Title: Horizontal spreading of planetary debris accreted by white
dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Bauer, Evan B.;
Toloza, Odette; Cukanovaite, Elena; Koester, Detlev; Farihi, Jay;
Freytag, Bernd; Gänsicke, Boris T.; Ludwig, Hans-Günter; Veras,
Dimitri
2021MNRAS.503.1646C Altcode: 2021arXiv210209564C; 2021MNRAS.tmp..586C
White dwarfs with metal-polluted atmospheres have been studied widely
in the context of the accretion of rocky debris from evolved planetary
systems. One open question is the geometry of accretion and how
material arrives and mixes in the white dwarf surface layers. Using the
three-dimensional (3D) radiation hydrodynamics code CO<SUP>5</SUP>BOLD,
we present the first transport coefficients in degenerate star
atmospheres that describe the advection-diffusion of a passive scalar
across the surface plane. We couple newly derived horizontal diffusion
coefficients with previously published vertical diffusion coefficients
to provide theoretical constraints on surface spreading of metals in
white dwarfs. Our grid of 3D simulations probes the vast majority of
the parameter space of convective white dwarfs, with pure-hydrogen
atmospheres in the effective temperature range of 6000-18 000 K and
pure-helium atmospheres in the range of 12 000-34 000 K. Our results
suggest that warm hydrogen-rich atmospheres (DA; ${\gtrsim} 13\, 000$
K) and helium-rich atmospheres (DB and DBA; ${\gtrsim} 30\, 000$ K) are
unable to efficiently spread the accreted metals across their surface,
regardless of the time dependence of accretion. This result may be at
odds with the current non-detection of surface abundance variations in
white dwarfs with debris discs. For cooler hydrogen- and helium-rich
atmospheres, we predict a largely homogeneous distribution of metals
across the surface within a vertical diffusion time-scale. This is
typically less than 0.1 per cent of disc lifetime estimates, a quantity
that is revisited in this paper using the overshoot results. These
results have relevance for studies of the bulk composition of evolved
planetary systems and models of accretion disc physics.
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Title: 3D spectroscopic analysis of helium-line white dwarfs
Authors: Cukanovaite, Elena; Tremblay, Pier-Emmanuel; Bergeron,
Pierre; Freytag, Bernd; Ludwig, Hans-Günter; Steffen, Matthias
2021MNRAS.501.5274C Altcode: 2020arXiv201112693C; 2020MNRAS.tmp.3465C
In this paper, we present corrections to the spectroscopic parameters
of DB and DBA white dwarfs with -10.0 ≤ log (H/He) ≤ -2.0, 7.5
≤ log g ≤ 9.0, and $12\, 000$ ≲ T<SUB>eff</SUB> $\lesssim 34\,
000\,\mathrm{ K}$ , based on 282 3D atmospheric models calculated with
the CO<SUP>5</SUP>BOLD radiation-hydrodynamics code. These corrections
arise due to a better physical treatment of convective energy transport
in 3D models when compared to the previously available 1D model
atmospheres. By applying the corrections to an existing Sloan Digital
Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant
corrections both for effective temperature and surface gravity. The
3D log g corrections are most significant for T<SUB>eff</SUB> ≲
18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this
low effective temperature range, the surface gravity determined from
the spectroscopic technique can also be significantly affected by
the treatment of the neutral van der Waals line broadening of helium
and by non-ideal effects due to the perturbation of helium by neutral
atoms. Thus, by removing uncertainties due to 1D convection, our work
showcases the need for improved description of microphysics for DB
and DBA model atmospheres. Overall, we find that our 3D spectroscopic
parameters for the SDSS sample are generally in agreement with Gaia
Data Release 2 absolute fluxes within 1σ-3σ for individual white
dwarfs. By comparing our results to DA white dwarfs, we determine
that the precision and accuracy of DB/DBA atmospheric models are
similar. For ease of user application of the correction functions,
we provide an example PYTHON code.
---------------------------------------------------------
Title: The signature of granulation in a solar power spectrum as
seen with CO<SUP>5</SUP>BOLD
Authors: Lundkvist, Mia S.; Ludwig, Hans-Günter; Collet, Remo;
Straus, Thomas
2021MNRAS.501.2512L Altcode: 2020arXiv201110045L; 2020MNRAS.tmp.3425L
The granulation background seen in the power spectrum of a solar-like
oscillator poses a serious challenge for extracting precise and detailed
information about the stellar oscillations. Using a 3D hydrodynamical
simulation of the Sun computed with CO<SUP>5</SUP>BOLD, we investigate
various background models to infer, using a Bayesian methodology,
which one provides the best fit to the background in the simulated
power spectrum. We find that the best fit is provided by an expression
including the overall power level and two characteristic frequencies,
one with an exponent of two and one with a free exponent taking on a
value around six. We assess the impact of the 3D hydro-code on this
result by repeating the analysis with a simulation from S TAGGER and
find that the main conclusion is unchanged. However, the details of
the resulting best fits differ slightly between the two codes, but we
explain this difference by studying the effect of the spatial resolution
and the duration of the simulation on the fit. Additionally, we look
into the impact of adding white noise to the simulated time series as
a simple way to mimic a real star. We find that, as long as the noise
level is not too low, the results are consistent with the no-noise case.
---------------------------------------------------------
Title: Velocity-intensity asymmetry reversal of solar radial p-modes
Authors: Philidet, J.; Belkacem, K.; Ludwig, H. -G.; Samadi, R.;
Barban, C.
2020A&A...644A.171P Altcode: 2020arXiv201102439P
The development of space-borne missions has significantly improved the
quality of the measured spectra of solar-like oscillators. Their p-mode
line profiles can now be resolved, and the asymmetries inferred for a
variety of stars other than the Sun. However, it has been known for a
long time that the asymmetries of solar p-modes are reversed between
the velocity and the intensity spectra. Understanding the origin of
this reversal is necessary in order to use asymmetries as a tool for
seismic diagnosis. For stars other than the Sun, only the intensity
power spectrum is sufficiently resolved to allow for an estimation
of mode asymmetries. We recently developed an approach designed to
model and predict these asymmetries in the velocity power spectrum
of the Sun and to successfully compare them to their observationally
derived counterpart. In this paper we expand our model and predict the
asymmetries featured in the intensity power spectrum. We find that the
shape of the mode line profiles in intensity is largely dependent on how
the oscillation-induced variations of the radiative flux are treated,
and that modelling it realistically is crucial to understanding
asymmetry reversal. Perturbing a solar-calibrated grey atmosphere
model, and adopting the quasi-adiabatic framework as a first step, we
reproduce the asymmetries observed in the solar intensity spectrum for
low-frequency modes. We conclude that, unlike previously thought, it
is not necessary to invoke an additional mechanism (e.g. non-adiabatic
effects, coherent non-resonant background signal) to explain asymmetry
reversal. This additional mechanism is necessary, however, to explain
asymmetry reversal for higher-order modes.
---------------------------------------------------------
Title: The solar gravitational redshift from HARPS-LFC Moon
spectra⋆. A test of the general theory of relativity
Authors: González Hernández, J. I.; Rebolo, R.; Pasquini, L.;
Lo Curto, G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Esposito, M.; Suárez Mascareño, A.; Toledo-Padrón, B.; Probst,
R. A.; Hänsch, T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.;
Udem, Th.; Wilken, T.
2020A&A...643A.146G Altcode: 2020arXiv200910558G
Context. The general theory of relativity predicts the redshift of
spectral lines in the solar photosphere as a consequence of the
gravitational potential of the Sun. This effect can be measured
from a solar disk-integrated flux spectrum of the Sun's reflected
light on Solar System bodies. <BR /> Aims: The laser frequency comb
(LFC) calibration system attached to the HARPS spectrograph offers
the possibility of performing an accurate measurement of the solar
gravitational redshift (GRS) by observing the Moon or other Solar System
bodies. Here, we analyse the line shift observed in Fe absorption lines
from five high-quality HARPS-LFC spectra of the Moon. <BR /> Methods:
We selected an initial sample of 326 photospheric Fe lines in the
spectral range between 476-585 nm and measured their line positions
and equivalent widths (EWs). Accurate line shifts were derived from
the wavelength position of the core of the lines compared with the
laboratory wavelengths of Fe lines. We also used a CO<SUP>5</SUP>BOLD
3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic
line profiles of a subsample of about 200 spectral Fe lines centred
at their laboratory wavelengths. We fit the observed relatively
weak spectral Fe lines (with EW< 180 mÅ) with the 3D synthetic
profiles. <BR /> Results: Convective motions in the solar photosphere
do not affect the line cores of Fe lines stronger than about ∼150
mÅ. In our sample, only 15 Fe I lines have EWs in the range 150<
EW(mÅ) < 550, providing a measurement of the solar GRS at 639 ±
14 m s<SUP>-1</SUP>, which is consistent with the expected theoretical
value on Earth of ∼633.1 m s<SUP>-1</SUP>. A final sample of about 97
weak Fe lines with EW < 180 mÅ allows us to derive a mean global
line shift of 638 ± 6 m s<SUP>-1</SUP>, which is in agreement with
the theoretical solar GRS. <BR /> Conclusions: These are the most
accurate measurements of the solar GRS obtained thus far. Ultrastable
spectrographs calibrated with the LFC over a larger spectral range,
such as HARPS or ESPRESSO, together with a further improvement on the
laboratory wavelengths, could provide a more robust measurement of the
solar GRS and further testing of 3D hydrodynamical models. <P />Tables
A.1 and A.2 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A146">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A146</A>
<P />Based on observations taken with the ESO 3.6 m telescope at La
Silla Observatory, Chile.
---------------------------------------------------------
Title: VizieR Online Data Catalog: The solar gravitational redshift
(Gonzalez Hernandez+, 2020)
Authors: Gonzalez Hernandez, J. I.; Rebolo, R.; Pasquini, L.; Lo Curto,
G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Esposito,
M.; Suarez Mascarenno, A.; Toledo-Padron, B.; Probst, R. A.; Hansch,
T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.; Udem, T.; Wilken, T.
2020yCat..36430146G Altcode:
Line data and velocity shifts of the FeI and FeII lines,
with laboratory wavelengths, λ<SUB>lab</SUB>, from Nave et
al. (1994ApJS...94..221N, 2013ApJS..204....1N) and excitation
potentials, oscillator strengths from the VALD database (Piskunov et
al. 1995A&AS..112..525P). <P />In Table A.1 we provide the mean line
core shifts, v<SUB>core</SUB>obs, measured on the spectral lines from
the observed HARPS-LFC spectra of the MOON and computed with respect to
the original laboratory wavelengths (Nave et al. 1994ApJS...94..221N,
2013ApJS..204....1N). <P />We also give the recalibrated wavelengths,
lambda_nist, computed from recalibrated wavenumber measurements and
Ritz wavelengths, lambda_ritz, computed from recalibrated energy levels,
with their corresponding wavelengths uncertainties, extracted from the
NIST database (Kramida et al. 2019APS..DMPN09004K). <P />In Table A.2,
we give the line core shifts measured on the observed spectral lines,
v<SUB>core</SUB>obs_n, estimated using the recalibrated wavelengths,
lambda_nist, as reference laboratory wavelengths, the 3D profiles,
v<SUB>core,3D, and the global line shifts, v</SUB>fit<SUB>3D</SUB>n,
from fitting the observed spectral lines using 3D profiles, and
corrected using the recalibrated wavelengths lambda_nist as reference
laboratory wavelengths. <P />Wavelengths are given in Angstroms,
wavelength uncertainties in miliAngstroems, excitation potentials in
eV, equivalent widths (EW) in miliAngstroems, and velocity shifts in
m/s. <P />(2 data files).
---------------------------------------------------------
Title: Chemically Peculiar A and F Stars with Enhanced s-process
and Iron-peak Elements: Stellar Radiative Acceleration at Work
Authors: Xiang, Mao-Sheng; Rix, Hans-Walter; Ting, Yuan-Sen; Ludwig,
Hans-Günter; Coronado, Johanna; Zhang, Meng; Zhang, Hua-Wei; Buder,
Sven; Tio, Piero Dal
2020ApJ...898...28X Altcode: 2020arXiv200603329X
We present ≳15,000 metal-rich ([Fe/H] > -0.2 dex) A and
F stars whose surface abundances deviate strongly from solar
abundance ratios and cannot plausibly reflect their birth material
composition. These stars are identified by their high [Ba/Fe] abundance
ratios ([Ba/Fe] > 1.0 dex) in the LAMOST DR5 spectra analyzed by
Xiang et al. They are almost exclusively main-sequence and subgiant
stars with T<SUB>eff</SUB> ≳ 6300 K. Their distribution in the Kiel
diagram (T<SUB>eff</SUB>- $\mathrm{log}g$ ) traces a sharp border at
low temperatures along a roughly fixed-mass trajectory (around 1.4
M<SUB>⊙</SUB>) that corresponds to an upper limit in convective
envelope mass fraction of around 10<SUP>-4</SUP>. Most of these
stars exhibit distinctly enhanced abundances of iron-peak elements
(Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational
velocity measurements from GALAH DR2 show that the majority of these
stars rotate slower than typical stars in an equivalent temperature
range. These characteristics suggest that they are related to the
so-called Am/Fm stars. Their abundance patterns are qualitatively
consistent with the predictions of stellar evolution models that
incorporate radiative acceleration, suggesting they are a consequence
of stellar internal evolution, particularly involving the competition
between gravitational settling and radiative acceleration. These
peculiar stars constitute 40% of the whole population of stars with
mass above 1.5 M<SUB>⊙</SUB>, affirming that "peculiar" photospheric
abundances due to stellar evolution effects are a ubiquitous phenomenon
for these intermediate-mass stars. This large sample of Ba-enhanced,
chemically peculiar A/F stars with individual element abundances
provides the statistics to test more stringently the mechanisms that
alter the surface abundances in stars with radiative envelopes.
---------------------------------------------------------
Title: Spatially Resolved Stellar Disk Spectra at Hyper-high
Resolution: Toward Earth-like Exoplanet Detection
Authors: Dravins, D.; Ludwig, H.
2020AAS...23613002D Altcode:
High-precision spectroscopy might find 'truly' Earth-like
exoplanets. Instrumental precisions are close to being achieved
but limitations arise in the complexities of spectral-line
formation. Spectral lines become somewhat asymmetric by being
formed in dynamic gas flows. Radial-velocity signatures differ
between different types of lines, change between stars, vary across
stellar disks, and are modulated by magnetic activity. Spectroscopy
across spatially resolved stellar disks has become possible by using
transiting exoplanets as occulting spatial probes, permitting to
test center-to-limb atmospheric hydrodynamics in stars also other
than the Sun. Additional suitable target stars will likely be found
in exoplanet surveys, and simulated observations are in progress to
identify strategies for their near-future observations. From a grid
of 3-D hydrodynamic CO5BOLD model atmospheres for solar-type stars,
synthetic spectra have been computed at hyper-high spectral resolution
(R greater than 1 million), for several center-to-limb locations across
stellar disks. (The term 'hyper-high' is used since 'ultra-high'
is already taken for lower-resolution data.) Such resolutions are
required to fully resolve intrinsic line asymmetries. To segregate
those from such arising due to blends, and also to obtain absolute
wavelength shifts irrespective of errors in laboratory wavelengths,
3-D spectra are matched against similar data from 1-D models. There,
unblended lines appear symmetric at their laboratory wavelength
positions, and differences to 3-D profiles isolate effects arising in
the dynamic photospheres. Synthetic spectra are surveyed for unblended
lines with different strengths, excitation potentials, and ionization
levels, each of which contribute characteristic signatures of line
asymmetries and apparent Doppler shifts. The hyper-high resolution
data are degraded to common spectrometer values to appreciate what
signatures may realistically be observed. An adequate understanding
of both line formation and of spectrometer performance should enable
to disentangle effects from variable stellar atmospheres from those
induced by even small Earth-like exoplanets.
---------------------------------------------------------
Title: Modelling the asymmetries of the Sun's radial p-mode line
profiles
Authors: Philidet, J.; Belkacem, K.; Samadi, R.; Barban, C.; Ludwig,
H. -G.
2020A&A...635A..81P Altcode: 2020arXiv200110271P
Context. The advent of space-borne missions has substantially increased
the number and quality of the measured power spectrum of solar-like
oscillators. It now allows for the p-mode line profiles to be resolved
and facilitates an estimation of their asymmetry. The fact that this
asymmetry can be measured for a variety of stars other than the Sun
calls for a revisiting of acoustic mode asymmetry modelling. This
asymmetry has been shown to be related to a highly localised source
of stochastic driving in layers just beneath the surface. However,
existing models assume a very simplified, point-like source of
excitation. Furthermore, mode asymmetry could also be impacted by a
correlation between the acoustic noise and the oscillating mode. Prior
studies have modelled this impact, but only in a parametrised fashion,
which deprives them of their predictive power. <BR /> Aims: In this
paper, we aim to develop a predictive model for solar radial p-mode
line profiles in the velocity spectrum. Unlike the approach favoured by
prior studies, this model is not described by free parameters and we
do not use fitting procedures to match the observations. Instead, we
use an analytical turbulence model coupled with constraints extracted
from a 3D hydrodynamic simulation of the solar atmosphere. We
then compare the resulting asymmetries with their observationally
derived counterpart. <BR /> Methods: We model the velocity power
spectral density by convolving a realistic stochastic source term
with the Green's function associated with the radial homogeneous wave
equation. We compute the Green's function by numerically integrating the
wave equation and we use theoretical considerations to model the source
term. We reconstruct the velocity power spectral density and extract
the line profile of radial p-modes as well as their asymmetry. <BR />
Results: We find that stochastic excitation localised beneath the
mode upper turning point generates negative asymmetry for ν <
ν<SUB>max</SUB> and positive asymmetry for ν > ν<SUB>max</SUB>. On
the other hand, stochastic excitation localised above this limit
generates negative asymmetry throughout the p-mode spectrum. As a
result of the spatial extent of the source of excitation, both cases
play a role in the total observed asymmetries. By taking this spatial
extent into account and using a realistic description of the spectrum of
turbulent kinetic energy, both a qualitative and quantitative agreement
can be found with solar observations performed by the GONG network. We
also find that the impact of the correlation between acoustic noise and
oscillation is negligible for mode asymmetry in the velocity spectrum.
---------------------------------------------------------
Title: A high-precision abundance analysis of the nuclear benchmark
star HD 20
Authors: Hanke, Michael; Hansen, Camilla Juul; Ludwig, Hans-Günter;
Cristallo, Sergio; McWilliam, Andrew; Grebel, Eva K.; Piersanti,
Luciano
2020A&A...635A.104H Altcode: 2020arXiv200111038H
Metal-poor stars with detailed information available about their
chemical inventory pose powerful empirical benchmarks for nuclear
astrophysics. Here we present our spectroscopic chemical abundance
investigation of the metal-poor ([Fe/H] = -1.60 ± 0.03 dex),
r-process-enriched ([Eu/Fe] = 0.73 ± 0.10 dex) halo star <ASTROBJ>HD
20</ASTROBJ>, using novel and archival high-resolution data at
outstanding signal-to-noise ratios (up to ∼1000 Å<SUP>-1</SUP>). By
combining one of the first asteroseismic gravity measurements in the
metal-poor regime from a TESS light curve with the spectroscopic
analysis of iron lines under non-local thermodynamic equilibrium
conditions, we derived a set of highly accurate and precise stellar
parameters. These allowed us to delineate a reliable chemical pattern
that is comprised of solid detections of 48 elements, including 28
neutron-capture elements. Hence, we establish HD 20 among the few
benchmark stars that have nearly complete patterns and low systematic
dependencies on the stellar parameters. Our light-element (Z ≤ 30)
abundances are representative of other, similarly metal-poor stars
in the Galactic halo that exhibit contributions from core-collapse
supernovae of type II. In the realm of the neutron-capture elements,
our comparison to the scaled solar r-pattern shows that the
lighter neutron-capture elements (Z ≲ 60) are poorly matched. In
particular, we find imprints of the weak r-process acting at low
metallicities. Nonetheless, by comparing our detailed abundances to
the observed metal-poor star BD +17 3248, we find a persistent residual
pattern involving mainly the elements Sr, Y, Zr, Ba, and La. These are
indicative of enrichment contributions from the s-process and we show
that mixing with material from predicted yields of massive, rotating
AGB stars at low metallicity improves the fit considerably. Based on
a solar ratio of heavy- to light-s elements - which is at odds with
model predictions for the i-process - and a missing clear residual
pattern with respect to other stars with claimed contributions from
this process, we refute (strong) contributions from such astrophysical
sites providing intermediate neutron densities. Finally, nuclear
cosmochronology is used to tie our detection of the radioactive
element Th to an age estimate for HD 20 of 11.0 ± 3.8 Gyr. <P />Full
Table C.1 is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/635/A104">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/635/A104</A>
<P />This paper includes data gathered with the 6.5 m Magellan
Telescopes located at Las Campanas Observatory, Chile. Based in part
on data products from observations made with ESO Telescopes under
program IDs 090.B-0605(A) (PI: Chanamé) and 60.A-9036(A).
---------------------------------------------------------
Title: VizieR Online Data Catalog: Chemical abundance analysis of
HD 20 (Hanke+, 2020)
Authors: Hanke, M.; Hansen, C. J.; Ludwig, H. -G.; Cristallo, S.;
McWilliam, A.; Grebel, E. K.; Piersanti, L.
2020yCat..36350104H Altcode:
Equivalent widths (EWs) measured from HD 20's spectra using EWCODE
are presented alongside individual transition parameters and deduced
abundances. Profiles for which a standard EW analysis was prohibited
were analyzed using spectrum synthesis. NLTE corrections were performed
and are listed whenever available. <P />(1 data file).
---------------------------------------------------------
Title: Calibration of the mixing-length theory for structures of
helium-dominated atmosphere white dwarfs
Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig,
H. -G.; Fontaine, G.; Brassard, P.; Toloza, O.; Koester, D.
2019MNRAS.490.1010C Altcode: 2019MNRAS.tmp.2282C; 2019arXiv190910532C
We perform a calibration of the mixing-length parameter at the bottom
boundary of the convection zone for helium-dominated atmospheres of
white dwarfs. This calibration is based on a grid of 3D DB (pure-helium)
and DBA (helium-dominated with traces of hydrogen) model atmospheres
computed with the CO5BOLD radiation-hydrodynamics code, and a grid
of 1D DB and DBA envelope structures. The 3D models span a parameter
space of hydrogen-to-helium abundances in the range -10.0 ≤ log
(H/He) ≤-2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0,
and effective temperatures in the range 12 000 K ≲ T<SUB>eff</SUB> ≲
34 000 K. The 1D envelopes cover a similar atmospheric parameter range,
but are also calculated with different values of the mixing-length
parameter, namely 0.4 ≤ ML2/α ≤ 1.4. The calibration is performed
based on two definitions of the bottom boundary of the convection zone:
the Schwarzschild and the zero convective flux boundaries. Thus, our
calibration is relevant for applications involving the bulk properties
of the convection zone including its total mass, which excludes the
spectroscopic technique. Overall, the calibrated ML2/α is smaller
than what is commonly used in evolutionary models and theoretical
determinations of the blue edge of the instability strip for pulsating
DB and DBA stars. With calibrated ML2/α we are able to deduce more
accurate convection zone sizes needed for studies of planetary debris
mixing and dredge-up of carbon from the core. We highlight this by
calculating examples of metal-rich 3D DBAZ models and finding their
convection zone masses. Mixing-length calibration represents the first
step of in-depth investigations of convective overshoot in white dwarfs
with helium-dominated atmospheres.
---------------------------------------------------------
Title: Convective overshoot and macroscopic diffusion in
pure-hydrogen-atmosphere white dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Freytag, Bernd;
Ludwig, Hans-Günter; Koester, Detlev
2019MNRAS.488.2503C Altcode: 2019MNRAS.tmp.1723C; 2019arXiv190611252C
We present a theoretical description of macroscopic diffusion
caused by convective overshoot in pure-hydrogen DA white dwarfs
using 3D, closed-bottom, radiation hydrodynamics CO<SUP>5</SUP>BOLD
simulations. We rely on a new grid of deep 3D white dwarf models
in the temperature range 11 400 ≤ T_{eff} ≤ 18 000 K where
tracer particles and a tracer density are used to derive macroscopic
diffusion coefficients driven by convective overshoot. These diffusion
coefficients are compared to microscopic diffusion coefficients from 1D
structures. We find that the mass of the fully mixed region is likely to
increase by up to 2.5 orders of magnitude while inferred accretion rates
increase by a more moderate order of magnitude. We present evidence
that an increase in settling time of up to 2 orders of magnitude is to
be expected, which is of significance for time-variability studies of
polluted white dwarfs. Our grid also provides the most robust constraint
on the onset of convective instabilities in DA white dwarfs to be in
the effective temperature range from 18 000 to 18 250 K.
---------------------------------------------------------
Title: The <SUP>6</SUP>Li/<SUP>7</SUP>Li isotopic ratio in the
metal-poor binary CS22876-032
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Ludwig, H. -G.; Steffen, M.; Monaco, L.; Cayrel, R.
2019A&A...628A.111G Altcode: 2019arXiv190705109G
<BR /> Aims: We present high-resolution and high-quality UVES
spectroscopic data of the metal-poor double-lined spectroscopic binary
CS 22876-032 ([Fe/H] approximately -3.7 dex). Our goal is to derive
the <SUP>6</SUP>Li/<SUP>7</SUP>Li isotopic ratio by analysing the
Li I λ 670.8 nm doublet. <BR /> Methods: We co-added all 28 useful
spectra normalised and corrected for radial velocity to the rest frame
of the primary star. We fitted the Li profile with a grid of the 3D
non-local thermodynamic equilibrium (NLTE) synthetic spectra to take
into account the line profile asymmetries induced by stellar convection,
and performed Monte Carlo simulations to evaluate the uncertainty of
the fit of the Li line profile. <BR /> Results: We checked that the
veiling factor does not affect the derived isotopic ratio, <SUP>6</SUP>
Li/<SUP>7</SUP>Li, and only modifies the Li abundance, A(Li), by
about 0.15 dex. The best fit of the Li profile of the primary star
provides A(Li) = 2.17 ± 0.01 dex and <SUP>6</SUP> Li/<SUP>7</SUP>Li =
8<SUB>-5</SUB><SUP>+2</SUP>% at 68% confidence level. In addition, we
improved the Li abundance of the secondary star at A(Li) = 1.55 ± 0.04
dex, which is about 0.6 dex lower than that of the primary star. <BR
/> Conclusions: The analysis of the Li profile of the primary star is
consistent with no detection of <SUP>6</SUP> Li and provides an upper
limit to the isotopic ratio of <SUP>6</SUP> Li/<SUP>7</SUP>Li <
10% at this very low metallicity, about 0.5 dex lower in metallicity
than previous attempts for detection of <SUP>6</SUP> Li in extremely
metal poor stars. These results do not solve or worsen the cosmological
<SUP>7</SUP> Li problem, nor do they support the need for non-standard
<SUP>6</SUP>Li production in the early Universe. <P />The two averaged
spectra are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A111">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A111</A>Based
on observations made with the Very Large Telescope (VLT) at ESO Paranal
Observatory, Chile, Programme 080.D-0333.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Li in BPS CS22876-032 spectrum
(Gonzalez Hernandez+, 2019)
Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Ludwig,
H. -G.; Steffen, M.; Monaco, L.; Cayrel, R.
2019yCat..36280111G Altcode:
Average co-added, rebinned spectra in the region around the LiI 670.8nm
resonance line of the two stellar components of the metal-poor binary
CS 22876-032 A and CS 22876-032 B. For each star, wavelength, normalised
flux and flux error are given. <P />(2 data files).
---------------------------------------------------------
Title: Masses of the Hyades white dwarfs. A gravitational redshift
measurement
Authors: Pasquini, L.; Pala, A. F.; Ludwig, H. -G.; Lẽao, I. C.;
de Medeiros, J. R.; Weiss, A.
2019A&A...627L...8P Altcode: 2019arXiv190701265P
Context. It is possible to accurately measure the masses of the white
dwarfs (WDs) in the Hyades cluster using gravitational redshift,
because the radial velocity of the stars can be obtained independently
of spectroscopy from astrometry and the cluster has a low velocity
dispersion. <BR /> Aims: We aim to obtain an accurate measurement of
the Hyades WD masses by determining the mass-to-radius ratio (M/R)
from the observed gravitational redshift, and to compare them with
masses derived from other methods. <BR /> Methods: We analyse archive
high-resolution UVES-VLT spectra of six WDs belonging to the Hyades
to measure their Doppler shift, from which M/R is determined after
subtracting the astrometric radial velocity. We estimate the radii
using Gaia photometry as well as literature data. <BR /> Results:
The M/R error associated to the gravitational redshift measurement is
about 5%. The radii estimates, evaluated with different methods, are
in very good agreement, though they can differ by up to 4% depending
on the quality of the data. The masses based on gravitational redshift
are systematically smaller than those derived from other methods, by
a minimum of ∼0.02 up to 0.05 solar masses. While this difference
is within our measurement uncertainty, the fact that it is systematic
indicates a likely real discrepancy between the different methods. <BR
/> Conclusions: We show that the M/R derived from gravitational redshift
measurements is a powerful tool to determine the masses of the Hyades
WDs and could reveal interesting properties of their atmospheres. The
technique can be improved by using dedicated spectrographs, and
can be extended to other clusters, making it unique in its ability
to accurately and empirically determine the masses of WDs in open
clusters. At the same time we prove that gravitational redshift in
WDs agrees with the predictions of stellar evolution models to within
a few percent. <P />Based on UVES data from the ESO VLT archive.
---------------------------------------------------------
Title: Analysis of surface effect on solar-like oscillation
frequencies using 3D hydrodynamical models
Authors: Sonoi, T.; Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau,
E.; Mosser, B.
2019EAS....82..253S Altcode:
We evaluate the frequency difference between standard stellar
models and models patched with 3D hydrodynamical models across the
T<SUB>eff</SUB>-g plane. It allows us to constrain frequency corrections
for surface effect. The coefficients in the correction functionals are
thus provided as functions of effective temperature and surface gravity.
---------------------------------------------------------
Title: Accurate effective temperature from Hα profiles
Authors: Giribaldi, R. E.; Ubaldo-Melo, M. L.; Porto de Mello, G. F.;
Pasquini, L.; Ludwig, H. -G.; Ulmer-Moll, S.; Lorenzo-Oliveira, D.
2019A&A...624A..10G Altcode: 2018arXiv181112274G
Context. The determination of stellar effective temperature
(T<SUB>eff</SUB>) in F, G, and K stars using Hα profile fitting
is a quite remarkable and powerful tool because it does not depend
on reddening and is only slightly sensitive to other atmospheric
parameters. Nevertheless, this technique is not frequently used because
of the complex procedure needed to recover the profile of broad lines in
echelle spectra. As a consequence, tests performed on different models
have sometimes provided ambiguous results. <BR /> Aims: The main aim of
this work is to test the ability of the Hα profile fitting technique
to derive T<SUB>eff</SUB>. We also aim to improve the applicability
of this technique to echelle spectra and to test how well 1D + LTE
models perform on a variety of F-K stars. We also apply the technique
to HARPS spectra and test the reliability and the stability of the
HARPS response over several years using the Sun. <BR /> Methods: We
have developed a normalization method for recovering undistorted Hα
profiles and we have first applied it to spectra acquired with the
single-order Coudé instrument (resolution R = 45 000) at do Pico
dos Dias Observatory to avoid the problem of blaze correction. The
continuum location around Hα is optimised using an iterative procedure,
where the identification of minute telluric features is performed. A
set of spectra was acquired with the MUSICOS echelle spectrograph
(R = 40 000) to independently validate the normalization method. The
accuracy of the method and of the 1D + LTE model is determined using
Coudé/HARPS/MUSICOS spectra of the Sun and Coudé-only spectra of
a sample of ten Gaia Benchmark Stars with T<SUB>eff</SUB> determined
from interferometric measurements. HARPS, Coudé, and MUSICOS spectra
are used to determine T<SUB>eff</SUB> of 43 sample stars. <BR />
Results: We find that a proper choice of spectral windows of fits
plus the identification of telluric features allow for a very careful
normalization of the spectra and produce reliable Hα profiles. We also
find that the most used solar atlases cannot be used as templates for
Hα temperature diagnostics without renormalization. The comparison with
the Sun shows that Hα profiles from 1D + LTE models underestimate the
solar T<SUB>eff</SUB> by 28 K. We find the same agreement between Hα
and interferometry and between Hα and Infrared Flux Method: a shallow
dependency on metallicity according to the relation T<SUB>eff</SUB> =
T<SUB>eff</SUB><SUP>Hα</SUP> - 159[Fe/H] + 28 K within the metallicity
range - 0.70 to + 0.40 dex. The comparison with the Infrared Flux
Method shows a scatter of 59 K dominated by photometric errors
(52 K). In order to investigate the origin of this dependency, we
analyzed spectra from 3D models and found that they produce hotter
temperatures, and that their use largely improves the agreement with
the interferometric and Infrared Flux Method measurements. Finally,
we find HARPS spectra to be fully suitable for Hα profile temperature
diagnostics; they are perfectly compatible with the Coudé spectra,
and lead to the same T<SUB>eff</SUB> for the Sun as that found when
analysing HARPS spectra over a timespan of more than 7 yr. <P />A
copy of the spectra is available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A10">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A10</A>Based
on observations collected at Observatório do Pico dos Dias (OPD),
operated by the Laboratório Nacional de Astrofísica, CNPq, Brazil
and on data from the ESO Science Archive Facility.
---------------------------------------------------------
Title: Two-dimensional non-LTE O I 777 nm line formation in radiation
hydrodynamics simulations of Cepheid atmospheres
Authors: Vasilyev, V.; Amarsi, A. M.; Ludwig, H. -G.; Lemasle, B.
2019A&A...624A..85V Altcode: 2019arXiv190302109V
Oxygen abundance measurements are important for understanding stellar
structure and evolution. Measured in Cepheids, they further provide
clues on the metallicity gradient and chemo-dynamical evolution
in the Galaxy. However, most of the abundance analyses of Cepheids
to date have been based on one-dimensional (1D) hydrostatic model
atmospheres. Here, we test the validity of this approach for the key
oxygen abundance diagnostic, the O I 777 nm triplet lines. We carry
out two-dimensional (2D) non-LTE radiative transfer calculations
across two different 2D radiation hydrodynamics simulations of Cepheid
atmospheres, having stellar parameters of T<SUB>eff</SUB> = 5600 K,
solar chemical compositions, and log g = 1.5 and 2.0, corresponding
to pulsation periods of 9 and 3 days, respectively. We find that
the 2D non-LTE versus 1D LTE abundance differences range from -1.0
to -0.25 dex depending on pulsational phase. The 2D non-LTE versus 1D
non-LTE abundance differences range from -0.2 to 0.8 dex. The abundance
differences are smallest when the Cepheid atmospheres are closest to
hydrostatic equilibrium, corresponding to phases of around 0.3-0.8, and
we recommend these phases for observers deriving the oxygen abundance
from O I 777 nm triplet with 1D hydrostatic models.
---------------------------------------------------------
Title: 4MOST: Project overview and information for the First Call
for Proposals
Authors: de Jong, R. S.; Agertz, O.; Berbel, A. A.; Aird, J.;
Alexander, D. A.; Amarsi, A.; Anders, F.; Andrae, R.; Ansarinejad,
B.; Ansorge, W.; Antilogus, P.; Anwand-Heerwart, H.; Arentsen, A.;
Arnadottir, A.; Asplund, M.; Auger, M.; Azais, N.; Baade, D.; Baker,
G.; Baker, S.; Balbinot, E.; Baldry, I. K.; Banerji, M.; Barden,
S.; Barklem, P.; Barthélémy-Mazot, E.; Battistini, C.; Bauer, S.;
Bell, C. P. M.; Bellido-Tirado, O.; Bellstedt, S.; Belokurov, V.;
Bensby, T.; Bergemann, M.; Bestenlehner, J. M.; Bielby, R.; Bilicki,
M.; Blake, C.; Bland-Hawthorn, J.; Boeche, C.; Boland, W.; Boller,
T.; Bongard, S.; Bongiorno, A.; Bonifacio, P.; Boudon, D.; Brooks,
D.; Brown, M. J. I.; Brown, R.; Brüggen, M.; Brynnel, J.; Brzeski,
J.; Buchert, T.; Buschkamp, P.; Caffau, E.; Caillier, P.; Carrick,
J.; Casagrande, L.; Case, S.; Casey, A.; Cesarini, I.; Cescutti, G.;
Chapuis, D.; Chiappini, C.; Childress, M.; Christlieb, N.; Church, R.;
Cioni, M. -R. L.; Cluver, M.; Colless, M.; Collett, T.; Comparat, J.;
Cooper, A.; Couch, W.; Courbin, F.; Croom, S.; Croton, D.; Daguisé,
E.; Dalton, G.; Davies, L. J. M.; Davis, T.; de Laverny, P.; Deason,
A.; Dionies, F.; Disseau, K.; Doel, P.; Döscher, D.; Driver, S. P.;
Dwelly, T.; Eckert, D.; Edge, A.; Edvardsson, B.; Youssoufi, D. E.;
Elhaddad, A.; Enke, H.; Erfanianfar, G.; Farrell, T.; Fechner, T.;
Feiz, C.; Feltzing, S.; Ferreras, I.; Feuerstein, D.; Feuillet, D.;
Finoguenov, A.; Ford, D.; Fotopoulou, S.; Fouesneau, M.; Frenk, C.;
Frey, S.; Gaessler, W.; Geier, S.; Gentile Fusillo, N.; Gerhard,
O.; Giannantonio, T.; Giannone, D.; Gibson, B.; Gillingham, P.;
González-Fernández, C.; Gonzalez-Solares, E.; Gottloeber, S.; Gould,
A.; Grebel, E. K.; Gueguen, A.; Guiglion, G.; Haehnelt, M.; Hahn, T.;
Hansen, C. J.; Hartman, H.; Hauptner, K.; Hawkins, K.; Haynes, D.;
Haynes, R.; Heiter, U.; Helmi, A.; Aguayo, C. H.; Hewett, P.; Hinton,
S.; Hobbs, D.; Hoenig, S.; Hofman, D.; Hook, I.; Hopgood, J.; Hopkins,
A.; Hourihane, A.; Howes, L.; Howlett, C.; Huet, T.; Irwin, M.; Iwert,
O.; Jablonka, P.; Jahn, T.; Jahnke, K.; Jarno, A.; Jin, S.; Jofre,
P.; Johl, D.; Jones, D.; Jönsson, H.; Jordan, C.; Karovicova, I.;
Khalatyan, A.; Kelz, A.; Kennicutt, R.; King, D.; Kitaura, F.; Klar,
J.; Klauser, U.; Kneib, J. -P.; Koch, A.; Koposov, S.; Kordopatis, G.;
Korn, A.; Kosmalski, J.; Kotak, R.; Kovalev, M.; Kreckel, K.; Kripak,
Y.; Krumpe, M.; Kuijken, K.; Kunder, A.; Kushniruk, I.; Lam, M. I.;
Lamer, G.; Laurent, F.; Lawrence, J.; Lehmitz, M.; Lemasle, B.; Lewis,
J.; Li, B.; Lidman, C.; Lind, K.; Liske, J.; Lizon, J. -L.; Loveday,
J.; Ludwig, H. -G.; McDermid, R. M.; Maguire, K.; Mainieri, V.; Mali,
S.; Mandel, H.; Mandel, K.; Mannering, L.; Martell, S.; Martinez
Delgado, D.; Matijevic, G.; McGregor, H.; McMahon, R.; McMillan,
P.; Mena, O.; Merloni, A.; Meyer, M. J.; Michel, C.; Micheva, G.;
Migniau, J. -E.; Minchev, I.; Monari, G.; Muller, R.; Murphy, D.;
Muthukrishna, D.; Nandra, K.; Navarro, R.; Ness, M.; Nichani, V.;
Nichol, R.; Nicklas, H.; Niederhofer, F.; Norberg, P.; Obreschkow, D.;
Oliver, S.; Owers, M.; Pai, N.; Pankratow, S.; Parkinson, D.; Paschke,
J.; Paterson, R.; Pecontal, A.; Parry, I.; Phillips, D.; Pillepich,
A.; Pinard, L.; Pirard, J.; Piskunov, N.; Plank, V.; Plüschke, D.;
Pons, E.; Popesso, P.; Power, C.; Pragt, J.; Pramskiy, A.; Pryer,
D.; Quattri, M.; Queiroz, A. B. d. A.; Quirrenbach, A.; Rahurkar,
S.; Raichoor, A.; Ramstedt, S.; Rau, A.; Recio-Blanco, A.; Reiss, R.;
Renaud, F.; Revaz, Y.; Rhode, P.; Richard, J.; Richter, A. D.; Rix,
H. -W.; Robotham, A. S. G.; Roelfsema, R.; Romaniello, M.; Rosario, D.;
Rothmaier, F.; Roukema, B.; Ruchti, G.; Rupprecht, G.; Rybizki, J.;
Ryde, N.; Saar, A.; Sadler, E.; Sahlén, M.; Salvato, M.; Sassolas,
B.; Saunders, W.; Saviauk, A.; Sbordone, L.; Schmidt, T.; Schnurr,
O.; Scholz, R. -D.; Schwope, A.; Seifert, W.; Shanks, T.; Sheinis,
A.; Sivov, T.; Skúladóttir, Á.; Smartt, S.; Smedley, S.; Smith,
G.; Smith, R.; Sorce, J.; Spitler, L.; Starkenburg, E.; Steinmetz,
M.; Stilz, I.; Storm, J.; Sullivan, M.; Sutherland, W.; Swann, E.;
Tamone, A.; Taylor, E. N.; Teillon, J.; Tempel, E.; ter Horst, R.;
Thi, W. -F.; Tolstoy, E.; Trager, S.; Traven, G.; Tremblay, P. -E.;
Tresse, L.; Valentini, M.; van de Weygaert, R.; van den Ancker, M.;
Veljanoski, J.; Venkatesan, S.; Wagner, L.; Wagner, K.; Walcher,
C. J.; Waller, L.; Walton, N.; Wang, L.; Winkler, R.; Wisotzki, L.;
Worley, C. C.; Worseck, G.; Xiang, M.; Xu, W.; Yong, D.; Zhao, C.;
Zheng, J.; Zscheyge, F.; Zucker, D.
2019Msngr.175....3D Altcode: 2019arXiv190302464D
We introduce the 4-metre Multi-Object Spectroscopic Telescope (4MOST),
a new high-multiplex, wide-field spectroscopic survey facility under
development for the four-metre-class Visible and Infrared Survey
Telescope for Astronomy (VISTA) at Paranal. Its key specifications
are: a large field of view (FoV) of 4.2 square degrees and a high
multiplex capability, with 1624 fibres feeding two low-resolution
spectrographs (R = λ/Δλ 6500), and 812 fibres transferring light
to the high-resolution spectrograph (R 20 000). After a description of
the instrument and its expected performance, a short overview is given
of its operational scheme and planned 4MOST Consortium science; these
aspects are covered in more detail in other articles in this edition
of The Messenger. Finally, the processes, schedules, and policies
concerning the selection of ESO Community Surveys are presented,
commencing with a singular opportunity to submit Letters of Intent
for Public Surveys during the first five years of 4MOST operations.
---------------------------------------------------------
Title: Spectroscopic and astrometric radial velocities: Hyades as
a benchmark
Authors: Leão, I. C.; Pasquini, L.; Ludwig, H. -G.; de Medeiros, J. R.
2019MNRAS.483.5026L Altcode: 2018arXiv181108771L; 2018MNRAS.tmp.3060L
We study the accuracy of spectroscopic radial velocities (RVs) by
comparing spectroscopic and astrometric RVs for stars of the Hyades
open cluster. We obtained High-Accuracy Radial velocity Planet Searcher
(HARPS) spectra for a large sample of Hyades stars and combined them
with accurate astrometric RVs based on Hipparcos and Gaia data. After
cleaning the sample from binaries, RV variables, and outliers, 71
stars remained. The distribution of the observed RV difference (between
spectroscopic and astrometric) is skewed and depends on the star right
ascension. This is consistent with the Hyades cluster rotating at 42.3 m
s<SUP>-1</SUP>pc<SUP>-1</SUP>. The two Hyades giants in the sample show,
as predicted by gravitational redshift (GR), a spectroscopic RV that
is blue-shifted with respect to the dwarfs, and the empirical GR slope
is of 626 ± 131 m s<SUP>-1</SUP>, in agreement with the theoretical
prediction. The difference between spectroscopic and astrometric
RVs is close to zero when considering the Gaia-based RVs corrected
for cluster rotation. The mean difference is of -33 m s<SUP>-1</SUP>
and the median is of -16 m s<SUP>-1</SUP>, with a standard deviation
of 347 m s<SUP>-1</SUP> being close to the expected cluster velocity
dispersion. We also determine a new value of the cluster centroid
spectroscopic RV: 39.36 ± 0.26 km s<SUP>-1</SUP>. We finally discuss
other phenomena that can influence the RV difference, such as cluster
expansion, stellar rotation, stellar activity, general relativity,
and Galactic potential. Clusters within the reach of current telescopes
are expected to show differences of several hundreds m s<SUP>-1</SUP>,
depending on their position in the Galaxy.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Normalized Halpha line profiles
of FGK stars (Giribaldi+, 2019)
Authors: Giribaldi, R. E.; Ubaldo-Melo, M. L.; Porto de Mello, G. F.;
Pasquini, L.; Ludwig, H. -G.; Ulmer-Moll, S.; Lorenzo-Oliveira, D.
2019yCat..36240010G Altcode:
Normalized observational Hα line profiles of 43 F- ,G- , and
K-type stars, including the Sun. The profiles were recorded in
spectra acquired by the single-order coude instrument at do Pico
dos Dias Observatory. The spectra are provided in .fits format,
and their observation conditions are registered in the headers
of the files. For some spectra, a version corrected from telluric
contamination is also provided but in ascii format only. These files
contain three columns: Wavelength (Angstrom), telluric-corrected flux,
and non telluric-corrected flux. Some telluric-corrected spectra may
present obvious correction errors. Some spectra may be contaminated by
cosmic rays. The names of the files indicate the Henry Draper catalog
number of the star, except for the Sun, for which the names of the
solar surrogates are indicated. The name of the stars are followed by
a number 1, 2, 3, or 4, which indicates a different spectrum of the
same star. <P />(3 data files).
---------------------------------------------------------
Title: 4MOST Consortium Survey 2: The Milky Way Halo High-Resolution
Survey
Authors: Christlieb, N.; Battistini, C.; Bonifacio, P.; Caffau, E.;
Ludwig, H. -G.; Asplund, M.; Barklem, P.; Bergemann, M.; Church, R.;
Feltzing, S.; Ford, D.; Grebel, E. K.; Hansen, C. J.; Helmi, A.;
Kordopatis, G.; Kovalev, M.; Korn, A.; Lind, K.; Quirrenbach, A.;
Rybizki, J.; Skúladóttir, Á.; Starkenburg, E.
2019Msngr.175...26C Altcode: 2019arXiv190302468C
We will study the formation history of the Milky Way, and the earliest
phases of its chemical enrichment, with a sample of more than 1.5
million stars at high galactic latitude. Elemental abundances of up to
20 elements with a precision of better than 0.2 dex will be derived
for these stars. The sample will include members of kinematically
coherent substructures, which we will associate with their possible
birthplaces by means of their abundance signatures and kinematics,
allowing us to test models of galaxy formation. Our target catalogue
is also expected to contain 30 000 stars at a metallicity of less than
one hundredth that of the Sun. This sample will therefore be almost
a factor of 100 larger than currently existing samples of metal-poor
stars for which precise elemental abundances are available (determined
from high-resolution spectroscopy), enabling us to study the early
chemical evolution of the Milky Way in unprecedented detail.
---------------------------------------------------------
Title: Systematic investigation of chemical abundances derived using
IR spectra obtained with GIANO
Authors: Caffau, E.; Bonifacio, P.; Oliva, E.; Korotin, S.; Capitanio,
L.; Andrievsky, S.; Collet, R.; Sbordone, L.; Duffau, S.; Sanna, N.;
Tozzi, A.; Origlia, L.; Ryde, N.; Ludwig, H. -G.
2019A&A...622A..68C Altcode: 2018arXiv181205100C
Context. Detailed chemical abundances of Galactic stars are needed in
order to improve our knowledge of the formation and evolution of our
galaxy, the Milky Way. <BR /> Aims: We took advantage of the GIANO
archive spectra to select a sample of Galactic disc stars in order
to derive their chemical inventory and to compare the abundances we
derived from these infrared spectra to the chemical pattern derived
from optical spectra. <BR /> Methods: We analysed high-quality spectra
of 40 stars observed with GIANO. We derived the stellar parameters from
the photometry and the Gaia data-release 2 (DR2) parallax; the chemical
abundances were derived with the code MyGIsFOS. For a subsample of stars
we compared the chemical pattern derived from the GIANO spectra with
the abundances derived from optical spectra. We derived P abundances
for all 40 stars, increasing the number of Galactic stars for which
phosphorus abundance is known. <BR /> Results: We could derive
abundances of 14 elements, 8 of which are also derived from optical
spectra. The comparison of the abundances derived from infrared and
optical spectra is very good. The chemical pattern of these stars is
the one expected for Galactic disc stars and is in agreement with the
results from the literature. <BR /> Conclusions: GIANO is providing
the astronomical community with an extremely useful instrument, able
to produce spectra with high resolution and a wide wavelength range
in the infrared. <P />GIANO programme A31TAC.
---------------------------------------------------------
Title: Calibration of mixing-length parameter α for MLT and FST
models by matching with CO<SUP>5</SUP>BOLD models
Authors: Sonoi, T.; Ludwig, H. -G.; Dupret, M. -A.; Montalbán, J.;
Samadi, R.; Belkacem, K.; Caffau, E.; Goupil, M. -J.
2019A&A...621A..84S Altcode: 2018arXiv181105229S
Context. Space observations by the CoRoT and Kepler missions have
provided a wealth of high-quality seismic data for a large number of
stars from the main sequence to the red giant phases. One main goal of
these missions is to take advantage of the rich spectra of solar-like
oscillations to perform precise determinations of stellar characteristic
parameters. To make the best of such data, we need theoretical stellar
models with a precise near-surface structure since a near-surface
structure of a solar-like star has significant influence on solar-like
oscillation frequencies. The mixing-length parameter is a key factor
to determine the near-surface structure of stellar models. In current
versions of the convection formulations used in stellar evolution
codes, the mixing-length parameter is a free parameter that needs to
be properly specified. <BR /> Aims: We aim at determining appropriate
values of the mixing-length parameter, α, to be used consistently with
the adopted convection formulation when computing stellar evolution
models across the Hertzsprung-Russell diagram. This determination
is based on 3D hydrodynamical simulation models. <BR /> Methods:
We calibrated α values by matching entropy profiles of 1D envelope
models with those of hydrodynamical 3D models of solar-like stars
produced by the CO<SUP>5</SUP>BOLD code. For such calibration, previous
works concentrated on the classical mixing-length theory (MLT). We
also analyzed full spectrum turbulence (FST) models. To construct the
atmosphere in the 1D models, we used the Eddington gray T(τ) relation
and that with the solar-calibrated Hopf-like function. <BR /> Results:
For both MLT and FST models with a mixing length l = αH<SUB>p</SUB>,
calibrated α values increase with increasing surface gravity or
decreasing effective temperature. For the FST models, we carried
out an additional calibration using an α<SUP>*</SUP> value defined
as l = r<SUB>top</SUB> - r + α<SUP>*</SUP>H<SUB>p, top</SUB>, where
α<SUP>*</SUP> is found to increase with surface gravity and effective
temperature. We provide tables of the calibrated α values across
the T<SUB>eff</SUB>-log g plane for solar metallicity. By computing
stellar evolution with varying α based on our 3D α calibration, we
find that the change from solar α to varying α shifts evolutionary
tracks particularly for the FST model. As for the correspondence
to the 3D models, the solar Hopf-like function generally gives a
photospheric-minimum entropy closer to a 3D model than the Eddington
T(τ). The structure below the photosphere depends on the adopted
convection model. However, we cannot obtain a definitive conclusion
about which convection model gives the best correspondence to the 3D
models. This is because each 1D physical quantity is related via an
equation of state (EoS), but it is not the case for the averaged 3D
quantities. Although the FST models with l = r<SUB>top</SUB> - r +
α<SUP>*</SUP>H<SUB>p, top</SUB> are found to give the oscillation
frequencies closest to the solar observed frequencies, their acoustic
cavities are formed with compensatory effects between deviating
density and temperature profiles near the top of the convective
envelope. In future work, an appropriate treatment of the top part of
the 1D convective envelope is necessary, for example, by considering
turbulent pressure and overshooting.
---------------------------------------------------------
Title: TOPoS. V. Abundance ratios in a sample of very metal-poor
turn-off stars
Authors: François, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite,
F.; Cayrel, R.; Christlieb, N.; Gallagher, A. J.; Klessen, R.; Koch,
A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S.
2018A&A...620A.187F Altcode: 2018arXiv181100035F
Context. Extremely metal-poor stars are keys to understand the early
evolution of our Galaxy. The ESO large programme TOPoS has been tailored
to analyse a new set of metal-poor turn-off stars, whereas most of
the previously known extremely metal-poor stars are giant stars. <BR
/> Aims: Sixty five turn-off stars (preselected from SDSS spectra)
have been observed with the X-shooter spectrograph at the ESO VLT Unit
Telescope 2, to derive accurate and detailed abundances of magnesium,
silicon, calcium, iron, strontium and barium. <BR /> Methods: We
analysed medium-resolution spectra (R ≃ 10 000) obtained with the
ESO X-shooter spectrograph and computed the abundances of several
α and neutron-capture elements using standard one-dimensional local
thermodynamic equilibrium (1D LTE) model atmospheres. <BR /> Results:
Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in
metal-poor turn-off stars as observed in metal-poor giant stars. We
found a significant spread of the [α/Fe] ratios with several stars
showing subsolar [Ca/Fe] ratios. We could measure the abundance of
strontium in 12 stars of the sample, leading to abundance ratios
[Sr/Fe] around the Solar value. We detected barium in two stars
of the sample. One of the stars (SDSS J114424-004658) shows both
very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex). <P
/>Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere under ESO programme ID
189.D-0165. <P />Equivalent widths of the Fe lines are only, and Tables
A.1 and A.2 are also available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187</A>
---------------------------------------------------------
Title: Pure-helium 3D model atmospheres of white dwarfs
Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig,
H. -G.; Bergeron, P.
2018MNRAS.481.1522C Altcode: 2018arXiv180900590C; 2018MNRAS.tmp.2259C
We present the first grid of 3D simulations for the pure-helium
atmospheres of DB white dwarfs. The simulations were computed with the
co<SUP>5</SUP>bold radiation-hydrodynamics code and cover effective
temperatures and surface gravities between 12 000 K ≲ T<SUB>eff</SUB>
≲ 34 000 K and 7.5 ≤ log g (cgs units) ≤ 9.0, respectively. In
this introductory work, synthetic spectra calculated from the 3D
simulations are compared to appropriate 1D model spectra under a
differential approach. This results in the derivation of 3D corrections
for the spectroscopically derived atmospheric parameters of DB stars
with respect to the 1D ML2/α = 1.25 mixing-length parametrization. No
significant T<SUB>eff</SUB> corrections are found for the V777 Her
instability strip region, and therefore no 3D revision is expected
for the empirical blue and red edges of the strip. However, large log
g corrections are found in the range 12 000 K < T<SUB>eff</SUB>
< 23 000 K for all log g values covered by the 3D grid. These
corrections indicate that 1D model atmospheres overpredict log g,
reminiscent of the results found from 3D simulations of pure-hydrogen
white dwarfs. The next step will be to compute 3D simulations with mixed
helium and hydrogen atmospheres to comprehend the full implications
for the stellar parameters of DB and DBA white dwarfs.
---------------------------------------------------------
Title: Influence of metallicity on the near-surface effect on
oscillation frequencies
Authors: Manchon, L.; Belkacem, K.; Samadi, R.; Sonoi, T.; Marques,
J. P. C.; Ludwig, H. -G.; Caffau, E.
2018A&A...620A.107M Altcode: 2018arXiv180908904M
Context. The CoRoT and Kepler missions have provided high-quality
measurements of the frequency spectra of solar-like pulsators,
enabling us to probe stellar interiors with a very high degree of
accuracy by comparing the observed and modelled frequencies. However,
the frequencies computed with 1D models suffer from systematic
errors related to the poor modelling of the uppermost layers of
stars. These biases are what is commonly named the near-surface
effect. The dominant effect is thought to be related to the turbulent
pressure that modifies the hydrostatic equilibrium and thus the
frequencies. This has already been investigated using grids of 3D
hydrodynamical simulations, which also were used to constrain the
parameters of the empirical correction models. However, the effect
of metallicity has not been considered so far. <BR /> Aims: We aim to
study the impact of metallicity on the surface effect, investigating
its influence across the Hertzsprung-Russell diagram, and providing
a method for accounting for it when using the empirical correction
models. <BR /> Methods: We computed a grid of patched 1D stellar
models with the stellar evolution code CESTAM in which poorly modelled
surface layers have been replaced by averaged stratification computed
with the 3D hydrodynamical code CO<SUP>5</SUP>BOLD. It allowed us to
investigate the dependence of both the surface effect and the empirical
correction functions on the metallicity. <BR /> Results: We found
that metallicity has a strong impact on the surface effect: keeping
T<SUB>eff</SUB> and log g constant, the frequency residuals can vary by
up to a factor of two (for instance from [Fe/H] = + 0.0 to [Fe/H] = +
0.5). Therefore, the influence of metallicity cannot be neglected. We
found that the correct way of accounting for it is to consider the
surface Rosseland mean opacity. It allowed us to give a physically
grounded justification as well as a scaling relation for the frequency
differences at ν<SUB>max</SUB> as a function of T<SUB>eff</SUB>, log g
and κ. Finally, we provide prescriptions for the fitting parameters of
the most commonly used correction functions. <BR /> Conclusions: We show
that the impact of metallicity through the Rosseland mean opacity must
be taken into account when studying and correcting the surface effect.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Very metal-poor turn-off stars
abundances (Francois+, 2018)
Authors: Francois, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite,
F.; Cayrel, R.; Christlieb, N.; Gallagher, A.; Klessen, R.; Koch,
A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S.
2018yCat..36200187F Altcode:
Sixty five turn-off stars (preselected from SDSS spectra) have been
observed with the X-Shooter spectrograph at the ESO VLT Unit Telescope
2, to derive accurate and detailed abundances of magnesium, silicon,
calcium, iron, strontium and barium. We analysed medium-resolution
spectra (R~10000) obtained with the ESO X-Shooter spectrograph and
computed the abundances of several alpha and neutron-capture elements
using standard one-dimensional local thermodynamic equilibrium (1D LTE)
model atmospheres. <P />(3 data files).
---------------------------------------------------------
Title: Calibration of the mixing length of the MLT and FST models
using 3D hydrodynamical models
Authors: Sonoi, T.; Ludwig, H. -G.; Dupret, M. -A.; Montalban, J.;
Belkacem, K.; Caffau, E.
2018phos.confE..27S Altcode:
Rich spectra of solar-like oscillations obtained with space observations
are expected to enable us to perform precise determinations of stellar
properties. To make the best of the spectra, we need theoretical
stellar models with precise near-surface structure, since the
near-surface structure has significant influence on solar-like
oscillation frequencies. The mixing-length parameter, α, is a key
factor to determine the near-surface structure. We aimed at determining
appropriate α values based on 3D radiation-coupled hydrodynamical
models produced by the CO^5BOLD code. For such calibration, previous
works concentrated on the classical mixing-length theory (MLT). Here
we also analyzed the full spectrum turbulence (FST) models. The
trends of the calibrated α values in the T<SUB>eff</SUB>-g plane is
found to be similar to those of previous calibrations with the other
grids of RHD models. A T(τ) relation based on the so-called VAL-C
solar-atmosphere model is found to give better correspondence to
the photospheric-minimum entropy in the 3D model than the Eddington
T(τ) relation. Although the structure below the photosphere depends
on convection models, not a single convection model gives the best
correspondence to the 3D model since physical quantities in the 3D
models are not necessarily related via an equation of states unlike
those in the 1D models. Although the FST model with a form of a mixing
length (l=r<SUB>top</SUB>-r+α<SUP>*</SUP>H<SUB>p,{top}</SUB>) is found
to give solar-oscillation frequencies apparently closest to the observed
ones, the acoustic cavity of this model is formed with compensatory
effects between deviating density and temperature profiles just below
the top of the convective envelope. In future work, a more sophisticated
treatment of the top part of the 1D convective envelope is necessary.
---------------------------------------------------------
Title: A physically-grounded relation between the metallicity and
the surface term affecting stellar oscillation frequencies
Authors: Manchon, Louis; Belkacem, Kevin; Samadi, Reza; Sonoi,
Takafumi; Marques, J. P. C.; Ludwig, Hans-Gunter; Caffau, E.
2018phos.confE..36M Altcode:
The CoRoT and Kepler missions have provided high-quality measurements
of the frequency spectra of solar-like pulsators, enabling us to probe
stellar interiors with a very high degree of accuracy by comparing the
observed and modeled frequencies. However, the frequencies computed with
1D models suffer from systematic errors related to the poor modeling of
the uppermost layers of stars. These biases are what is commonly named
the near surface effect. The dominant effect is thought to be related
to the turbulent pressure that modifies the hydrostatic equilibrium and
thus the frequencies. This has already been investigated using grids
of 3D hydrodynamical simulations, however, the effect of metallicity
has not been considered so far. We aim at studying the impact of
metallicity on the surface effect, investigating its influence across
the Hertzsprung–Russell diagram, and providing a relation between
the frequency differences and global parameters. We computed a grid
of 29 patched 1D stellar models with the stellar evolution code
CESTAM in which poorly modeled surface layers have been replaced by
averaged stratification computed with the 3D hydrodynamical code CO 5
BOLD. It allowed us to study the dependence of the surface effect on
the metallicity. We found that a correct way of accounting for it is
to consider the surface Rosseland mean opacity. It allowed us to give a
physically-grounded justification as well as a scaling relation for the
frequency differences at ν max as a function of T eff , log g and κ.
---------------------------------------------------------
Title: Abundance of zinc in the red giants of Galactic globular
cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Bonifacio,
P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2018A&A...616A.142C Altcode: 2018arXiv180603132C
<BR /> Aims: We investigate possible relations between the abundances
of zinc and the light elements sodium, magnesium, and potassium
in the atmospheres of red giant branch (RGB) stars of the Galactic
globular cluster 47 Tuc and study connections between the chemical
composition and dynamical properties of the cluster RGB stars. <BR
/> Methods: The abundance of zinc was determined in 27 RGB stars
of 47 Tuc using 1D local thermal equilibrium (LTE) synthetic line
profile fitting to the high-resolution 2dF/HERMES spectra obtained
with the Anglo-Australian Telescope (AAT). Synthetic spectra used
in the fitting procedure were computed with the SYNTHE code and
1D ATLAS9 stellar model atmospheres. <BR /> Results: The average
1D LTE zinc-to-iron abundance ratio and its RMS variations due to
star-to-star abundance spread determined in the sample of 27 RGB stars
is <[Zn/Fe]><SUP>1D LTE</SUP> = 0.11 ± 0.09. We did not detect
any statistically significant relations between the abundances of
zinc and those of light elements. Neither did we find any significant
correlation or anticorrelation between the zinc abundance in individual
stars and their projected distance from the cluster center. Finally,
no statistically significant relation between the absolute radial
velocities of individual stars and the abundance of zinc in their
atmospheres was detected. The obtained average [Zn/Fe]<SUP>1DLTE</SUP>
ratio agrees well with those determined in this cluster in earlier
studies and nearly coincides with that of Galactic field stars at this
metallicity. All these results suggest that nucleosynthesis of zinc
and light elements proceeded in separate, unrelated pathways in 47 Tuc.
---------------------------------------------------------
Title: Spatially resolved spectroscopy across stellar
surfaces. III. Photospheric Fe I lines across HD 189733A (K1 V)
Authors: Dravins, Dainis; Gustavsson, Martin; Ludwig, Hans-Günter
2018A&A...616A.144D Altcode: 2018arXiv180600012D
Context. Spectroscopy across spatially resolved stellar surfaces reveals
spectral line profiles free from rotational broadening, whose gradual
changes from disk center toward the stellar limb reflect an atmospheric
fine structure that is possible to model by 3D hydrodynamics. <BR />
Aims: Previous studies of photospheric spectral lines across stellar
disks exist for the Sun and <ASTROBJ>HD 209458</ASTROBJ> (G0 V) and
are now extended to the planet-hosting <ASTROBJ>HD 189733A</ASTROBJ>
to sample a cooler K-type star and explore the future potential of
the method. <BR /> Methods: During exoplanet transit, stellar surface
portions successively become hidden and differential spectroscopy
between various transit phases uncovers spectra of small surface
segments temporarily hidden behind the planet. The method was elaborated
in Paper I, in which observable signatures were predicted quantitatively
from hydrodynamic simulations. <BR /> Results: From observations of
<ASTROBJ>HD 189733A</ASTROBJ> with the ESO HARPS spectrometer at
λ/Δλ 115 000, profiles for stronger and weaker Fe I lines are
retrieved at several center-to-limb positions, reaching adequate
S/N after averaging over numerous similar lines. <BR /> Conclusions:
Retrieved line profile widths and depths are compared to synthetic
ones from models with parameters bracketing those of the target star
and are found to be consistent with 3D simulations. Center-to-limb
changes strongly depend on the surface granulation structure and much
greater line-width variation is predicted in hotter F-type stars
with vigorous granulation than in cooler K-types. Such parameters,
obtained from fits to full line profiles, are realistic to retrieve
for brighter planet-hosting stars, while their hydrodynamic modeling
offers previously unexplored diagnostics for stellar atmospheric fine
structure and 3D line formation. Precise modeling may be required in
searches for Earth-analog exoplanets around K-type stars, whose more
tranquil surface granulation and lower ensuing microvariability may
enable such detections.
---------------------------------------------------------
Title: Carbon-enhanced metal-poor 3D model atmospheres
Authors: Steffen, M.; Gallagher, A. J.; Caffau, E.; Bonifacio, P.;
Ludwig, H. -G.
2018IAUS..334..364S Altcode: 2017arXiv170805686S
We present our latest 3D model atmospheres for carbon-enhanced
metal-poor (CEMP) stars computed with the CO5BOLD code. The stellar
parameters are representative of hot turn-off objects (T<SUB>eff</SUB>
~ 6250 K, log g = 4.0, [Fe/H]=-3). The main purpose of these models
is to investigate the role of 3D effects on synthetic spectra of the
CH G-band (4140-4400 Å), the CN BX-band (3870-3890 Å), and several
UV OH transitions (3122-3128 Å). By comparison with the synthetic
spectra from standard 1D model atmospheres (assuming local thermodynamic
equilibrium, LTE), we derive 3D abundance corrections for carbon and
oxygen of up to -0.5 and -0.7 dex, respectively.
---------------------------------------------------------
Title: Abundances of Mg and K in the atmospheres of turn-off starsin
Galactic globular cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Dobrovolskas,
V.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2018A&A...615A.173C Altcode: 2018arXiv180410033C
<BR /> Aims: We determined abundances of Mg and K in the atmospheres
of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular
cluster 47 Tuc. The obtained abundances, together with those of Li,
O, and Na that we had earlier determined for the same sample of stars,
were used to search for possible relations between the abundances of K
and other light elements, Li, O, Na, and Mg, as well as the connections
between the chemical composition of TO stars and their kinematical
properties. <BR /> Methods: Abundances of Mg and K were determined using
archival high resolution VLT FLAMES/GIRAFFE spectra, in combination
with the one-dimensional (1D) non-local thermodynamic equilibrium
(NLTE) spectral synthesis methodology. Spectral line profiles were
computed with the MULTI code, using 1D hydrostatic ATLAS9 stellar model
atmospheres. We also utilized three-dimensional (3D) hydrodynamical
CO<SUP>5</SUP>BOLD and 1D hydrostatic LHD model atmospheres for
computing 3D-1D LTE abundance corrections for the spectral lines
of Mg and K, in order to assess the influence of convection on their
formation in the atmospheres of TO stars. <BR /> Results: The determined
average abundance-to-iron ratios and their root mean square variations
due to star-to-star abundance spreads were <[Mg/Fe]><SUP>1D
NLTE</SUP> = 0.47 ± 0.12, and <[K/Fe]><SUP>1D NLTE</SUP> = 0.39
± 0.09. Although the data suggest the possible existence of a weak
correlation in the [K/Fe]-[Na/Fe] plane, its statistical significance
is low. No statistically significant relations between the abundance
of K and other light elements were detected. Also, we did not find any
significant correlations or anti-correlations between the [Mg/Fe] and
[K/Fe] ratios and projected distance from the cluster center. Similarly,
no relations between the absolute radial velocities of individual stars
and abundances of Mg and K in their atmospheres were detected. The
3D-1D abundance corrections were found to be small (≤0.1 dex) for
the lines of Mg and K used in this study, thus indicating that the
influence of convection on their formation is small.
---------------------------------------------------------
Title: Observations of the solar chromosphere with ALMA and comparison
with theoretical models
Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold O.;
Kuhar, Matej; Kobelski, Adam; Wedemeyer, Sven; White, Stephen M.;
Ludwig, Hans-G.; Temmer, Manuela; Saar, Steven H.; Selhorst, Caius L.
2018csss.confE..37B Altcode: 2018arXiv181207293B
In this work we use solar observations with the ALMA radio telescope
at the wavelength of 1.21 mm. The aim of the analysisis to improve
understanding of the solar chromosphere, a dynamic layer in the
solar atmosphere between the photosphere andcorona. The study has
an observational and a modeling part. In the observational part
full-disc solar images are analyzed.Based on a modied FAL atmospheric
model, radiation models for various observed solar structures are
developed. Finally, theobservational and modeling results are compared
and discussed.
---------------------------------------------------------
Title: Spectroscopic Properties of a Two-Dimensional Cepheid Model
Authors: Vasilyev, Valeriy; Ludwig, Hans-Günter; Freytag, Bernd;
Lemasle, Bertrand; Marconi, Marcella
2018pas6.conf..222V Altcode: 2018pas..conf..222V
The chemical composition of Cepheid variables can provide information on
the chemo-dynamical evolution of the Galaxy and beyond. The standard
method for determining atmospheric parameters and abundances of
Cepheids is based on one-dimensional plane-parallel hydrostatic
model atmospheres, where convection is treated by Mixing Length
Theory. We check the validity of the quasi-static approach against
a two-dimensional dynamical Cepheid model computed with CO5BOLD. The
spectroscopic investigation of the two-dimensional Cepheid model allowed
to derive projection factors and to explain the residual line-of-sight
velocity of Galactic Cepheids, long known as the “K-term”, by line
shifts of convective origin. Moreover, hydrostatic 1D model atmospheres
can provide unbiased estimates of stellar parameters and abundances
of Cepheids for particular phases of their pulsations.
---------------------------------------------------------
Title: Using the CIFIST grid of CO<SUP>5</SUP>BOLD 3D model
atmospheres to study the effects of stellar granulation on photometric
colours. II. The role of convection across the H-R diagram
Authors: Kučinskas, A.; Klevas, J.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.; Caffau, E.
2018A&A...613A..24K Altcode: 2018arXiv180200073K
<BR /> Aims: We studied the influence of convection on the spectral
energy distributions (SEDs), photometric magnitudes, and colour
indices of different types of stars across the H-R diagram. <BR />
Methods: The 3D hydrodynamical CO<SUP>5</SUP>BOLD, averaged ⟨3D⟩,
and 1D hydrostatic LHD model atmospheres were used to compute SEDs
of stars on the main sequence (MS), main sequence turn-off (TO),
subgiant branch (SGB), and red giant branch (RGB), in each case at
two different effective temperatures and two metallicities, [M/H] =
0.0 and - 2.0. Using the obtained SEDs, we calculated photometric
magnitudes and colour indices in the broad-band Johnson-Cousins
UBVRI and 2MASS JHK<SUB>s</SUB>, and the medium-band Strömgren
uvby photometric systems. <BR /> Results: The 3D-1D differences in
photometric magnitudes and colour indices are small in both photometric
systems and typically do not exceed ± 0.03 mag. Only in the case of the
coolest giants located on the upper RGB are the differences in the U and
u bands able reach ≈-0.2 mag at [M/H] = 0.0 and ≈-0.1 mag at [M/H]
= -2.0. Generally, the 3D-1D differences are largest in the blue-UV
part of the spectrum and decrease towards longer wavelengths. They
are also sensitive to the effective temperature and are significantly
smaller in hotter stars. Metallicity also plays a role and leads to
slightly larger 3D-1D differences at [M/H] = 0.0. All these patterns are
caused by a complex interplay between the radiation field, opacities,
and horizontal temperature fluctuations that occur due to convective
motions in stellar atmospheres. Although small, the 3D-1D differences
in the magnitudes and colour indices are nevertheless comparable to
or larger than typical photometric uncertainties and may therefore
cause non-negligible systematic differences in the estimated effective
temperatures.
---------------------------------------------------------
Title: TOPoS. IV. Chemical abundances from high-resolution
observations of seven extremely metal-poor stars
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; Sbordone,
L.; Monaco, L.; François, P.; Plez, B.; Molaro, P.; Gallagher, A. J.;
Cayrel, R.; Christlieb, N.; Klessen, R. S.; Koch, A.; Ludwig, H. -G.;
Steffen, M.; Zaggia, S.; Abate, C.
2018A&A...612A..65B Altcode: 2018arXiv180103935B
Context. Extremely metal-poor (EMP) stars provide us with indirect
information on the first generations of massive stars. The TOPoS
survey has been designed to increase the census of these stars and to
provide a chemical inventory that is as detailed as possible. <BR />
Aims: Seven of the most iron-poor stars have been observed with the
UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine
their chemical composition. <BR /> Methods: We analysed the spectra
based on 1D LTE model atmospheres, but also used 3D hydrodynamical
simulations of stellar atmospheres. <BR /> Results: We measured carbon
in six of the seven stars: all are carbon-enhanced and belong to the
low-carbon band, defined in the TOPoS II paper. We measured lithium
(A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H]
<-5.2). We were also able to measure Li in three stars at [Fe/H]
-4.0, two of which lie on the Spite plateau. We confirm that SDSS
J1349+1407 is extremely rich in Mg, but not in Ca. It is also very
rich in Na. Several of our stars are characterised by low α-to-iron
ratios. <BR /> Conclusions: The lack of high-carbon band stars at low
metallicity can be understood in terms of evolutionary timescales
of binary systems. The detection of Li in SDSS J1035+0641 places a
strong constraint on theories that aim at solving the cosmological
lithium problem. The Li abundance of the two warmer stars at [Fe/H]
-4.0 places them on the Spite plateau, while the third, cooler star,
lies below. We argue that this suggests that the temperature at which
Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407
may belong to a class of Mg-rich EMP stars. We cannot assess if there
is a scatter in α-to-iron ratios among the EMP stars or if there are
several discrete populations. However, the existence of stars with
low α-to-iron ratios is supported by our observations. <P />Based
on observations obtained at ESO Paranal Observatory, Programmes
189.D-0165,090.D-0306, 093.D-0136, and 096.D-0468.
---------------------------------------------------------
Title: Using the CIFIST grid of CO<SUP>5</SUP>BOLD 3D model
atmospheres to study the effects of stellar granulation on photometric
colours. I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS,
Gaia, and SDSS systems
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Kučinskas, A.; Prakapavičius, D.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2018A&A...611A..68B Altcode: 2017arXiv171200024B
Context. The atmospheres of cool stars are temporally and spatially
inhomogeneous due to the effects of convection. The influence of
this inhomogeneity, referred to as granulation, on colours has never
been investigated over a large range of effective temperatures and
gravities. Aim. We aim to study, in a quantitative way, the impact of
granulation on colours. <BR /> Methods: We use the CIFIST (Cosmological
Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the
COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2,
3) hydrodynamical models to compute emerging fluxes. These in turn are
used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS,
Gaia and SDSS systems. Every CO5BOLD model has a corresponding one
dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model
computed for the same atmospheric parameters, which we used to define
a "3D correction" that can be applied to colours computed from fluxes
computed from any 1D model atmosphere code. As an example, we illustrate
these corrections applied to colours computed from ATLAS models. <BR />
Results: The 3D corrections on colours are generally small, of the order
of a few hundredths of a magnitude, yet they are far from negligible. We
find that ignoring granulation effects can lead to underestimation of
Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when
using colours as diagnostics. We have identified a major shortcoming in
how scattering is treated in the current version of the CIFIST grid,
which could lead to offsets of the order 0.01 mag, especially for
colours involving blue and UV bands. We have investigated the Gaia and
HIPPARCOS photometric systems and found that the (G - H<SUB>p</SUB>),
(BP - RP) diagram is immune to the effects of granulation. In addition,
we point to the potential of the RVS photometry as a metallicity
diagnostic. <BR /> Conclusions: Our investigation shows that the
effects of granulation should not be neglected if one wants to use
colours as diagnostics of the stellar parameters of F, G, K stars. A
limitation is that scattering is treated as true absorption in our
current computations, thus our 3D corrections are likely an upper
limit to the true effect. We are already computing the next generation
of the CIFIST grid, using an approximate treatment of scattering. <P
/>The appendix tables are only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://cdsarc.u-strasbg.fr">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68</A>
---------------------------------------------------------
Title: Spectroscopic properties of a two-dimensional time-dependent
Cepheid model. II. Determination of stellar parameters and abundances
Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.;
Marconi, M.
2018A&A...611A..19V Altcode: 2017arXiv171100236V
Context. Standard spectroscopic analyses of variable stars are based
on hydrostatic 1D model atmospheres. This quasi-static approach has
not been theoretically validated. Aim. We aim at investigating the
validity of the quasi-static approximation for Cepheid variables. We
focus on the spectroscopic determination of the effective temperature
T<SUB>eff</SUB>, surface gravity log g, microturbulent velocity
ξ<SUB>t</SUB>, and a generic metal abundance log A, here taken
as iron. <BR /> Methods: We calculated a grid of 1D hydrostatic
plane-parallel models covering the ranges in effective temperature and
gravity that are encountered during the evolution of a 2D time-dependent
envelope model of a Cepheid computed with the radiation-hydrodynamics
code CO5BOLD. We performed 1D spectral syntheses for artificial iron
lines in local thermodynamic equilibrium by varying the microturbulent
velocity and abundance. We fit the resulting equivalent widths
to corresponding values obtained from our dynamical model for 150
instances in time, covering six pulsational cycles. In addition,
we considered 99 instances during the initial non-pulsating stage
of the temporal evolution of the 2D model. In the most general case,
we treated T<SUB>eff</SUB>, log g, ξ<SUB>t</SUB>, and log A as free
parameters, and in two more limited cases, we fixed T<SUB>eff</SUB>
and log g by independent constraints. We argue analytically that our
approach of fitting equivalent widths is closely related to current
standard procedures focusing on line-by-line abundances. <BR />
Results: For the four-parametric case, the stellar parameters are
typically underestimated and exhibit a bias in the iron abundance of
≈-0.2 dex. To avoid biases of this type, it is favorable to restrict
the spectroscopic analysis to photometric phases ϕ<SUB>ph</SUB>
≈ 0.3…0.65 using additional information to fix the effective
temperature and surface gravity. <BR /> Conclusions: Hydrostatic 1D
model atmospheres can provide unbiased estimates of stellar parameters
and abundances of Cepheid variables for particular phases of their
pulsations. We identified convective inhomogeneities as the main driver
behind potential biases. To obtain a complete view on the effects
when determining stellar parameters with 1D models, multidimensional
Cepheid atmosphere models are necessary for variables of longer period
than investigated here.
---------------------------------------------------------
Title: Impact of magnetic fields on the structure of convective
atmospheres of red giant stars
Authors: Klevas, J.; Kučinskas, A.; Wedemeyer, S.; Ludwig, H. -G.
2018CoSka..48..280K Altcode:
We use 3D magnetohydrodynamic CO<SUP>5</SUP>BOLD model atmospheres
to study the interplay between magnetic fields and convection in the
atmospheres of red giant stars. We find that vortex-like structures
occur prominently in stars with stronger magnetic fields and lead to
alterations of their thermal structures.
---------------------------------------------------------
Title: A comparison of solar ALMA observations and model based
predictions of the brightness temperature
Authors: Brajša, R.; Kuhar, M.; Benz, A. O.; Skokić, I.; Sudar,
D.; Wedemeyer, S.; Báarta, M.; De Pontieu, B.; Kim, S.; Kobelski,
A.; Shimojo, M.; White, S.; Yagoubov, P.; Yan, Y.; Ludwig, H. G.;
Temmer, M.; Saar, S. H.; Selhorst, C. L.; Beuc, R.
2018CEAB...42....1B Altcode:
The new facility Atacama Large Millimeter/submillimeter Array (ALMA) is
capable of observing the Sun in the wavelength range from 0.3 mm to 10
mm with an unprecedented spatial, temporal and spectral resolution. The
first aim of the present work is to identify different structures
in the solar atmosphere (quiet Sun, active regions, filaments on the
disc, and coronal holes) in a full disc solar ALMA image at 1.21 mm
obtained on December 18, 2015 during a CSV-EOC campaign. It is compared
with full disc solar images from the same day in the Hα line (Cerro
Tololo Observatory, NISP), and at three EUV wavelengths (30.4 nm,
21.1 nm, 17.1 nm; a composite SDO image). Positions of the quiet Sun
areas, active regions, filaments on the disc, and coronal holes are
identified in the ALMA image. To interpret solar observations with ALMA
it is important to compare the measured and calculated intensities
of various solar structures. So, the second aim of this work is to
calculate the intensity (brightness temperature) for those structures
(quiet Sun, active regions, filaments on the disc, and coronal holes)
for a broad wavelength range (from 0.3 mm to 10 mm), closely related
to that of the ALMA, and to compare the results with available
ALMA observations. Thermal bremsstrahlung is the dominant radiation
mechanism for explanation of the observed phenomena. A procedure for
calculating the brightness temperature for a given wavelength and
model atmosphere, which integrates the radiative transfer equation
for thermal bremsstrahlung, is used. At the wavelength of 1.21 mm
active regions appear as bright areas, while filaments on the disc and
coronal holes are not discernible from the quiet Sun background. The
models generally agree with the observed results: Active regions are
bright primarily due to higher densities, filaments can appear bright,
dark or not at all and coronal holes cannot be easily identified.
---------------------------------------------------------
Title: VizieR Online Data Catalog: 3D correction in 5 photometric
systems (Bonifacio+, 2018)
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Kucinskas, A.; Prakapavicius, D.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2018yCat..36110068B Altcode:
We have used the CIFIST grid of CO5BOLD models to investigate the
effects of granulation on fluxes and colours of stars of spectral
type F, G, and K. <P />We publish tables with 3D corrections that
can be applied to colours computed from any 1D model atmosphere. For
Teff>=5000K, the corrections are smooth enough, as a function
of atmospheric parameters, that it is possible to interpolate the
corrections between grid points; thus the coarseness of the CIFIST
grid should not be a major limitation. However at the cool end there
are still far too few models to allow a reliable interpolation. <P
/>(20 data files).
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. VI. First chromosphere model of a
late-type giant
Authors: Wedemeyer, Sven; Kučinskas, Arūnas; Klevas, Jonas; Ludwig,
Hans-Günter
2017A&A...606A..26W Altcode: 2017arXiv170509641W
<BR /> Aims: Although observational data unequivocally point to
the presence of chromospheres in red giant stars, no attempts
have been made so far to model them using 3D hydrodynamical model
atmospheres. We therefore compute an exploratory 3D hydrodynamical model
atmosphere for a cool red giant in order to study the dynamical and
thermodynamic properties of its chromosphere, as well as the influence
of the chromosphere on its observable properties. <BR /> Methods:
Three-dimensional radiation hydrodynamics simulations are carried out
with the CO<SUP>5</SUP>BOLD model atmosphere code for a star with the
atmospheric parameters (T<SUB>eff</SUB> ≈ 4010 K, log g = 1.5, [ M / H
] = 0.0), which are similar to those of the K-type giant star Aldebaran
(α Tau). The computational domain extends from the upper convection
zone into the chromosphere (7.4 ≥ log τ<SUB>Ross</SUB> ≥ - 12.8)
and covers several granules in each horizontal direction. Using this
model atmosphere, we compute the emergent continuum intensity maps at
different wavelengths, spectral line profiles of Ca II K, the Ca II
infrared triplet line at 854.2 nm, and Hα, as well as the spectral
energy distribution (SED) of the emergent radiative flux. <BR />
Results: The initial model quickly develops a dynamical chromosphere
that is characterised by propagating and interacting shock waves. The
peak temperatures in the chromospheric shock fronts reach values of
up to 5000 K, although the shock fronts remain quite narrow. Similar
to the Sun, the gas temperature distribution in the upper layers
of red giant stars is composed of a cool component due to adiabatic
cooling in the expanding post-shock regions and a hot component due
to shock waves. For this red giant model, the hot component is a
rather flat high-temperature tail, which nevertheless affects the
resulting average temperatures significantly. <BR /> Conclusions:
The simulations show that the atmospheres of red giant stars are
dynamic and intermittent. Consequently, many observable properties
cannot be reproduced with static 1D models, but require advanced 3D
hydrodynamical modelling. Furthermore, including a chromosphere in the
models might produce significant contributions to the emergent UV flux.
---------------------------------------------------------
Title: Spectroscopic properties of a two-dimensional time-dependent
Cepheid model. I. Description and validation of the model
Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.;
Marconi, M.
2017A&A...606A.140V Altcode: 2017arXiv170903905V
Context. Standard spectroscopic analyses of Cepheid variables are based
on hydrostatic one-dimensional model atmospheres, with convection
treated using various formulations of mixing-length theory. <BR />
Aims: This paper aims to carry out an investigation of the validity of
the quasi-static approximation in the context of pulsating stars. We
check the adequacy of a two-dimensional time-dependent model of a
Cepheid-like variable with focus on its spectroscopic properties. <BR />
Methods: With the radiation-hydrodynamics code CO5BOLD, we construct
a two-dimensional time-dependent envelope model of a Cepheid with
T<SUB>eff</SUB> = 5600 K, log g = 2.0, solar metallicity, and a
2.8-day pulsation period. Subsequently, we perform extensive spectral
syntheses of a set of artificial iron lines in local thermodynamic
equilibrium. The set of lines allows us to systematically study effects
of line strength, ionization stage, and excitation potential. <BR />
Results: We evaluate the microturbulent velocity, line asymmetry,
projection factor, and Doppler shifts. The microturbulent velocity,
averaged over all lines, depends on the pulsational phase and varies
between 1.5 and 2.7 km s<SUP>-1</SUP>. The derived projection factor
lies between 1.23 and 1.27, which agrees with observational results. The
mean Doppler shift is non-zero and negative, -1 km s<SUP>-1</SUP>,
after averaging over several full periods and lines. This residual
line-of-sight velocity (related to the "K-term") is primarily caused
by horizontal inhomogeneities, and consequently we interpret it as the
familiar convective blueshift ubiquitously present in non-pulsating
late-type stars. Limited statistics prevent firm conclusions on the line
asymmetries. <BR /> Conclusions: Our two-dimensional model provides
a reasonably accurate representation of the spectroscopic properties
of a short-period Cepheid-like variable star. Some properties are
primarily controlled by convective inhomogeneities rather than by
the Cepheid-defining pulsations. Extended multi-dimensional modelling
offers new insight into the nature of pulsating stars.
---------------------------------------------------------
Title: Stellar atmospheres behind transiting exoplanets
Authors: Dravins, D.; Ludwig, H. -G.; Dahlén, E.; Gustavsson, M.;
Pazira, H.
2017EPSC...11...21D Altcode:
Stellar surfaces are covered with brighter and darker structures, just
like on the Sun. While solar surface details can be easily studied
with telescopes, stellar surfaces cannot thus be resolved. However,
one can use planets that happen to pass in front of distant stars as
"shades" that successively block out small portions of the stellar
surface behind. By measuring how the light from the star changes during
such a transit, one can deduce stellar surface properties. Knowing those
is required not only to study the star as such, but also to deduce the
chemical composition of the planet that is passing in front of it,
where some of the detected starlight has been filtered through the
planet's atmosphere.
---------------------------------------------------------
Title: Spatially resolved spectroscopy across stellar
surfaces. II. High-resolution spectra across HD 209458 (G0 V)
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik;
Pazira, Hiva
2017A&A...605A..91D Altcode: 2017arXiv170801618D
Context. High-resolution spectroscopy across spatially resolved
stellar surfaces aims at obtaining spectral-line profiles that
are free from rotational broadening; the gradual changes of these
profiles from disk center toward the stellar limb reveal properties
of atmospheric fine structure, which are possible to model with 3D
hydrodynamics. <BR /> Aims: Previous such studies have only been
carried out for the Sun but are now extended to other stars. In this
work, profiles of photospheric spectral lines are retrieved across
the disk of the planet-hosting star HD 209458 (G0 V). <BR /> Methods:
During exoplanet transit, stellar surface portions successively become
hidden and differential spectroscopy provides spectra of small surface
segments temporarily hidden behind the planet. The method was elaborated
in Paper I, with observable signatures quantitatively predicted from
hydrodynamic simulations. <BR /> Results: From observations of HD
209458 with spectral resolution λ/ Δλ 80 000, photospheric Fe
I line profiles are obtained at several center-to-limb positions,
reaching adequately high S/N after averaging over numerous similar
lines. <BR /> Conclusions: Retrieved line profiles are compared
to synthetic line profiles. Hydrodynamic 3D models predict, and
current observations confirm, that photospheric absorption lines
become broader and shallower toward the stellar limb, reflecting that
horizontal velocities in stellar granulation are greater than vertical
velocities. Additional types of 3D signatures will become observable
with the highest resolution spectrometers at large telescopes.
---------------------------------------------------------
Title: Spatially resolved spectroscopy across stellar
surfaces. I. Using exoplanet transits to analyze 3D stellar
atmospheres
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik;
Pazira, Hiva
2017A&A...605A..90D Altcode: 2017arXiv170801616D
Context. High-precision stellar analyses require hydrodynamic modeling
to interpret chemical abundances or oscillation modes. Exoplanet
atmosphere studies require stellar background spectra to be known
along the transit path while detection of Earth analogs require
stellar microvariability to be understood. Hydrodynamic 3D models can
be computed for widely different stars but have been tested in detail
only for the Sun with its resolved surface features. Model predictions
include spectral line shapes, asymmetries, and wavelength shifts,
and their center-to-limb changes across stellar disks. <BR /> Aims: We
observe high-resolution spectral line profiles across spatially highly
resolved stellar surfaces, which are free from the effects of spatial
smearing and rotational broadening present in full-disk spectra,
enabling comparisons to synthetic profiles from 3D models. <BR />
Methods: During exoplanet transits, successive stellar surface portions
become hidden and differential spectroscopy between various transit
phases provides spectra of small surface segments temporarily hidden
behind the planet. Planets cover no more than 1% of any main-sequence
star, enabling high spatial resolution but demanding very precise
observations. Realistically measurable quantities are identified
through simulated observations of synthetic spectral lines. <BR />
Results: In normal stars, line profile ratios between various transit
phases may vary by 0.5%, requiring S/N ≳ 5000 for meaningful spectral
reconstruction. While not yet realistic for individual spectral lines,
this is achievable for cool stars by averaging over numerous lines
with similar parameters. <BR /> Conclusions: For bright host stars of
large transiting planets, spatially resolved spectroscopy is currently
practical. More observable targets are likely to be found in the near
future by ongoing photometric searches.
---------------------------------------------------------
Title: The Gaia-ESO Survey: Galactic evolution of sulphur and zinc
Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.;
Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco, L.;
François, P.; Skúladóttir, Á.; Bragaglia, A.; Donati, P.; Spina,
L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.;
Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.;
Friel, E.; Jiménez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino,
E.; Tang, B.; Tautvaišienė, G.; Vallenari, A.; Hawkins, K.; Gilmore,
G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic,
R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani,
F.; Franciosini, E.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis,
J.; Morbidelli, L.; Sousa, S. G.; Worley, C. C.
2017A&A...604A.128D Altcode: 2017arXiv170402981D
Context. Due to their volatile nature, when sulphur and zinc are
observed in external galaxies, their determined abundances represent
the gas-phase abundances in the interstellar medium. This implies
that they can be used as tracers of the chemical enrichment of matter
in the Universe at high redshift. Comparable observations in stars
are more difficult and, until recently, plagued by small number
statistics. <BR /> Aims: We wish to exploit the Gaia-ESO Survey
(GES) data to study the behaviour of sulphur and zinc abundances
of a large number of Galactic stars, in a homogeneous way. <BR />
Methods: By using the UVES spectra of the GES sample, we are able to
assemble a sample of 1301 Galactic stars, including stars in open and
globular clusters in which both sulphur and zinc were measured. <BR
/> Results: We confirm the results from the literature that sulphur
behaves as an α-element. We find a large scatter in [Zn/Fe] ratios
among giant stars around solar metallicity. The lower ratios are
observed in giant stars at Galactocentric distances less than 7.5
kpc. No such effect is observed among dwarf stars, since they do not
extend to that radius. <BR /> Conclusions: Given the sample selection,
giants and dwarfs are observed at different Galactic locations, and it
is plausible, and compatible with simple calculations, that Zn-poor
giants trace a younger population more polluted by SN Ia yields. It
is necessary to extend observations in order to observe both giants
and dwarfs at the same Galactic location. Further theoretical work on
the evolution of zinc is also necessary. <P />Based on observations
collected at the European Organisation for Astronomical Research in the
Southern Hemisphere under ESO programmes 188.B-3002, 193.B-0936.The full
table of S abundances is only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A128">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/604/A128</A>
---------------------------------------------------------
Title: An Investigation of the Formation and Line Properties of MgH
in 3D Hydrodynamical Model Stellar Atmospheres
Authors: Thygesen, Anders O.; Kirby, Evan N.; Gallagher, Andrew J.;
Ludwig, Hans-G.; Caffau, Elisabetta; Bonifacio, Piercarlo; Sbordone,
Luca
2017ApJ...843..144T Altcode: 2017arXiv170604218T
Studies of the isotopic composition of magnesium in cool stars have so
far relied upon the use of 1D model atmospheres. Since the isotopic
ratios derived are based on asymmetries of optical MgH lines, it
is important to test the impact from other effects affecting line
asymmetries, like stellar convection. Here, we present a theoretical
investigation of the effects of including self-consistent modeling
of convection. Using spectral syntheses based on 3D hydrodynamical
CO<SUP>5</SUP>BOLD models of dwarfs (4000 K ≲ T <SUB>eff</SUB> ≲
5160 K, 4.0 ≤ {log}g ≤ 4.5, -3.0≤slant [{Fe}/{{H}}]≤slant
-1.0) and giants (T <SUB>eff</SUB> ∼ 4000 K, {log}g = 1.5,
-3.0≤slant [{Fe}/{{H}}]≤slant -1.0), we perform a detailed
analysis comparing 3D and 1D syntheses. We describe the impact on the
formation and behavior of MgH lines from using 3D models, and perform
a qualitative assessment of the systematics introduced by the use of 1D
syntheses. Using 3D model atmospheres significantly affect the strength
of the MgH lines, especially in dwarfs, with 1D syntheses requiring
an abundance correction of up to +0.69 dex, with the largest for our
5000 K models. The corrections are correlated with T <SUB>eff</SUB>
and are also affected by the metallicity. The shape of the strong
<SUP>24</SUP>MgH component in the 3D syntheses is poorly reproduced in
1D. This results in 1D syntheses underestimating <SUP>25</SUP>Mg by
up to ∼5 percentage points and overestimating <SUP>24</SUP>Mg by a
similar amount for dwarfs. This discrepancy increases with decreasing
metallicity. <SUP>26</SUP>Mg is recovered relatively well, with the
largest difference being ∼2 percentage points. The use of 3D for
giants has less impact, due to smaller differences in the atmospheric
structure and a better reproduction of the line shape in 1D.
---------------------------------------------------------
Title: Abundances of Na, Mg, and K in the atmospheres of red giant
branch stars of Galactic globular cluster 47 Tucanae
Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Prakapavičius,
D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2017A&A...604A..35C Altcode: 2017arXiv170402751C
<BR /> Aims: We study the abundances of Na, Mg, and K in the atmospheres
of 32 red giant branch (RGB) stars in the Galactic globular cluster
(GGC) 47 Tuc, with the goal to investigate the possible existence of
Na-K and Mg-K correlations/anti-correlations, similar to those that
were recently discovered in two other GGCs, NGC 2419 and 2808. <BR
/> Methods: The abundances of K, Na, and Mg were determined using
high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian
Telescope (AAT). The one-dimensional (1D) NLTE abundance estimates
were obtained using 1D hydrostatic ATLAS9 model atmospheres and
spectral line profiles synthesized with the MULTI package. We also
used three-dimensional (3D) hydrodynamical CO<SUP>5</SUP>BOLD and
1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance
corrections, Δ<SUB>3D - 1D LTE</SUB>, for the spectral lines of Na,
Mg, and K used in our study. These abundance corrections were used to
understand the role of convection in the formation of spectral lines,
as well as to estimate the differences in the abundances obtained with
the 3D hydrodynamical and 1D hydrostatic model atmospheres. <BR />
Results: The average element-to-iron abundance ratios and their RMS
variations due to star-to-star abundance spreads determined in our
sample of RGB stars were ⟨ [ Na / Fe ] ⟩ <SUP>1D NLTE</SUP> =
0.42 ± 0.13, ⟨ [ Mg / Fe ] ⟩ <SUP>1D NLTE</SUP> = 0.41 ± 0.11,
and ⟨ [ K / Fe ] ⟩ <SUP>1D NLTE</SUP> = 0.05 ± 0.14. We found no
statistically significant relations between the abundances of the three
elements studied here. Also, there were no abundance trends with the
distance from the cluster center, nor any statistically significant
relations between the abundance/abundance ratios and absolute radial
velocities of individual stars. All these facts suggest the similarity
of K abundance in stars that belong to different generations in 47
Tuc which, in turn, may hint that evolution of K in this particular
cluster was unrelated to the nucleosynthesis of Na and/or Mg.
---------------------------------------------------------
Title: VizieR Online Data Catalog: S abundances for 1301 stars from
GES (Duffau+, 2017)
Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.;
Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco,
L.; Francois, P.; Skuladottir, A.; Bragaglia, A.; Donati, P.; Spina,
L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.;
Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.;
Friel, E.; Jimenez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino,
E.; Tang, B.; Tautvaisiene, G.; Vallenari, A.; Hawkins, K.; Gilmore,
G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic,
R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani,
F.; Franciosini, E.; Hourihane, A.; Jofre, P.; Lardo, C.; Lewis, J.;
Morbidelli, L.; Sousa, S. G.; Worley, C. C.
2017yCat..36040128D Altcode:
GES internal star identifier (CNAME), Sulphur abundances and NLTE
corrections to the Sulphur abundances for 1301 stars. Sulphur
abundances are expressed in the customary logarithmic form:
A(S)=log_10(N(S)/N(H))+12. The abundances delivered are the LTE
ones. NLTEabundances can be determined by directly summing the NLTE
correction delivered: A(S)<SUB>NLTE = A(S) + NLTE</SUB>C. So that a
negative NLTE correction indicates that the NLTE abundance is lower
than the LTE one. <P />(1 data file).
---------------------------------------------------------
Title: Computation of eigenfrequencies for equilibrium models
including turbulent pressure
Authors: Sonoi, T.; Belkacem, K.; Dupret, M. -A.; Samadi, R.; Ludwig,
H. -G.; Caffau, E.; Mosser, B.
2017A&A...600A..31S Altcode: 2017arXiv170107244S
Context. The space-borne missions CoRoT and Kepler have provided a
wealth of highly accurate data. However, our inability to properly
model the upper-most region of solar-like stars prevents us from
making the best of these observations. This problem is called "surface
effect" and a key ingredient to solve it is turbulent pressure for the
computation of both the equilibrium models and the oscillations. While
3D hydrodynamic simulations help to include properly the turbulent
pressure in the equilibrium models, the way this surface effect is
included in the computation of stellar oscillations is still subject
to uncertainties. <BR /> Aims: We aim at determining how to properly
include the effect of turbulent pressure and its Lagrangian perturbation
in the adiabatic computation of the oscillations. We also discuss the
validity of the gas-gamma model and reduced gamma model approximations,
which have been used to compute adiabatic oscillations of equilibrium
models including turbulent pressure. <BR /> Methods: We use a patched
model of the Sun with an inner part constructed by a 1D stellar
evolution code (CESTAM) and an outer part by the 3D hydrodynamical
code (CO<SUP>5</SUP>BOLD). Then, the adiabatic oscillations are
computed using the ADIPLS code for the gas-gamma and reduced gamma
model approximations and with the MAD code imposing the adiabatic
condition on an existing time-dependent convection formalism. Finally,
all those results are compared to the observed solar frequencies. <BR
/> Results: We show that the computation of the oscillations using the
time-dependent convection formalism in the adiabatic limit improves
significantly the agreement with the observed frequencies compared to
the gas-gamma and reduced gamma model approximations. Of the components
of the perturbation of the turbulent pressure, the perturbation of the
density and advection term is found to contribute most to the frequency
shift. <BR /> Conclusions: The turbulent pressure is certainly the
dominant factor responsible for the surface effects. Its inclusion into
the equilibrium models is thus necessary but not sufficient. Indeed,
the perturbation of the turbulent pressure must be properly taken
into account for computing adiabatic oscillation frequencies. We
propose a formalism to evaluate the frequency shift due to the
inclusion of the term with the turbulent pressure perturbation in
the variational principle in order to extrapolate our result to other
stars at various evolutionary stages. Although this work is limited to
adiabatic oscillations and the inclusion of the turbulent pressure,
future works will have to account for the nonadiabatic effect and
convective backwarming.
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. V. Oxygen abundance in the metal-poor
giant HD 122563 from OH UV lines
Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas,
J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M.
2017A&A...599A.128P Altcode: 2016arXiv161103283P
Context. Although oxygen is an important tracer of the early Galactic
evolution, its abundance trends with metallicity are still relatively
poorly known at [Fe/H] ≲ -2.5. This is in part due to a lack of
reliable oxygen abundance indicators in the metal-poor stars, and in
part due to shortcomings in 1D LTE abundance analyses where different
abundance indicators, such as OH lines located in the UV and IR or
the forbidden [O I] line at 630 nm, frequently provide inconsistent
results. <BR /> Aims: In this study, we determined the oxygen abundance
in the metal-poor halo giant HD 122563 using a 3D hydrodynamical
CO<SUP>5</SUP>BOLD model atmosphere. Our main goal was to understand
whether a 3D LTE analysis can help to improve the reliability of oxygen
abundances that are determined from OH UV lines in comparison to those
obtained using standard 1D LTE methodology. <BR /> Methods: The oxygen
abundance in HD 122563 was determined using 71 OH UV lines located in
the wavelength range between 308-330 nm. The analysis was performed
using a high-resolution VLT UVES spectrum with a 1D LTE spectral line
synthesis performed using the SYNTHE package and classical ATLAS9
model atmosphere. Subsequently, a 3D hydrodynamical CO<SUP>5</SUP>BOLD
and 1D hydrostatic LHD model atmospheres were used to compute 3D-1D
abundance corrections. For this, the microturbulence velocity used
with the 1D LHD model atmosphere was derived from the hydrodynamical
CO<SUP>5</SUP>BOLD model atmosphere of HD 122563. The obtained abundance
corrections were then applied to determine 3D LTE oxygen abundances from
each individual OH UV line. <BR /> Results: As in previous studies,
we found trends in the 1D LTE oxygen abundances determined from OH UV
lines with line parameters, such as the line excitation potential, χ,
and the line equivalent width, W. These trends become significantly less
pronounced in 3D LTE. Using OH UV lines, we determined a 3D LTE oxygen
abundance in HD 122563 of A(O)<SUB>3D LTE</SUB> = 6.23 ± 0.13 ([O/Fe]
= 0.07 ± 0.13). This is in fair agreement with the oxygen abundance
obtained from OH IR lines, A(O)<SUB>3D LTE</SUB> = 6.39 ± 0.11 ([O/Fe]
= 0.23 ± 0.11), but it is noticeably lower than that determined when
using the forbidden [O I] line, A(O)<SUB>3D LTE</SUB> = 6.53 ± 0.15
([O/Fe] = 0.37 ± 0.15). While the exact cause of this discrepancy
remains unclear, it is very likely that non-LTE effects may play a
decisive role here. Oxygen-to-iron ratios determined in HD 122563
using OH UV/IR lines and the forbidden [O I] line fall on the lower
boundary of the [O/Fe] distribution as observed in the Galactic field
stars at this metallicity and suggest a very mild oxygen overabundance
with respect to iron, [O/Fe] ≲ 0.4.
---------------------------------------------------------
Title: VizieR Online Data Catalog: NGC104 RGB Na, Mg, and K abundances
(Cerniauskas+, 2017)
Authors: Cerniauskas, A.; Kucinskas, A.; Klevas, J.; Prakapavicius,
D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2017yCat..36040035C Altcode:
We used 2dF/HERMES spectra obtained in two wavelength regions,
564.9-587.3nm (GREEN) and 758.5-788.7nm (IR), using the spectral
resolution of R~28000 and exposure time of 1200s. The observations
were carried out during the period of Oct 22 - Dec 20, 2013 <P />(1
data file).
---------------------------------------------------------
Title: An in-depth spectroscopic examination of molecular bands from
3D hydrodynamical model atmospheres. II. Carbon-enhanced metal-poor
3D model atmospheres
Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.; Homeier, D.; Plez, B.
2017A&A...598L..10G Altcode: 2017arXiv170109102G
Context. Tighter constraints on metal-poor stars we observe are
needed to better understand the chemical processes of the early
Universe. Computing a stellar spectrum in 3D allows one to model complex
stellar behaviours, which cannot be replicated in 1D. <BR /> Aims:
We examine the effect that the intrinsic CNO abundances have on a 3D
model structure and the resulting 3D spectrum synthesis. <BR /> Methods:
Model atmospheres were computed in 3D for three distinct CNO chemical
compositions using the CO<SUP>5</SUP>BOLD model atmosphere code,
and their internal structures were examined. Synthetic spectra were
computed from these models using Linfor3D and they were compared. New
3D abundance corrections for the G-band and a selection of UV OH lines
were also computed. <BR /> Results: The varying CNO abundances change
the metal content of the 3D models. This had an effect on the model
structure and the resulting synthesis. However, it was found that
the C/O ratio had a larger effect than the overall metal content of
a model. <BR /> Conclusions: Our results suggest that varying the
C/O ratio has a substantial impact on the internal structure of the
3D model, even in the hot turn-off star models explored here. This
suggests that bespoke 3D models, for specific CNO abundances should
be sought. Such effects are not seen in 1D at these temperature regimes.
---------------------------------------------------------
Title: Convective overshoot and metal accretion onto white dwarfs.
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Koester, D.;
Fontaine, G.
2017MmSAI..88..104T Altcode:
A large fraction of white dwarfs host evolved planetary systems and show
evidence of accretion from planetary debris. The accretion-diffusion
model is the preferred method to understand the metal pollution in these
otherwise hydrogen- and helium-rich white dwarf atmospheres. In this
scenario, the accreted material first settles on the atmosphere. If
the outer stellar layers are unstable to convection, the metals are
then rapidly mixed up within the convection zone. In the classical
1D approach, it is generally assumed that the convection zone has a
sharp bottom boundary, below which microscopic diffusion is unhampered
and slowly removes metals from the visible layers. More realistic
3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD
demonstrate, however, that the bottom of the convection zone does not
have a sharp boundary, and that velocities decay exponentially below
the unstable convective layers with a velocity scale height of the
order of one pressure scale height. This has a potentially dramatic
effect on the inferred mass of accreted materiel, hence on the chemical
composition and size of planetary debris around white dwarfs.
---------------------------------------------------------
Title: Using CO5BOLD models to predict the effects of granulation
on colours .
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
Castelli, F.; Gallagher, A. J.; Prakapavičius, D.; Kučinskas, A.;
Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
2017MmSAI..88...90B Altcode:
In order to investigate the effects of granulation on fluxes and
colours, we computed the emerging fluxes from the models in the
CO5BOLD grid with metallicities [M/H]=0.0,-1.0,-2.0 and -3.0. These
fluxes have been used to compute colours in different photometric
systems. We explain here how our computations have been performed and
provide some results.
---------------------------------------------------------
Title: Enhanced methods for computing spectra from CO5BOLD models
using Linfor3D. Molecular bands in metal-poor stars
Authors: Gallagher, A. J.; Steffen, M.; Caffau, E.; Bonifacio, P.;
Ludwig, H. -G.; Freytag, B.
2017MmSAI..88...82G Altcode: 2016arXiv161004427G
Molecular features such as the G-band, CN-band and NH-band are important
diagnostics for measuring a star's carbon and nitrogen abundances,
especially in metal-poor stars where atomic lines are no longer visible
in stellar spectra. Unlike atomic transitions, molecular features
tend to form in bands, which cover large wavelength regions in a
spectrum. While it is a trivial matter to compute carbon and nitrogen
molecular bands under the assumption of 1D, it is extremely time
consuming in 3D. In this contribution to the 2016 COBOLD workshop we
review the improvements made to the 3D spectral synthesis code Linfor3D,
and discuss the new challenges found when computing molecular features
in 3D.
---------------------------------------------------------
Title: The influence of convection on OH UV line formation in the
atmosphere of the metal-poor red giant HD 122563.
Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas,
J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M.
2017MmSAI..88...77P Altcode:
We utilized high-resolution spectra of the metal-poor red giant star
HD 122563 and classical 1D hydrostatic ATLAS9 model atmosphere to
derive the 1D LTE oxygen abundance from OH UV lines. The obtained
average 1D LTE oxygen abundance is xtmean {ensuremath {A(O)_{1D
LTE}} = 6.41 ± 0.16. We also used 3D hydrodynamical COBOLD and 1D
hydrostatic LHD model atmospheres to correct the 1D LTE abundances for
convection-related effects and to determine average 3D LTE abundance,
xtmean {ensuremath {A(O)_{3D LTE}} = 6.23 ± 0.13. We found that while
the oxygen abundances determined using 1D hydrostatic model atmospheres
showed trends both with the line strength and excitation potential,
these trends essentially disappeared in the 3D LTE case. The average 3D
LTE oxygen abundance obtained from the OH UV lines agrees reasonably
well with the oxygen abundances obtained in the earlier analyses from
the OH IR lines and the 630.0 nm [O I] line, while the remaining small
discrepancies may possibly be attributed to NLTE effects.
---------------------------------------------------------
Title: Investigation of the solar centre-to-limb variation of oxygen
and lithium spectral features
Authors: Caffau, E.; Malherbe, J. -M.; Steffen, M.; Ludwig, H. -G.;
Mott, A.
2017MmSAI..88...45C Altcode:
We compare intensity spectra of the Sun observed at different limb
angles in the wavelength range covering the forbidden oxygen lines and
the lithium resonance feature with line formation computations performed
on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. Among
the prime oxygen abundance indicators, the forbidden line at 630 nm
is contaminated with a significant Ni I blend. The availability of
observations at different positions on the solar disc allows us to
disentangle the contributions of oxygen and nickel and to derive their
individual abundances. We derived in the past, from the [OI] line,
A(O)=8.73± 0.05 with a nickel abundance of A(Ni)=6.1± 0.04. From
the observations here presented, we obtain A(O)=8.71 and A(Ni)=6.09,
in excellent agreement with the previous result. For lithium, we
investigated the Li doublet at 670.7 nm and compared synthetic spectra
of the Li spectra range based on different line-lists available in the
literature to the observed data. With these observations, we are still
unable to conclude on which is the best line-list to be used for the
blending lines.
---------------------------------------------------------
Title: 3D hydrodynamical COBOLD simulations of a chromosphere of a
red giant
Authors: Klevas, J.; Wedemeyer, S.; Kučinskas, A.; Ludwig, H. -G.
2017MmSAI..88..100K Altcode:
We present the results of a 3D hydrodynamical simulation of a cool red
giant star with a chromosphere. The simulation was performed using
a 3D hydrodynamic COBOLD model atmosphere (T<SUB>eff</SUB>=4000 K,
{log g}=1.5 cgs, [M/H]=0.0 [dex]), which was extended outwards to
include chromospheric layers. We synthesized the spectral energy
distribution of a model atmosphere including chromosphere and compare
it to the spectral energy distribution of a model atmosphere without
the chromosphere. We find that adding a model chromosphere leads to a
significant increase of the radiative flux at wavelengths smaller than
300 nm. The increase in the UV flux is attributed to the presence of
shock waves, which provide additional heating in the chromosphere.
---------------------------------------------------------
Title: TOPoS. III. An ultra iron-poor multiple CEMP system
Authors: Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Monaco, L.;
Sbordone, L.; François, P.; Gallagher, A. J.; Plez, B.; Zaggia, S.;
Ludwig, H. -G.; Cayrel, R.; Koch, A.; Steffen, M.; Salvadori, S.;
Klessen, R.; Glover, S.; Christlieb, N.
2016A&A...595L...6C Altcode: 2016arXiv161004106C
<BR /> Aims: One of the primary objectives of the TOPoS survey
is to search for the most metal-poor stars. Our search has led
to the discovery of one of the most iron-poor objects known, SDSS
J092912.32+023817.0. This object is a multiple system, in which two
components are clearly detected in the spectrum. <BR /> Methods:
We have analysed 16 high-resolution spectra obtained using the UVES
spectrograph at the ESO 8.2 m VLT telescope to measure radial velocities
and determine the chemical composition of the system. <BR /> Results:
Cross correlation of the spectra with a synthetic template yields a
double-peaked cross-correlation function (CCF) for eight spectra, and
in one case there is evidence for the presence of a third peak. Chemical
analysis of the spectrum obtained by averaging all the spectra for which
the CCF showed a single peak found that the iron abundance is [Fe/H] =
-4.97. The system is also carbon enhanced with [C/Fe] = +3.91 (A(C) =
7.44). From the permitted oxygen triplet we determined an upper limit
for oxygen of [O/Fe] < +3.52 such that C/O > 1.3. We are also
able to provide more stringent upper limits on the Sr and Ba abundances
([Sr/Fe] < +0.70, and [Ba/Fe] < +1.46, respectively). <P />Based
on observations made with ESO Telescopes at the La Silla Paranal
Observatory under programme ID 094.D-0488 and 096.D-0616.
---------------------------------------------------------
Title: An in-depth spectroscopic examination of molecular bands from
3D hydrodynamical model atmospheres. I. Formation of the G-band in
metal-poor dwarf stars
Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.; Spite, M.
2016A&A...593A..48G Altcode: 2016arXiv160507215G
Context. Recent developments in the three-dimensional (3D) spectral
synthesis code Linfor3D have meant that for the first time, large
spectral wavelength regions, such as molecular bands, can be synthesised
with it in a short amount of time. <BR /> Aims: A detailed spectral
analysis of the synthetic G-band for several dwarf turn-off-type 3D
atmospheres (5850 ≲ T<SUB>eff</SUB> [ K ] ≲ 6550, 4.0 ≤ log g
≤ 4.5, - 3.0 ≤ [Fe/H] ≤-1.0) was conducted, under the assumption
of local thermodynamic equilibrium. We also examine carbon and oxygen
molecule formation at various metallicity regimes and discuss the impact
it has on the G-band. <BR /> Methods: Using a qualitative approach,
we describe the different behaviours between the 3D atmospheres and
the traditional one-dimensional (1D) atmospheres and how the different
physics involved inevitably leads to abundance corrections, which
differ over varying metallicities. Spectra computed in 1D were fit to
every 3D spectrum to determine the 3D abundance correction. <BR />
Results: Early analysis revealed that the CH molecules that make up
the G-band exhibited an oxygen abundance dependency; a higher oxygen
abundance leads to weaker CH features. Nitrogen abundances showed zero
impact to CH formation. The 3D corrections are also stronger at lower
metallicity. Analysis of the 3D corrections to the G-band allows us to
assign estimations of the 3D abundance correction to most dwarf stars
presented in the literature. <BR /> Conclusions: The 3D corrections
suggest that A(C) in carbon-enhanced metal-poor (CEMP) stars with high
A(C) would remain unchanged, but would decrease in CEMP stars with lower
A(C). It was found that the C/O ratio is an important parameter to the
G-band in 3D. Additional testing confirmed that the C/O ratio is an
equally important parameter for OH transitions under 3D. This presents
a clear interrelation between the carbon and oxygen abundances in 3D
atmospheres through their molecular species, which is not seen in 1D.
---------------------------------------------------------
Title: Hydrodynamical model atmospheres: Their impact on stellar
spectroscopy and asteroseismology of late-type stars
Authors: Ludwig, H. -G.; Steffen, M.
2016AN....337..844L Altcode: 2016arXiv160900219L
Hydrodynamical, i.e. multi-dimensional and time-dependent,
model atmospheres of late-type stars have reached a high level
of realism. They are commonly applied in high-fidelity work on
stellar abundances but also allow the study of processes that
are not modelled in standard, one-dimensional hydrostatic model
atmospheres. Here, we discuss two observational aspects that emerge
from such processes, the photometric granulation background and the
spectroscopic microturbulence. We use CO5BOLD hydrodynamical model
atmospheres to characterize the total granular brightness fluctuations
and characteristic time scale for FGK stars. Emphasis is put on the
diagnostic potential of the granulation background for constraining
the fundamental atmospheric parameters. We find a clear metallicity
dependence of the granulation background. The comparison between
the model predictions and available observational constraints at
solar metallicity shows significant differences, that need further
clarification. Concerning microturbulence, we report on the derivation
of a theoretical calibration based on CO5BOLD models, which shows
good correspondence with the measurements for stars in the Hyades. We
emphasize the importance of a consistent procedure when determining
the microturbulence, and point to limitations of the commonly applied
description of microturbulence in hydrostatic model atmospheres.
---------------------------------------------------------
Title: Exoplanet Transits Enable High-Resolution Spectroscopy Across
Spatially Resolved Stellar Surfaces
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik;
Pazira, Hiva
2016csss.confE..66D Altcode: 2016arXiv160703489D
Observations of stellar surfaces ndash; except for the Sun ndash;
are hampered by their tiny angular extent, while observed spectral
lines are smeared by averaging over the stellar surface, and by stellar
rotation. Exoplanet transits can be used to analyze stellar atmospheric
structure, yielding high-resolution spectra across spatially highly
resolved stellar surfaces, free from effects of spatial smearing and the
rotational wavelength broadening present in full-disk spectra. During
a transit, stellar surface portions successively become hidden, and
differential spectroscopy between various transit phases provides
spectra of those surface segments then hidden behind the planet. The
small area subtended by even a large planet (about 1% of a main-sequence
star) offers high spatial resolution but demands very precise
observations. We demonstrate the reconstruction of photospheric Fe I
line profilesnbsp;at a spectral resolution R=80,000 across the surface
of the solar-type star HD 209458. Any detailed understanding of stellar
atmospheres requires modeling with 3-dimensional hydrodynamics. The
properties predicted by such models are mapped onto the precise
spectral-line shapes, asymmetries and wavelength shifts, and their
variation from the center to the limb across any stellar disk. This
method provides a tool for testing and verifying such models. The
method will soon become applicable to more diverse types of stars,
thanks to new spectrometers on very large telescopes, and since ongoing
photometric searches are expected to discover additional bright host
stars of transiting exoplanets.>
---------------------------------------------------------
Title: Spatially Resolved Spectroscopy Across HD189733 (K1V) Using
Exoplanet Transits
Authors: Gustavsson, Martin; Dravins, Dainis; Ludwig, Hans-Günter
2016csss.confE..53G Altcode:
For testing 3-dimensional models of stellar atmospheres, spectroscopy
across spatially resolved stellar surfaces would be desired with
a spectral resolution of(R = 100,000) or more. Hydrodynamic models
predict variations in line profile shapes, strengths, wavelength
positions and asymmetries. These variations vary systematically between
disk center and limb and as a function of line strength, excitation
potential and wavelength region. However, except for a few supergiants
and the Sun, current telescopes are not yet capable of resolving
any stellar surfaces. One alternative method to resolve distant
stellar surfaces, feasible already now, is differential spectroscopy
of transiting exoplanet systems. By subtracting in-transit spectra
from the spectrum outside of transit, the spectra from stellar surface
portions temporarily hidden behind the planet can be disentangled. Since
transiting planets cover only a small portion of the stellar surface,
the method requires a very high signal-to-noise ratio, obtainable by
averaging numerous similar spectral lines. We apply such differential
spectroscopy on the 7.7 mag K1V star HD 189733 ('Alopex'*); its
transiting planet covers ∼ 3% of its host star's surface, which
is the deepest known transit among the brighter systems. Archival
data from the ESO HARPS spectrometerare used to construct averaged
profiles of photospheric Fe I lines, with the aim of comparing spatially
resolved profiles to analogous synthetic line profiles computed from the
3-dimensional hydrodynamic CO<SUP>5</SUP>BOLD model.<BR /> * We refer
to HD 189733 as 'Alopex' (from the Greek 'αλɛπού'), denoting a
fox related to the one that gave name to its constellation of Vulpecula.
---------------------------------------------------------
Title: Chemical abundances in a high-velocity RR Lyrae star near
the bulge
Authors: Hansen, C. J.; Rich, R. M.; Koch, A.; Xu, S.; Kunder, A.;
Ludwig, H. -G.
2016A&A...590A..39H Altcode: 2016arXiv160305654H
Low-mass variable high-velocity stars are interesting study cases
for many aspects of Galactic structure and evolution. Until recently,
the only known high- or hyper-velocity stars were young stars thought
to originate from the Galactic center. Wide-area surveys such as
APOGEE and BRAVA have found several low-mass stars in the bulge with
Galactic rest-frame velocities higher than 350 km s<SUP>-1</SUP>. In
this study we present the first abundance analysis of a low-mass RR
Lyrae star that is located close to the Galactic bulge, with a space
motion of ~-400 km s<SUP>-1</SUP>. Using medium-resolution spectra,
we derived abundances (including upper limits) of 11 elements. These
allowed us to chemically tag the star and discuss its origin, although
our derived abundances and metallicity, at [Fe/H] =-0.9 dex, do not
point toward one unambiguous answer. Based on the chemical tagging,
we cannot exclude that it originated in the bulge. However, its
retrograde orbit and the derived abundances combined suggest that the
star was accelerated from the outskirts of the inner (or even outer)
halo during many-body interactions. Other possible origins include
the bulge itself, or the star might have been stripped from a stellar
cluster or the Sagittarius dwarf galaxy when it merged with the Milky
Way. <P />The data presented herein were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership among the
California Institute of Technology, the University of California and the
National Aeronautics and Space Administration. The Observatory was made
possible by the generous financial support of the W. M. Keck Foundation.
---------------------------------------------------------
Title: The chemical composition of red giants in 47
Tucanae. II. Magnesium isotopes and pollution scenarios
Authors: Thygesen, A. O.; Sbordone, L.; Ludwig, H. -G.; Ventura, P.;
Yong, D.; Collet, R.; Christlieb, N.; Melendez, J.; Zaggia, S.
2016A&A...588A..66T Altcode: 2016arXiv160200058T
Context. The phenomenon of multiple populations in globular clusters
is still far from understood, with several proposed mechanisms to
explain the observed behaviour. The study of elemental and isotopic
abundance patterns are crucial for investigating the differences
among candidate pollution mechanisms. <BR /> Aims: We derive magnesium
isotopic ratios for 13 stars in the globular cluster 47 Tucanae (NGC
104) to provide new, detailed information about the nucleosynthesis
that has occurred within the cluster. For the first time, the impact
of 3D model stellar atmospheres on the derived Mg isotopic ratios
is investigated. <BR /> Methods: Using both tailored 1D atmospheric
models and 3D hydrodynamical models, we derive magnesium isotopic
ratios from four features of MgH near 5135 Å in 13 giants near the tip
of the red giant branch, using high signal-to-noise, high-resolution
spectra. <BR /> Results: We derive the magnesium isotopic ratios for
all stars and find no significant offset of the isotopic distribution
between the pristine and the polluted populations. Furthermore, we do
not detect any statistically significant differences in the spread in
the Mg isotopes in either population. No trends were found between
the Mg isotopes and [Al/Fe]. The inclusion of 3D atmospheres has a
significant impact on the derived <SUP>25</SUP>Mg/<SUP>24</SUP>Mg
ratio, increasing it by a factor of up to 2.5, compared to 1D. The
<SUP>26</SUP>Mg/<SUP>24</SUP>Mg ratio, on the other hand, essentially
remains unchanged. <BR /> Conclusions: We confirm the results seen from
other globular clusters, where no strong variation in the isotopic
ratios is observed between stellar populations, for observed ranges
in [Al/Fe]. We see no evidence for any significant activation of the
Mg-Al burning chain. The use of 3D atmospheres causes an increase of
a factor of up to 2.5 in the fraction of <SUP>25</SUP>Mg, resolving
part of the discrepancy between the observed isotopic fraction and
the predictions from pollution models. <P />Based on observations
made with the ESO Very Large Telescope at Paranal Observatory, Chile
(Programmes 084.B-0810 and 086.B-0237).
---------------------------------------------------------
Title: Solar Science with the Atacama Large Millimeter/Submillimeter
Array—A New View of Our Sun
Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.;
Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu,
B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin,
P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz,
A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.;
Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary,
D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van
der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.;
Selhorst, C. L.; Barta, M.
2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W
The Atacama Large Millimeter/submillimeter Array (ALMA) is a new
powerful tool for observing the Sun at high spatial, temporal, and
spectral resolution. These capabilities can address a broad range
of fundamental scientific questions in solar physics. The radiation
observed by ALMA originates mostly from the chromosphere—a complex
and dynamic region between the photosphere and corona, which plays a
crucial role in the transport of energy and matter and, ultimately,
the heating of the outer layers of the solar atmosphere. Based on
first solar test observations, strategies for regular solar campaigns
are currently being developed. State-of-the-art numerical simulations
of the solar atmosphere and modeling of instrumental effects can help
constrain and optimize future observing modes for ALMA. Here we present
a short technical description of ALMA and an overview of past efforts
and future possibilities for solar observations at submillimeter and
millimeter wavelengths. In addition, selected numerical simulations
and observations at other wavelengths demonstrate ALMA's scientific
potential for studying the Sun for a large range of science cases.
---------------------------------------------------------
Title: Lithium spectral line formation in stellar atmospheres. The
impact of convection and NLTE effects
Authors: Klevas, J.; Kučinskas, A.; Steffen, M.; Caffau, E.; Ludwig,
H. -G.
2016A&A...586A.156K Altcode: 2015arXiv151208999K
<BR /> Aims: Because of the complexities involved in treating
spectral line formation in full 3D and non-local thermodynamic
equilibrium (NLTE), different simplified approaches are sometimes
used to account for the NLTE effects with 3D hydrodynamical model
atmospheres. In certain cases, chemical abundances are derived in
1D NLTE and then corrected for the 3D effects by adding 3D-1D LTE
(Local Thermodynamic Equilibrium, LTE) abundance corrections (3D+NLTE
approach). Alternatively, average ⟨3D⟩ model atmospheres are
sometimes used to substitute for the full 3D hydrodynamical models. <BR
/> Methods: In this work we tested whether the results obtained using
these simplified schemes (3D+NLTE, ⟨3D⟩ NLTE) may reproduce those
derived using the full 3D NLTE computations. The tests were made using
3D hydrodynamical CO<SUP>5</SUP>BOLD model atmospheres of the main
sequence (MS), main sequence turn-off (TO), subgiant (SGB), and red
giant branch (RGB) stars, all at two metallicities, [ M / H ] = 0.0
and -2.0. Our goal was to investigate the role of 3D and NLTE effects
on the formation of the 670.8 nm lithium resonance line. This was done
by assessing differences in the strengths of synthetic 670.8 nm line
profiles, which were computed using 3D/1D NLTE/LTE approaches. <BR />
Results: Our results show that Li 670.8 nm line strengths obtained
using different methodologies differ only slightly in most of the
models at solar metallicity studied here. However, the line strengths
predicted with the 3D NLTE and 3D+NLTE approaches become significantly
different at subsolar metallicities. At [ M / H ] = -2.0, this may lead
to (3D NLTE) - (3D+NLTE) differences in the predicted lithium abundance
of ~0.46 and ~0.31 dex in the TO and RGB stars respectively. On the
other hand, NLTE line strengths computed with the average ⟨3D⟩ and
1D model atmospheres are similar to those obtained with the full 3D
NLTE approach for MS, TO, SGB, and RGB stars, at all metallicities;
3D - ⟨3D⟩ and 3D - 1D differences in the predicted abundances
are always less than ~0.04 dex and ~0.08 dex, respectively. However,
neither of the simplified approaches can reliably substitute 3D NLTE
spectral synthesis when precision is required.
---------------------------------------------------------
Title: New insights on pulsating white dwarfs from 3D
radiation-hydrodynamical simulations
Authors: Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig,
Hans-Günter; Gianninas, Alexandros; Kilic, Mukremin
2016IAUFM..29B.667T Altcode:
We have recently computed a grid of 3D radiation-hydrodynamical
simulations for the atmosphere of pure-hydrogen DA white dwarfs in
the range 5.0 < log g < 9.0. Our grid covers the full ZZ Ceti
instability strip where pulsating DA white dwarfs are located. We
have significantly improved the theoretical framework to study these
objects by removing the free parameters of 1D convection, which were
previously a major modeling hurdle. We present improved atmospheric
parameter determinations based on spectroscopic fits with 3D model
spectra, allowing for an updated definition of the empirical edges
of the ZZ Ceti instability strip. Our 3D simulations also precisely
predict the depth of the convection zones, narrowing down the internal
layers where pulsation are being driven. We hope that these 3D effects
will be included in asteroseismic models in the future to predict the
region of the HR diagram where white dwarfs are expected to pulsate.
---------------------------------------------------------
Title: GIANO Y-band spectroscopy of dwarf stars: Phosphorus, sulphur,
and strontium abundances
Authors: Caffau, E.; Andrievsky, S.; Korotin, S.; Origlia, L.; Oliva,
E.; Sanna, N.; Ludwig, H. -G.; Bonifacio, P.
2016A&A...585A..16C Altcode: 2015arXiv151006396C
Context. In recent years a number of poorly studied chemical elements,
such as phosphorus, sulphur, and strontium, have received special
attention as important tracers of the Galactic chemical evolution. <BR
/> Aims: By exploiting the capabilities of the infrared echelle
spectrograph GIANO mounted at the Telescopio Nazionale Galileo,
we acquired high resolution spectra of four Galactic dwarf stars
spanning the metallicity range between about one-third and twice
the solar value. We performed a detailed feasibility study about
the effectiveness of the P, S, and Sr line diagnostics in the Y band
between 1.03 and 1.10 μm. <BR /> Methods: Accurate chemical abundances
have been derived using one-dimensional model atmospheres computed in
local thermodynamic equilibrium (LTE). We computed the line formation
assuming LTE for P, while we performed non-LTE analysis to derive S
and Sr abundances. <BR /> Results: We were able to derive phosphorus
abundance for three stars and an upper limit for one star, while we
obtained the abundance of sulphur and strontium for all of the stars. We
find [P/Fe] and [S/Fe] abundance ratios consistent with solar-scaled
or slightly depleted values, while the [Sr/Fe] abundance ratios are
more scattered (by ±0.2 dex) around the solar-scaled value. This is
fully consistent with previous studies using both optical and infrared
spectroscopy. <BR /> Conclusions: We verified that high-resolution,
Y-band spectroscopy as provided by GIANO is a powerful tool to study
the chemical evolution of P, S, and Sr in dwarf stars. <P />Based on
observations obtained with GIANO.
---------------------------------------------------------
Title: Synthetic activity indicators for M-type dwarf stars
Authors: Wedemeyer, Sven; Ludwig, Hans-Günter
2016IAUS..320..303W Altcode: 2015arXiv151106153W
Here, we present a set of time-dependent 3D RMHD simulations of a
M-dwarf star representative of AD Leo, which extend from the upper
convection zone into the chromosphere. The 3D model atmospheres are
characterized by a very dynamic and intermittent structure on small
spatial and temporal scales and a wealth of physical processes,
which by nature cannot be described by means of 1D static model
atmospheres. Artificial observations of these models imply that a
combination of complementary diagnostics such as Ca II lines and the
continuum intensity from UV to millimeter wavelengths, probe various
properties of the dynamics, thermal and magnetic structure of the
photosphere and the chromosphere and thus provide measures of stellar
activity, which can be compared to observations. The complicated
magnetic field structure and its imprint in synthetic diagnostics may
have important implications for the understanding and characterization
of stellar activity and with it possibly for the evaluation of planetary
habitability around active M-dwarf stars.
---------------------------------------------------------
Title: Book Review: Astronomical Spectroscopy — An Introduction
to the Atomic and Molecular Physics of Astronomical Spectra<SUP></SUP>
Authors: Ludwig, Hans-Günter
2015JAI.....480001L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The photospheric solar oxygen project. IV. 3D-NLTE
investigation of the 777 nm triplet lines
Authors: Steffen, M.; Prakapavičius, D.; Caffau, E.; Ludwig, H. -G.;
Bonifacio, P.; Cayrel, R.; Kučinskas, A.; Livingston, W. C.
2015A&A...583A..57S Altcode: 2015arXiv150803487S
Context. The solar photospheric oxygen abundance is still widely
debated. Adopting the solar chemical composition based on the "low"
oxygen abundance, as determined with the use of three-dimensional (3D)
hydrodynamical model atmospheres, results in a well-known mismatch
between theoretical solar models and helioseismic measurements
that is so far unresolved. <BR /> Aims: We carry out an independent
redetermination of the solar oxygen abundance by investigating the
center-to-limb variation of the O i IR triplet lines at 777 nm in
different sets of spectra. <BR /> Methods: The high-resolution and high
signal-to-noise solar center-to-limb spectra are analyzed with the
help of detailed synthetic line profiles based on 3D hydrodynamical
CO5BOLD model atmospheres and 3D non-LTE line formation calculations
with NLTE3D. The idea is to exploit the information contained in the
observations at different limb angles to simultaneously derive the
oxygen abundance, A(O), and the scaling factor S<SUB>H</SUB> that
describes the cross-sections for inelastic collisions with neutral
hydrogen relative to the classical Drawin formula. Using the same
codes and methods, we compare our 3D results with those obtained from
the semi-empirical Holweger-Müller model atmosphere as well as from
different one-dimensional (1D) reference models. <BR /> Results: With
the CO5BOLD 3D solar model, the best fit of the center-to-limb variation
of the triplet lines is obtained when the collisions by neutral hydrogen
atoms are assumed to be efficient, i.e., when the scaling factor
S<SUB>H</SUB> is between 1.2 and 1.8, depending on the choice of the
observed spectrum and the triplet component used in the analysis. The
line profile fits achieved with standard 1D model atmospheres (with
fixed microturbulence, independent of disk position μ) are clearly
of inferior quality compared to the 3D case, and give the best match
to the observations when ignoring collisions with neutral hydrogen
(S<SUB>H</SUB> = 0). The results derived with the Holweger-Müller model
are intermediate between 3D and standard 1D. <BR /> Conclusions: The
analysis of various observations of the triplet lines with different
methods yields oxygen abundance values (on a logarithmic scale where
A(H) = 12) that fall in the range 8.74 <A(O) < 8.78, and our
best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76
± 0.02. All 1D non-LTE models give much lower oxygen abundances,
by up to -0.15 dex. This is mainly a consequence of the assumption
of a μ-independent microturbulence. An independent determination of
the relevant collisional cross-sections is essential to substantially
improve the accuracy of the oxygen abundance derived from the O i IR
triplet. <P />Appendices E and F are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526406/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Surface-effect corrections for solar-like oscillations using
3D hydrodynamical simulations. I. Adiabatic oscillations
Authors: Sonoi, T.; Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau,
E.; Mosser, B.
2015A&A...583A.112S Altcode: 2015arXiv151000300S
Context. The CoRoT and Kepler space-borne missions have provided us with
a wealth of high-quality observational data that allows for seismic
inferences of stellar interiors. This requires the computation of
precise and accurate theoretical frequencies, but imperfect modeling of
the uppermost stellar layers introduces systematic errors. To overcome
this problem, an empirical correction has been introduced by Kjeldsen
et al. (2008, ApJ, 683, L175) and is now commonly used for seismic
inferences. Nevertheless, we still lack a physical justification
allowing for the quantification of the surface-effect corrections. <BR
/> Aims: Our aim is to constrain the surface-effect corrections across
the Hertzsprung-Russell (HR) diagram using a set of 3D hydrodynamical
simulations. <BR /> Methods: We used a grid of these simulations
computed with the CO<SUP>5</SUP>BOLD code to model the outer layers of
solar-like stars. Upper layers of the corresponding 1D standard models
were then replaced by the layers obtained from the horizontally averaged
3D models. The frequency differences between these patched models
and the 1D standard models were then calculated using the adiabatic
approximation and allowed us to constrain the Kjeldsen et al. power law,
as well as a Lorentzian formulation. <BR /> Results: We find that the
surface effects on modal frequencies depend significantly on both the
effective temperature and the surface gravity. We further provide the
variation in the parameters related to the surface-effect corrections
using their power law as well as a Lorentzian formulation. Scaling
relations between these parameters and the elevation (related to the
Mach number) is also provided. The Lorentzian formulation is shown to
be more robust for the whole frequency spectrum, while the power law
is not suitable for the frequency shifts in the frequency range above
ν<SUB>max</SUB>. Finally, we show that, owing to turbulent pressure,
the elevation of the uppermost layers modifies the location of the
hydrogen ionization zone and consequently introduces glitches in
the surface effects for models with high (low) effective temperature
(surface gravity). <BR /> Conclusions: Surface-effect corrections vary
significantly across the HR diagram. Therefore, empirical relations
like those by Kjeldsen et al. must not be calibrated on the Sun but
should instead be constrained using realistic physical modeling as
provided by 3D hydrodynamical simulations.
---------------------------------------------------------
Title: On the Evolution of Magnetic White Dwarfs
Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.;
Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P.
2015ApJ...812...19T Altcode: 2015arXiv150905398T
We present the first radiation magnetohydrodynamic simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy
transfer is seriously impeded by magnetic fields when the plasma-β
parameter, the thermal-to-magnetic-pressure ratio, becomes smaller
than unity. The critical field strength that inhibits convection
in the photosphere of white dwarfs is in the range B = 1-50 kG,
which is much smaller than the typical 1-1000 MG field strengths
observed in magnetic white dwarfs, implying that these objects have
radiative atmospheres. We have employed evolutionary models to study the
cooling process of high-field magnetic white dwarfs, where convection
is entirely suppressed during the full evolution (B ≳ 10 MG). We
find that the inhibition of convection has no effect on cooling rates
until the effective temperature (T<SUB>eff</SUB>) reaches a value of
around 5500 K. In this regime, the standard convective sequences start
to deviate from the ones without convection due to the convective
coupling between the outer layers and the degenerate reservoir of
thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, the effects of magnetism on cooling rates are not expected
to be observed. This result contrasts with a recent suggestion
that magnetic white dwarfs with T<SUB>eff</SUB> ≲ 10,000 K cool
significantly slower than non-magnetic degenerates.
---------------------------------------------------------
Title: Stellar science from a blue wavelength range. A possible
design for the blue arm of 4MOST
Authors: Hansen, C. J.; Ludwig, H. -G.; Seifert, W.; Koch, A.; Xu,
W.; Caffau, E.; Christlieb, N.; Korn, A. J.; Lind, K.; Sbordone, L.;
Ruchti, G.; Feltzing, S.; de Jong, R. S.; Barden, S.
2015AN....336..665H Altcode: 2015arXiv150802714H
From stellar spectra, a variety of physical properties of stars
can be derived. In particular, the chemical composition of stellar
atmospheres can be inferred from absorption line analyses. These
provide key information on large scales, such as the formation of our
Galaxy, down to the small-scale nucleosynthesis processes that take
place in stars and supernovae. By extending the observed wavelength
range toward bluer wavelengths, we optimize such studies to also
include critical absorption lines in metal-poor stars, and allow
for studies of heavy elements (Z\ensuremath{g}e 38) whose formation
processes remain poorly constrained. In this context, spectrographs
optimized for observing blue wavelength ranges are essential, since many
absorption lines at redder wavelengths are too weak to be detected in
metal-poor stars. This means that some elements cannot be studied in
the visual-redder regions, and important scientific tracers and science
cases are lost. The present era of large public surveys will target
millions of stars. It is therefore important that the next generation
of spectrographs are designed such that they cover a wide wavelength
range and can observe a large number of stars simultaneously. Only
then, we can gain the full information from stellar spectra, from
both metal-poor to metal-rich ones, that will allow us to understand
the aforementioned formation scenarios in greater detail. Here we
describe the requirements driving the design of the forthcoming survey
instrument 4MOST, a multi-object spectrograph commissioned for the
ESO VISTA 4 m-telescope. While 4MOST is also intended for studies of
active galactic nuclei, baryonic acoustic oscillations, weak lensing,
cosmological constants, supernovae and other transients, we focus here
on high-density, wide-area survey of stars and the science that can
be achieved with high-resolution stellar spectroscopy. Scientific and
technical requirements that governed the design are described along with
a thorough line blending analysis. For the high-resolution spectrograph,
we find that a sampling of {\ensuremath{g}e 2.5} (pixels per resolving
element), spectral resolution of 18 000 or higher, and a wavelength
range covering 393-436 nm, is the most well-balanced solution for
the instrument. A spectrograph with these characteristics will enable
accurate abundance analysis (± 0.1 dex) in the blue and allow us to
confront the outlined scientific questions.
---------------------------------------------------------
Title: Collective electronic pulsation around giant nuclei in the
Thomas-Fermi model
Authors: Ludwig, H.; Ruffini, R.; Xue, S. -S.
2015NuPhA.941....1L Altcode: 2014arXiv1402.3468L
Based on the Thomas-Fermi solution for compressed electron gas around a
giant nucleus, we study electric pulsations of electron number-density,
pressure and electric fields, which could be caused by an external
perturbations acting on the nucleus or the electrons themselves. We
numerically obtain the eigen-frequencies and eigen-functions for
stationary pulsation modes that fulfill the boundary-value problem
established by electron-number and energy-momentum conservation,
equation of state, and Maxwell's equations, as well as physical
boundary conditions, and assume the nucleons in β-equilibrium
at nuclear density. We particularly study the configuration of
ultra-relativistic electrons with a large fraction contained within
the nucleus. Such configurations can be realized for a giant nucleus or
high external compression on the electrons. The lowest modes turn out
to be heavily influenced by the relativistic plasma frequency induced
by the positive charge background in the nucleus. Our results can be
applied to heavy nuclei in the neutron star crust, as well as to the
whole core of a neutron star. We discuss the possibility to apply our
results to dynamic nuclei using the spectral method.
---------------------------------------------------------
Title: Synthetic activity indicators for M-type dwarf stars
Authors: Wedemeyer, Sven; Ludwig, Hans-Günter; Hauschildt, Peter;
De Gennaro Aquino, Ivan
2015IAUGA..2255174W Altcode:
Our understanding of the Sun has been substantially progressed
owing to the advances in high-resolution observations during the
last decades. These observations guided the development of numerical
simulation codes for stellar atmospheres towards unprecedented levels
of realism and complexity. Such 3D radiation magnetohydrodynamic (RMHD)
codes can be applied and adapted to cooler stars. Here, we present a
set of time-dependent 3D RMHD simulations for dwarf stars of spectral
type M (representative of AD Leo). "M-dwarfs" are the most abundant
stars in our galaxy and known to exhibit mega-flares. Comparisons
of M-dwarf models with the Sun as fundamental reference case reveal
differences and similarities, which lead to important insights into
the structure and dynamics of quiescent "background" atmospheres. The
models, which extend from the upper convection zone into the
chromosphere, have different initial magnetic field strengths (up to
500G) and topologies, representing regions with different activity
levels. The 3D model atmospheres are characterized by a very dynamic
and intermittent structure on small spatial and temporal scales,
final field strengths reaching a few kG and a wealth of physical
processes, which by nature cannot be described by means of 1D static
model atmospheres.Synthetic observables, i.e. spectra and intensity
images, are calculated by using these models as input for detailed
radiative transfer calculations and can be combined into synthetic full
stellar disks, thus simulating spatially unresolved observations of
M-dwarfs. The considered diagnostics, like, e.g., Halpha, Ca II lines,
or the continuum intensity from UV to millimeter wavelengths, sample
various properties of the dynamics, thermal and magnetic structure
of the photosphere and the chromosphere and thus provide measures of
stellar activity, which can be compared to observations. The complicated
magnetic field structure and its imprint in synthetic diagnostics may
have important implications for the understanding and characterization
of stellar activity and with it possibly for the evaluation of planetary
habitability around active M-dwarf stars.
---------------------------------------------------------
Title: 3D Model Atmospheres for Extremely Low-mass White Dwarfs
Authors: Tremblay, P. -E.; Gianninas, A.; Kilic, M.; Ludwig, H. -G.;
Steffen, M.; Freytag, B.; Hermes, J. J.
2015ApJ...809..148T Altcode: 2015arXiv150701927T
We present an extended grid of mean three-dimensional (3D) spectra
for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use
CO5BOLD radiation-hydrodynamics 3D simulations covering T<SUB>eff</SUB>
= 6000-11,500 K and log g = 5-6.5 (g in cm s<SUP>-2</SUP>) to derive
analytical functions to convert spectroscopically determined 1D
temperatures and surface gravities to 3D atmospheric parameters. Along
with the previously published 3D models, the 1D to 3D corrections are
now available for essentially all known convective DA WDs (i.e., log g
= 5-9). For low-mass WDs, the correction in temperature is relatively
small (a few percent at the most), but the surface gravities measured
from the 3D models are lower by as much as 0.35 dex. We revisit
the spectroscopic analysis of the extremely low-mass (ELM) WDs, and
demonstrate that the 3D models largely resolve the discrepancies seen
in the radius and mass measurements for relatively cool ELM WDs in
eclipsing double WD and WD + millisecond pulsar binary systems. We
also use the 3D corrections to revise the boundaries of the ZZ Ceti
instability strip, including the recently found ELM pulsators.
---------------------------------------------------------
Title: Stellar Spectroscopy during Exoplanet Transits: Revealing
structures across stellar surfaces
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik
2015IAUGA..2233688D Altcode:
Exoplanet transits permit to study stellar surface portions that
successively become hidden behind the planet. Differential spectroscopy
between various transit phases reveals spectra of those stellar
surface segments that were hidden. The deduced center-to-limb behavior
of stellar spectral line shapes, asymmetries and wavelength shifts
enables detailed tests of 3-dimensional hydrodynamic models of stellar
atmospheres, such that are required for any precise determination
of abundances or seismic properties. Such models can now be computed
for widely different classes of stars (including metal-poor ones and
white dwarfs), but have been feasible to test and verify only for the
Sun with its resolved surface structure. Exoplanet transits may also
occur across features such as starspots, whose magnetic signatures will
be retrieved from spectra of sufficient fidelity.Knowing the precise
background stellar spectra, also properties of exoplanet atmospheres
are better constrained: e.g., the Rossiter-McLaughlin effect becomes
resolved as not only a simple change of stellar wavelength, but as a
variation of the full line profiles and their asymmetries.Such studies
are challenging since exoplanets cover only a tiny fraction of the
stellar disk. Current work, analyzing sequences of high-fidelity ESO
UVES spectra, demonstrate that such spatially resolved stellar spectra
can already be (marginally) retrieved in a few cases with the brightest
host stars. Already in a near future, ongoing exoplanet surveys are
likely to find further bright hosts that will enable such studies for
various stellar types. http://arxiv.org/abs/1408.1402
---------------------------------------------------------
Title: New Insights on Pulsating White Dwarfs from 3D
Radiation-Hydrodynamical Simulations
Authors: Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig,
Hans-Günter
2015IAUGA..2253532T Altcode:
We have recently computed a grid of 3D radiation-hydrodynamical
simulations for the atmosphere of 70 pure-hydrogen DA white dwarfs
in the range 7.0 < log g < 9.0. This includes the full ZZ Ceti
instability strip where DA white dwarfs are pulsating, by far the most
common type of degenerate pulsators. We have significantly improved
the theoretical framework to study these objects by removing the free
parameters of 1D convection, which were previously a major modeling
hurdle. We will compare our new models with the observed sample
of ZZ Ceti stars and highlight the improved derived properties of
these objects. In particular, the new spectroscopically determined 3D
atmospheric parameters allow for an improved definition of instability
strip edges. We have also made new predictions for the size of
convection zones, which significantly impact the position where the
pulsations are driven, and the region of the HR diagram where white
dwarfs are expected to pulsate. Finally, we will present new results
from non-adiabatic pulsation calculations.
---------------------------------------------------------
Title: A three-dimensional hydrodynamical line profile analysis of
iron lines and barium isotopes in HD 140283
Authors: Gallagher, A. J.; Ludwig, H. -G.; Ryan, S. G.; Aoki, W.
2015A&A...579A..94G Altcode: 2015arXiv150402353G
Context. Heavy-elements, i.e. those beyond the iron peak, mostly form
via two neutron capture processes: the slow (s-) and the rapid (r-)
process. Metal-poor stars should contain fewer isotopes that form
via the s-process, according to currently accepted theory. It has been
shown in several investigations that theory and observation do not agree
well, raising questions on the validity of either the methodology or the
theory. <BR /> Aims: We analyse the metal-poor star HD 140283, for which
we have a high quality spectrum. We test whether a three-dimensional
(3D) local thermodynamic equilibrium (LTE) stellar atmosphere and
spectrum synthesis code permits a more reliable analysis of the iron
abundance and barium isotope ratio than a one-dimensional (1D) LTE
analysis. <BR /> Methods: Using 3D hydrodynamical model atmospheres,
we examine 91 iron lines of varying strength and formation depth. This
provides us with the star's rotational speed. With this, we model the
barium isotope ratio by exploiting the hyperfine structure of the
singly ionised 4554 Å resonance line, and study the impact of the
uncertainties in the stellar parameters. <BR /> Results: The star's
rotational speed was found to be 1.65 ± 0.05 km s<SUP>-1</SUP>. Barium
isotopes under the 3D paradigm show a dominant r-process signature
as 77 ± 6 ± 17% (f<SUB>odd</SUB> = 0.38 ± 0.02 ± 0.06) of barium
isotopes form via the r-process, where errors represent the assigned
random and systematic errors, respectively. We find that 3D LTE fits
reproduce iron line profiles better than those in 1D, but do not provide
a unique abundance (within the uncertainties). However, we demonstrate
that the isotopic ratio is robust against this shortcoming. <BR />
Conclusions: Our barium isotope result agrees well with currently
accepted theory regarding the formation of the heavy-elements during
the early Galaxy. The improved fit to the asymmetric iron line profiles
suggests that the current state of 3D LTE modelling provides excellent
simulations of fluid flows. However, the abundances they provide are
not yet self-consistent. This may improve with non-LTE considerations
and higher resolution models. <P />Based on observations taken using
the Subaru Telescope High Dispersion Spectrograph (HDS), operated by
the National Astronomical Observatory of Japan.
---------------------------------------------------------
Title: The photospheric solar oxygen project. III. Investigation of
the centre-to-limb variation of the 630 nm [O I]-Ni I blend
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Livingston, W.;
Bonifacio, P.; Malherbe, J. -M.; Doerr, H. -P.; Schmidt, W.
2015A&A...579A..88C Altcode: 2015arXiv150600931C
Context. The solar photospheric abundance of oxygen is still a matter
of debate. For about ten years some determinations have favoured a
low oxygen abundance which is at variance with the value inferred by
helioseismology. Among the oxygen abundance indicators, the forbidden
line at 630 nm has often been considered the most reliable even
though it is blended with a Ni i line. In Papers I and II of this
series we reported a discrepancy in the oxygen abundance derived
from the 630 nm and the subordinate [O I] line at 636 nm in dwarf
stars, including the Sun. <BR /> Aims: Here we analyse several,
in part new, solar observations of the centre-to-limb variation
of the spectral region including the blend at 630 nm in order to
separate the individual contributions of oxygen and nickel. <BR />
Methods: We analyse intensity spectra observed at different limb
angles in comparison with line formation computations performed on a
CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. <BR />
Results: The oxygen abundances obtained from the forbidden line at
different limb angles are inconsistent if the commonly adopted nickel
abundance of 6.25 is assumed in our local thermodynamic equilibrium
computations. With a slightly lower nickel abundance, A(Ni) ≈ 6.1,
we obtain consistent fits indicating an oxygen abundance of A(O) = 8.73
± 0.05. At this value the discrepancy with the subordinate oxygen
line remains. <BR /> Conclusions: The derived value of the oxygen
abundance supports the notion of a rather low oxygen abundance in the
solar photosphere. However, it is disconcerting that the forbidden
oxygen lines at 630 and 636 nm give noticeably different results,
and that the nickel abundance derived here from the 630 nm blend is
lower than expected from other nickel lines.
---------------------------------------------------------
Title: TOPoS . II. On the bimodality of carbon abundance in CEMP
stars Implications on the early chemical evolution of galaxies
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi,
A.; Klessen, R. S.; François, P.; Molaro, P.; Ludwig, H. -G.; Zaggia,
S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.;
Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.;
Steffen, M.
2015A&A...579A..28B Altcode: 2015arXiv150405963B
Context. In the course of the Turn Off Primordial Stars (TOPoS) survey,
aimed at discovering the lowest metallicity stars, we have found several
carbon-enhanced metal-poor (CEMP) stars. These stars are very common
among the stars of extremely low metallicity and provide important
clues to the star formation processes. We here present our analysis
of six CEMP stars. <BR /> Aims: We want to provide the most complete
chemical inventory for these six stars in order to constrain the
nucleosynthesis processes responsible for the abundance patterns. <BR
/> Methods: We analyse both X-Shooter and UVES spectra acquired at the
VLT. We used a traditional abundance analysis based on OSMARCS 1D local
thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum
line formation code. <BR /> Results: Calcium and carbon are the only
elements that can be measured in all six stars. The range is -5.0 ≤
[Ca/H] <-2.1 and 7.12 ≤ A(C) ≤ 8.65. For star SDSS J1742+2531
we were able to detect three Fe i lines from which we deduced [Fe/H]
= -4.80, from four Ca ii lines we derived [Ca/H] = -4.56, and from
synthesis of the G-band we derived A(C) = 7.26. For SDSS J1035+0641 we
were not able to detect any iron lines, yet we could place a robust
(3σ) upper limit of [Fe/H] < -5.0 and measure the Ca abundance,
with [Ca/H] = -5.0, and carbon, A(C) = 6.90, suggesting that this star
could be even more metal-poor than SDSS J1742+2531. This makes these
two stars the seventh and eighth stars known so far with [Fe/H] <
-4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected
in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a
robust upper limit of A(Li) < 1.8 for both stars. <BR /> Conclusions:
Our measured carbon abundances confirm the bimodal distribution of
carbon in CEMP stars, identifying a high-carbon band and a low-carbon
band. We propose an interpretation of this bimodality according to which
the stars on the high-carbon band are the result of mass transfer from
an AGB companion, while the stars on the low-carbon band are genuine
fossil records of a gas cloud that has also been enriched by a faint
supernova (SN) providing carbon and the lighter elements. The abundance
pattern of the UIP stars shows a large star-to-star scatter in the
[X/Ca] ratios for all elements up to aluminium (up to 1 dex), but
this scatter drops for heavier elements and is at most of the order
of a factor of two. We propose that this can be explained if these
stars are formed from gas that has been chemically enriched by several
SNe, that produce the roughly constant [X/Ca] ratios for the heavier
elements, and in some cases the gas has also been polluted by the
ejecta of a faint SN that contributes the lighter elements in variable
amounts. The absence of lithium in four of the five known unevolved
UIP stars can be explained by a dominant role of fragmentation in the
formation of these stars. This would result either in a destruction
of lithium in the pre-main-sequence phase, through rotational mixing
or to a lack of late accretion from a reservoir of fresh gas. The
phenomenon should have varying degrees of efficiency. <P />Based on
observations obtained at ESO Paranal Observatory, programme 091.D-0288,
091.D-0305, 189.D-0165.Appendix A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201425266/olm">http://www.aanda.org</A>Tables
4 is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A28">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A28</A>
---------------------------------------------------------
Title: Calibration of the Mixing-Length Free Parameter for White
Dwarf Structures
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
G.; Steffen, M.; Brassard, P.
2015ASPC..493...89T Altcode:
We present a comparison of our grid of 3D radiation-hydrodynamical
simulations for 70 pure-hydrogen DA white dwarfs, in the surface
gravity range 7.0 ≤log g≤ 9.0, with 1D envelope models based on the
mixing-length theory (MLT) for convection. We perform a calibration
of the mixing-length parameter for the lower part of the convection
zone. The 3D simulations are often restricted to the upper convective
layers, and in those cases, we rely on the asymptotic entropy value of
the adiabatic 3D upflows to calibrate 1D envelopes. Our results can be
applied to 1D structure calculations, and in particular for pulsation
and convective mixing studies. We demonstrate that while the 1D MLT
only provides a bottom boundary of the convection zone based on the
Schwarzschild criterion, the 3D stratifications are more complex. There
is a large overshoot region below the convective layers that is likely
critical for chemical diffusion applications.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Abundances of 3 CEMP stars
(Bonifacio+, 2015)
Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi,
A.; Klessen, R. S.; Francois, P.; Molaro, P.; Ludwig, H. -G.; Zaggia,
S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.;
Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.;
Steffen, M.
2015yCat..35790028B Altcode:
We analyse both X-Shooter and UVES spectra acquired at the VLT. We used
a traditional abundance analysis based on OSMARCS 1D Local Thermodynamic
Equilibrium (LTE) model atmospheres and the TURBOSPECTRUM line formation
code. <P />(2 data files).
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. IV. Oxygen diagnostics in extremely
metal-poor red giants with infrared OH lines
Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Caffau,
E.; Ludwig, H. -G.; Steffen, M.; Spite, M.
2015A&A...576A.128D Altcode: 2015arXiv150206587D
Context. Although oxygen is an important tracer of Galactic chemical
evolution, measurements of its abundance in the atmospheres of the
oldest Galactic stars are still scarce and rather imprecise. This
is mainly because only a few spectral lines are available for the
abundance diagnostics. At the lowest end of the metallicity scale,
oxygen can only be measured in giant stars and in most of cases such
measurements rely on a single forbidden [O i] 630 nm line that is very
weak and frequently blended with telluric lines. Although molecular
OH lines located in the ultraviolet and infrared could also be used
for the diagnostics, oxygen abundances obtained from the OH lines and
the [O i] 630 nm line are usually discrepant to a level of ~ 0.3-0.4
dex. <BR /> Aims: We study the influence of convection on the formation
of the infrared (IR) OH lines and the forbidden [O i] 630 nm line in the
atmospheres of extremely metal-poor (EMP) red giant stars. Our ultimate
goal is to clarify whether a realistic treatment of convection with
state-of-the-art 3D hydrodynamical model atmospheres may help to bring
the oxygen abundances obtained using the two indicators into closer
agreement. <BR /> Methods: We used high-resolution (R = 50 000) and high
signal-to-noise ratio (S/N ≈ 200-600) spectra of four EMP red giant
stars obtained with the VLT CRIRES spectrograph. For each EMP star,
4-14 IR OH vibrational-rotational lines located in the spectral range of
1514-1548 and 1595-1632 nm were used to determine oxygen abundances by
employing standard 1D local thermodynamic equilibrium (LTE) abundance
analysis methodology. We then corrected the 1D LTE abundances
obtained from each individual OH line for the 3D hydrodynamical
effects, which was done by applying 3D-1D LTE abundance corrections
that were determined using 3D hydrodynamical CO<SUP>5</SUP>BOLD and
1D hydrostatic LHD model atmospheres. <BR /> Results: We find that
the influence of convection on the formation of [O i] 630 nm line in
the atmospheres of EMP giants studied here is minor, which leads to
very small 3D-1D abundance corrections (Δ<SUB>3D-1D</SUB> ≤ -0.01
dex). On the contrary, IR OH lines are strongly affected by convection
and thus the abundance corrections for these lines are significant,
Δ<SUB>3D-1D</SUB> ≈ -0.2···-0.3 dex. These abundance corrections
do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained
using IR OH lines into better agreement with those determined from the
[O i] 630 nm line. Since in the EMP red giants IR OH lines are typically
at least a factor of two stronger than the [O i] line, OH lines may be
useful indicators of oxygen abundances in the EMP stars, provided that
the analysis is based on 3D hydrodynamical model atmospheres. <P />Based
on observations obtained at the European Southern Observatory (ESO)
Very Large Telescope (VLT) at Paranal Observatory, Chile (observing
programme 089.D-0079).Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424885/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Calibration of the Mixing-length Theory for Convective White
Dwarf Envelopes
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
G.; Steffen, M.; Brassard, P.
2015ApJ...799..142T Altcode: 2014arXiv1412.1789T
A calibration of the mixing-length parameter in the local mixing-length
theory (MLT) is presented for the lower part of the convection zone in
pure-hydrogen-atmosphere white dwarfs. The parameterization is performed
from a comparison of three-dimensional (3D) CO5BOLD simulations with
a grid of one-dimensional (1D) envelopes with a varying mixing-length
parameter. In many instances, the 3D simulations are restricted to the
upper part of the convection zone. The hydrodynamical calculations
suggest, in those cases, that the entropy of the upflows does not
change significantly from the bottom of the convection zone to regions
immediately below the photosphere. We rely on this asymptotic entropy
value, characteristic of the deep and adiabatically stratified layers,
to calibrate 1D envelopes. The calibration encompasses the convective
hydrogen-line (DA) white dwarfs in the effective temperature range
6000 <= T <SUB>eff</SUB> (K) <=15, 000 and the surface gravity
range 7.0 <= log g <= 9.0. It is established that the local
MLT is unable to reproduce simultaneously the thermodynamical, flux,
and dynamical properties of the 3D simulations. We therefore propose
three different parameterizations for these quantities. The resulting
calibration can be applied to structure and envelope calculations,
in particular for pulsation, chemical diffusion, and convective mixing
studies. On the other hand, convection has no effect on the white dwarf
cooling rates until there is a convective coupling with the degenerate
core below T <SUB>eff</SUB> ~ 5000 K. In this regime, the 1D structures
are insensitive to the MLT parameterization and converge to the mean
3D results, hence they remain fully appropriate for age determinations.
---------------------------------------------------------
Title: Oxygen in the Early Galaxy: OH Lines as Tracers of Oxygen
Abundance in Extremely Metal-Poor Giant Stars
Authors: Kucinskas, A.; Dobrovolskas, V.; Bonifacio, P.; Caffau, E.;
Ludwig, H. -G.; Steffen, M.; Spite, M.
2015csss...18..327K Altcode: 2014arXiv1409.3153K
Oxygen is a powerful tracer element of Galactic chemical
evolution. Unfortunately, only a few oxygen lines are available in the
ultraviolet-infrared stellar spectra for the reliable determination of
its abundance. Moreover, oxygen abundances obtained using different
spectral lines often disagree significantly. In this contribution we
therefore investigate whether the inadequate treatment of convection in
1D hydrostatic model atmospheres used in the abundance determinations
may be responsible for this disagreement. For this purpose, we used VLT
CRIRES spectra of three EMP giants, as well as 3D hydrodynamical COBOLD
and 1D hydrostatic LHD model atmospheres, to investigate the role of
convection in the formation of infrared (IR) OH lines. Our results show
that the presence of convection leads to significantly stronger IR OH
lines. As a result, the difference in the oxygen abundance determined
from IR OH lines with 3D hydrodynamical and classical 1D hydrostatic
model atmospheres may reach -0.2 dots -0.3 dex. In case of the three
EMP giants studied here, we obtain a good agrement between the 3D LTE
oxygen abundances determined by us using vibrational-rotational IR
OH lines in the spectral range of 1514-1626 nm, and oxygen abundances
determined from forbidden [O I] 630 nm line in previous studies.
---------------------------------------------------------
Title: Stellar Spectroscopy During Exoplanet Transits: Dissecting
Fine Structure Across Stellar Surfaces
Authors: Dravins, Dainis; Ludwig, Hans-Gunter; Dahlen, Erik; Pazira,
Hiva
2015csss...18..853D Altcode: 2014arXiv1408.1402D
Differential spectroscopy during exoplanet transits permits to
reconstruct spectra of small stellar surface portions that successively
become hidden behind the planet. The center-to-limb behavior of stellar
line shapes, asymmetries and wavelength shifts will enable detailed
tests of 3-dimensional hydrodynamic models of stellar atmospheres,
such that are required for any precise determination of abundances or
seismic properties. Such models can now be computed for widely different
stars but have been feasible to test in detail only for the Sun with
its resolved surface structure. Although very high quality spectra are
required, already current data permit reconstructions of line profiles
in the brightest transit host stars such as HD 209458 (G0 V).
---------------------------------------------------------
Title: Nucleus Driven Electronic Pulsation
Authors: Ludwig, H.; Ruffini, R.; Xue, S. -S.
2014arXiv1412.5455L Altcode:
We derive and solve by the spectral method the equations for a neutral
system of ultra-relativistic electrons that are compressed to the
radius of the nucleus and subject to a driving force. This driving
force can be thought of as originating from a nuclear breathing mode,
a possibility we discuss in detail.
---------------------------------------------------------
Title: The chemical composition of red giants in 47
Tucanae. I. Fundamental parameters and chemical abundance patterns
Authors: Thygesen, A. O.; Sbordone, L.; Andrievsky, S.; Korotin,
S.; Yong, D.; Zaggia, S.; Ludwig, H. -G.; Collet, R.; Asplund, M.;
Ventura, P.; D'Antona, F.; Meléndez, J.; D'Ercole, A.
2014A&A...572A.108T Altcode: 2014arXiv1409.4694T
Context. The study of chemical abundance patterns in globular
clusters is key importance to constraining the different candidates
for intracluster pollution of light elements. <BR /> Aims: We aim
at deriving accurate abundances for a wide range of elements in the
globular cluster 47 Tucanae (NGC 104) to add new constraints to the
pollution scenarios for this particular cluster, expanding the range of
previously derived element abundances. <BR /> Methods: Using tailored
1D local thermodynamic equilibrium (LTE) atmospheric models, together
with a combination of equivalent width measurements, LTE, and NLTE
synthesis, we derive stellar parameters and element abundances from
high-resolution, high signal-to-noise spectra of 13 red giant stars
near the tip of the RGB. <BR /> Results: We derive abundances of a
total 27 elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co,
Ni, Cu, Zn, Y, Zr, Mo, Ru, Ba, La, Ce, Pr, Nd, Eu, Dy). Departures
from LTE were taken into account for Na, Al, and Ba. We find a mean
[Fe/H] = -0.78 ± 0.07 and [ α/ Fe ] = 0.34 ± 0.03 in good agreement
with previous studies. The remaining elements show good agreement with
the literature, but including NLTE for Al has a significant impact on
the behavior of this key element. <BR /> Conclusions: We confirm the
presence of an Na-O anti-correlation in 47 Tucanae found by several
other works. Our NLTE analysis of Al shifts the [Al/Fe] to lower values,
indicating that this may be overestimated in earlier works. No evidence
of an intrinsic variation is found in any of the remaining elements. <P
/>Based on observations made with the ESO Very Large Telescope at
Paranal Observatory, Chile (Programmes 084.B-0810 and 086.B-0237).Full
Tables 2, 5, and 9 are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A108">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A108</A>Appendix
A is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424533/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: The Gaia-ESO Survey: The analysis of high-resolution UVES
spectra of FGK-type stars
Authors: Smiljanic, R.; Korn, A. J.; Bergemann, M.; Frasca, A.;
Magrini, L.; Masseron, T.; Pancino, E.; Ruchti, G.; San Roman,
I.; Sbordone, L.; Sousa, S. G.; Tabernero, H.; Tautvaišienė,
G.; Valentini, M.; Weber, M.; Worley, C. C.; Adibekyan, V. Zh.;
Allende Prieto, C.; Barisevičius, G.; Biazzo, K.; Blanco-Cuaresma,
S.; Bonifacio, P.; Bragaglia, A.; Caffau, E.; Cantat-Gaudin, T.;
Chorniy, Y.; de Laverny, P.; Delgado-Mena, E.; Donati, P.; Duffau,
S.; Franciosini, E.; Friel, E.; Geisler, D.; González Hernández,
J. I.; Gruyters, P.; Guiglion, G.; Hansen, C. J.; Heiter, U.; Hill, V.;
Jacobson, H. R.; Jofre, P.; Jönsson, H.; Lanzafame, A. C.; Lardo, C.;
Ludwig, H. -G.; Maiorca, E.; Mikolaitis, Š.; Montes, D.; Morel, T.;
Mucciarelli, A.; Muñoz, C.; Nordlander, T.; Pasquini, L.; Puzeras,
E.; Recio-Blanco, A.; Ryde, N.; Sacco, G.; Santos, N. C.; Serenelli,
A. M.; Sordo, R.; Soubiran, C.; Spina, L.; Steffen, M.; Vallenari,
A.; Van Eck, S.; Villanova, S.; Gilmore, G.; Randich, S.; Asplund,
M.; Binney, J.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.;
Micela, G.; Negueruela, I.; Prusti, T.; Rix, H. -W.; Alfaro, E.;
Babusiaux, C.; Bensby, T.; Blomme, R.; Flaccomio, E.; François, P.;
Irwin, M.; Koposov, S.; Walton, N.; Bayo, A.; Carraro, G.; Costado,
M. T.; Damiani, F.; Edvardsson, B.; Hourihane, A.; Jackson, R.; Lewis,
J.; Lind, K.; Marconi, G.; Martayan, C.; Monaco, L.; Morbidelli, L.;
Prisinzano, L.; Zaggia, S.
2014A&A...570A.122S Altcode: 2014arXiv1409.0568S
Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using
FLAMES at the VLT to obtain high-quality medium-resolution Giraffe
spectra for about 10<SUP>5</SUP> stars and high-resolution UVES spectra
for about 5000 stars. With UVES, the Survey has already observed
1447 FGK-type stars. <BR /> Aims: These UVES spectra are analyzed
in parallel by several state-of-the-art methodologies. Our aim is
to present how these analyses were implemented, to discuss their
results, and to describe how a final recommended parameter scale is
defined. We also discuss the precision (method-to-method dispersion)
and accuracy (biases with respect to the reference values) of the
final parameters. These results are part of the Gaia-ESO second
internal release and will be part of its first public release of
advanced data products. <BR /> Methods: The final parameter scale is
tied to the scale defined by the Gaia benchmark stars, a set of stars
with fundamental atmospheric parameters. In addition, a set of open
and globular clusters is used to evaluate the physical soundness
of the results. Each of the implemented methodologies is judged
against the benchmark stars to define weights in three different
regions of the parameter space. The final recommended results are
the weighted medians of those from the individual methods. <BR />
Results: The recommended results successfully reproduce the atmospheric
parameters of the benchmark stars and the expected T<SUB>eff</SUB>-log
g relation of the calibrating clusters. Atmospheric parameters and
abundances have been determined for 1301 FGK-type stars observed with
UVES. The median of the method-to-method dispersion of the atmospheric
parameters is 55 K for T<SUB>eff</SUB>, 0.13 dex for log g and 0.07
dex for [Fe/H]. Systematic biases are estimated to be between 50-100
K for T<SUB>eff</SUB>, 0.10-0.25 dex for log g and 0.05-0.10 dex for
[Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al,
Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and
Eu. The typical method-to-method dispersion of the abundances varies
between 0.10 and 0.20 dex. <BR /> Conclusions: The Gaia-ESO sample of
high-resolution spectra of FGK-type stars will be among the largest of
its kind analyzed in a homogeneous way. The extensive list of elemental
abundances derived in these stars will enable significant advances in
the areas of stellar evolution and Milky Way formation and evolution. <P
/>Based on observations made with the ESO/VLT, at Paranal Observatory,
under program 188.B-3002 (The Gaia-ESO Public Spectroscopic Survey, PIs
Gilmore and Randich). Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201423937/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: VizieR Online Data Catalog: 47 Tuc red giants chemical
composition (Thygesen+, 2014)
Authors: Thygesen, A. O.; Sbordone, L.; Andrievsky, S.; Korotin,
S.; Yong, D.; Zaggia, S.; Ludwig, H. -G.; Collet, R.; Asplund, M.;
Ventura, P.; D'Antona, F.; Melendez, J.; D'Ercole, A.
2014yCat..35720108T Altcode: 2014yCat..35729108T
Table 2 contains linelist of the non-HFS elements. These lines were used
for equivalent width measurements. Sorted by element number. Table 5
contains all the lines used for HFS synthesis. With the exception of
Ba, all lines are split into individual HFS components, including
isotopes. log(gf) values reflect this. Solar isotope mixture is
assumed. Sorted by element number. Table 8 contains the fundamental
stellar parameters. Table 9 contains [Fe/H] as well as abundance ratios
[X/Fe] for all stars. All measurements have an associated uncertainty
sig[X/Fe] and the number of lines used in each individual star. In
cases where no measurement exist, the value "-9.99" is given. <P />(5
data files).
---------------------------------------------------------
Title: Clues on the Galactic evolution of sulphur from star clusters
Authors: Caffau, E.; Monaco, L.; Spite, M.; Bonifacio, P.; Carraro,
G.; Ludwig, H. -G.; Villanova, S.; Beletsky, Y.; Sbordone, L.
2014A&A...568A..29C Altcode: 2014arXiv1407.0485C
Context. The abundances of α-elements are a powerful diagnostic of the
star formation history and chemical evolution of a galaxy. Sulphur,
being moderately volatile, can be reliably measured in the
interstellar medium (ISM) of damped Ly-α galaxies and extragalactic
H ii regions. Measurements in stars of different metallicity in our
Galaxy can then be readily compared to the abundances in external
galaxies. Such a comparison is not possible for Si or Ca that suffer
depletion onto dust in the ISM. Furthermore, studying sulphur is
interesting because it probes nucleosynthetic conditions that are
very different from those of O or Mg. In this context measurements
in star clusters are a reliable tracers of the Galactic evolution of
sulphur. <BR /> Aims: The aim of this paper is to determine sulphur
abundances in several Galactic clusters that span a metallicity range
-1.5 < [Fe/H] < 0.0. <BR /> Methods: We use a standard abundance
analysis, based on 1D model atmospheres in local thermodynamical
equilibrium (LTE) and literature corrections for non-LTE (NLTE), as
well as 3D corrections based on hydrodynamical model atmospheres, to
derive sulphur abundances in a sample of stars in the globular cluster
M 4, and the open clusters Trumpler 5, NGC 2477, and NGC 5822. <BR />
Results: We find ⟨ A(S) ⟩ <SUB>NLTE</SUB> = 6.11 ± 0.04 for M 4,
⟨ A(S) ⟩ <SUB>NLTE</SUB> = 7.17 ± 0.02 for NGC 2477, and ⟨ A(S)
⟩ <SUB>NLTE</SUB> = 7.13 ± 0.06 for NGC 5822. For the only star
studied in Trumpler 5 we find A(S)<SUB>NLTE</SUB> = 6.43 ± 0.03 and
A(S)<SUB>LTE</SUB> = 6.94 ± 0.05. <BR /> Conclusions: Our measurements
show that, by and large, the S abundances in Galactic clusters trace
reliably those in field stars. The only possible exception is Trumpler
5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by
roughly 0.4 dex than observed in field stars of comparable metallicity,
even though its LTE sulphur abundance is in line with abundances of
field stars. Moreover the LTE sulphur abundance is consistent only
with the abundance of another α-element, Mg, in the same star,
while the low NLTE value is consistent with Si and Ca. We believe
that further investigation of departures from LTE is necessary,
as well as observation of other S i lines in this star and in other
stars of the same cluster, before one can conclude that the sulphur
abundance in Trumpler 5 is indeed 0.4 dex lower than in field stars
of comparable metallicity. The S abundances in our sample of stars
in clusters imply that the clusters are chemically homogeneous for S
within 0.05 dex. <P />Based on observations made with ESO Telescopes
at the La Silla Paranal Observatory under programme ID 085.D-0537(A),
088.D-0045(A), 089.D-0062(B).
---------------------------------------------------------
Title: 4MOST: 4-metre Multi-Object Spectroscopic Telescope
Authors: de Jong, Roelof S.; Barden, Sam; Bellido-Tirado, Olga;
Brynnel, Joar; Chiappini, Cristina; Depagne, Éric; Haynes, Roger;
Johl, Diana; Phillips, Daniel P.; Schnurr, Olivier; Schwope, Axel D.;
Walcher, Jakob; Bauer, Svend M.; Cescutti, Gabriele; Cioni, Maria-Rosa
L.; Dionies, Frank; Enke, Harry; Haynes, Dionne M.; Kelz, Andreas;
Kitaura, Francisco S.; Lamer, Georg; Minchev, Ivan; Müller, Volker;
Nuza, Sebastián. E.; Olaya, Jean-Christophe; Piffl, Tilmann; Popow,
Emil; Saviauk, Allar; Steinmetz, Matthias; Ural, Uǧur; Valentini,
Monica; Winkler, Roland; Wisotzki, Lutz; Ansorge, Wolfgang R.; Banerji,
Manda; Gonzalez Solares, Eduardo; Irwin, Mike; Kennicutt, Robert C.;
King, David M. P.; McMahon, Richard; Koposov, Sergey; Parry, Ian R.;
Sun, Xiaowei; Walton, Nicholas A.; Finger, Gert; Iwert, Olaf; Krumpe,
Mirko; Lizon, Jean-Louis; Mainieri, Vincenzo; Amans, Jean-Philippe;
Bonifacio, Piercarlo; Cohen, Matthieu; François, Patrick; Jagourel,
Pascal; Mignot, Shan B.; Royer, Frédéric; Sartoretti, Paola; Bender,
Ralf; Hess, Hans-Joachim; Lang-Bardl, Florian; Muschielok, Bernard;
Schlichter, Jörg; Böhringer, Hans; Boller, Thomas; Bongiorno,
Angela; Brusa, Marcella; Dwelly, Tom; Merloni, Andrea; Nandra, Kirpal;
Salvato, Mara; Pragt, Johannes H.; Navarro, Ramón; Gerlofsma, Gerrit;
Roelfsema, Ronald; Dalton, Gavin B.; Middleton, Kevin F.; Tosh,
Ian A.; Boeche, Corrado; Caffau, Elisabetta; Christlieb, Norbert;
Grebel, Eva K.; Hansen, Camilla J.; Koch, Andreas; Ludwig, Hans-G.;
Mandel, Holger; Quirrenbach, Andreas; Sbordone, Luca; Seifert, Walter;
Thimm, Guido; Helmi, Amina; trager, Scott C.; Bensby, Thomas; Feltzing,
Sofia; Ruchti, Gregory; Edvardsson, Bengt; Korn, Andreas; Lind, Karin;
Boland, Wilfried; Colless, Matthew; Frost, Gabriella; Gilbert, James;
Gillingham, Peter; Lawrence, Jon; Legg, Neville; Saunders, Will;
Sheinis, Andrew; Driver, Simon; Robotham, Aaron; Bacon, Roland;
Caillier, Patrick; Kosmalski, Johan; Laurent, Florence; Richard, Johan
2014SPIE.9147E..0MD Altcode:
4MOST is a wide-field, high-multiplex spectroscopic survey facility
under development for the VISTA telescope of the European Southern
Observatory (ESO). Its main science drivers are in the fields
of galactic archeology, high-energy physics, galaxy evolution
and cosmology. 4MOST will in particular provide the spectroscopic
complements to the large area surveys coming from space missions like
Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like
VISTA, VST, DES, LSST and SKA. The 4MOST baseline concept features a 2.5
degree diameter field-of-view with ~2400 fibres in the focal surface
that are configured by a fibre positioner based on the tilting spine
principle. The fibres feed two types of spectrographs; ~1600 fibres go
to two spectrographs with resolution R<5000 (λ~390-930 nm) and ~800
fibres to a spectrograph with R>18,000 (λ~392-437 nm and 515-572 nm
and 605-675 nm). Both types of spectrographs are fixed-configuration,
three-channel spectrographs. 4MOST will have an unique operations
concept in which 5 year public surveys from both the consortium and
the ESO community will be combined and observed in parallel during each
exposure, resulting in more than 25 million spectra of targets spread
over a large fraction of the southern sky. The 4MOST Facility Simulator
(4FS) was developed to demonstrate the feasibility of this observing
concept. 4MOST has been accepted for implementation by ESO with
operations expected to start by the end of 2020. This paper provides
a top-level overview of the 4MOST facility, while other papers in
these proceedings provide more detailed descriptions of the instrument
concept[1], the instrument requirements development[2], the systems
engineering implementation[3], the instrument model[4], the fibre
positioner concepts[5], the fibre feed[6], and the spectrographs[7].
---------------------------------------------------------
Title: VizieR Online Data Catalog: Abundances of 47 Tuc turn-off stars
(Dobrovolskas+, 2014)
Authors: Dobrovolskas, V.; Kucinskas, A.; Bonifacio, P.; Korotin,
S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer,
F.; Prakapavicius, D.
2014yCat..35650121D Altcode: 2014yCat..35659121D
Spectra of the TO stars in 47 Tuc investigated in this work were
obtained with the GIRAFFE spectrograph in August-September, 2008,
under the programme 081.D-0287(A) (PI: Shen). The same data set
was independently analysed by D'Orazi et al. (2010ApJ...713L...1D,
Cat. J/ApJ/713/L1). <P />(1 data file).
---------------------------------------------------------
Title: White Dwarfs in the UKIRT Infrared Deep Sky Survey Data
Release 9
Authors: Tremblay, P. -E.; Leggett, S. K.; Lodieu, N.; Freytag, B.;
Bergeron, P.; Kalirai, J. S.; Ludwig, H. -G.
2014ApJ...788..103T Altcode:
We have identified 8 to 10 new cool white dwarfs from the Large Area
Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope
(UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired
with the Sloan Digital Sky Survey to obtain proper motions and a broad
ugrizYJHK wavelength coverage. Optical spectroscopic observations were
secured at Gemini Observatory and confirm the degenerate status for
eight of our targets. The final sample includes two additional white
dwarf candidates with no spectroscopic observations. We rely on improved
one-dimensional model atmospheres and new multi-dimensional simulations
with CO5BOLD to review the stellar parameters of the published LAS
white dwarf sample along with our additional discoveries. Most of the
new objects possess very cool atmospheres with effective temperatures
below 5000 K, including two pure-hydrogen remnants with a cooling age
between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km
s<SUP>-1</SUP> <=v <SUB>tan</SUB> <= 60 km s<SUP>-1</SUP>. They
are likely thick disk 10-11 Gyr old objects. In addition, we find
a resolved double degenerate system with v <SUB>tan</SUB> ~ 155 km
s<SUP>-1</SUP> and a cooling age between 3.0 and 5.0 Gyr. These white
dwarfs could be disk remnants with a very high velocity or former halo
G stars. We also compare the LAS sample with earlier studies of very
cool degenerates and observe a similar deficit of helium-dominated
atmospheres in the range 5000 < T <SUB>eff</SUB> (K) < 6000. We
review the possible explanations for the spectral evolution from
helium-dominated toward hydrogen-rich atmospheres at low temperatures.
---------------------------------------------------------
Title: White Dwarfs In The UKIRT Infrared Deep Sky Survey Data
Release 9
Authors: Tremblay, P. -E.; Leggett, S. K.; Lodieu, N.; Freytag, B.;
Bergeron, P.; Kalirai, J. S.; Ludwig, H. -G.
2014arXiv1405.0266T Altcode:
We have identified eight to ten new cool white dwarfs from the Large
Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed
Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data
set was paired with the Sloan Digital Sky Survey (SDSS) to obtain
proper motions and a broad ugrizYJHK wavelength coverage. Optical
spectroscopic observations were secured at Gemini Observatory
and confirm the degenerate status for eight of our targets. The
final sample includes two additional white dwarf candidates with no
spectroscopic observations. We rely on improved 1D model atmospheres
and new multi-dimensional simulations with CO5BOLD to review the
stellar parameters of the published LAS white dwarf sample along with
our additional discoveries. Most of the new objects possess very cool
atmospheres with effective temperatures below 5000 K, including two
pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr,
and tangential velocities in the range 40 km/s < vtan < 60
km/s. They are likely thick disk 10-11 Gyr-old objects. In addition
we find a resolved double degenerate system with vtan ~ 155 km/s and a
cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk
remnants with a very high velocity or former halo G stars. We also
compare the LAS sample with earlier studies of very cool degenerates
and observe a similar deficit of helium-dominated atmospheres in the
range 5000 < Teff (K) < 6000. We review the possible explanations
for the spectral evolution from helium-dominated towards hydrogen-rich
atmospheres at low temperatures.
---------------------------------------------------------
Title: Abundances of lithium, oxygen, and sodium in the turn-off
stars of Galactic globular cluster 47 Tucanae
Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Korotin,
S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer,
F.; Prakapavičius, D.
2014A&A...565A.121D Altcode: 2013arXiv1311.1072D
Context. The cluster 47 Tuc is among the most metal-rich Galactic
globular clusters and its metallicity is similar to that of metal-poor
disc stars and open clusters. Like other globular clusters, it
displays variations in the abundances of elements lighter than Si,
which is generally interpreted as evidence of the presence of multiple
stellar populations. <BR /> Aims: We aim to determine abundances of
Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order
to study the evolution of light elements in this cluster and to put
our results in perspective with observations of other globular and
open clusters, as well as with field stars. <BR /> Methods: We use
medium resolution spectra obtained with the GIRAFFE spectrograph at
the ESO 8.2 m Kueyen VLT telescope and use state of the art 1D model
atmospheres and NLTE line transfer to determine the abundances. We
also employ CO<SUP>5</SUP>BOLD hydrodynamical simulations to assess
the impact of stellar granulation on the line formation and inferred
abundances. <BR /> Results: Our results confirm the existence of
Na-O abundance anti-correlation and hint towards a possible Li-O
anti-correlation in the TO stars of 47 Tuc. At the same time,
we find no convincing evidence supporting the existence of Li-Na
correlation. The obtained 3D NLTE mean lithium abundance in a sample
of 94 TO stars where Li lines were detected reliably, ⟨A(Li)<SUB>3D
NLTE</SUB>⟩ = 1.78 ± 0.18 dex, appears to be significantly lower
than what is observed in other globular clusters. At the same time,
star-to-star spread in Li abundance is also larger than seen in other
clusters. The highest Li abundance observed in 47 Tuc is about 0.1
dex lower than the lowest Li abundance observed among the un-depleted
stars of the metal-poor open cluster NGC 2243. <BR /> Conclusions: The
correlations/anti-correlations among light element abundances confirm
that chemical enrichment history of 47 Tuc was similar to that of other
globular clusters, despite the higher metallicity of 47 Tuc. The lithium
abundances in 47 Tuc, when put into context with observations in other
clusters and field stars, suggest that stars that are more metal-rich
than [Fe/H] ~ -1.0 experience significant lithium depletion during
their lifetime on the main sequence, while the more metal-poor stars
do not. Rather strikingly, our results suggest that initial lithium
abundance with which the star was created may only depend on its age
(the younger the star, the higher its Li content) and not on its
metallicity. <P />Appendices are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322868/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: 3D modeling of stellar atmospheres and the impact on the
understanding of the reliability of elemental abundances in stars
as tracers of galactic chemical evolution
Authors: Ludwig, H. -G.; Steffen, M.; Bonifacio, P.; Caffau, E.;
Kučinskas, A.; Freytag, B.
2014IAUS..298..343L Altcode:
We present a critical review of the construction of 3D model atmospheres
with emphasis on modeling challenges. We discuss the basic physical
processes which give rise to the effects which set 3D models apart
from 1D standard models. We consider elemental abundances derived
from molecular features, and the determination of the microturbulence
with 3D models. The examples serve as illustration of the limitations
inherent to 1D, however, also to 3D modeling. We find that 3D models
can provide constraints on the microturbulence parameter, and predict
substantial corrections for abundances derived from molecular species.
---------------------------------------------------------
Title: 3D Model Atmospheres of White Dwarfs
Authors: Tremblay, Pier-Emmanuel; Ludwig, H.; Steffen, M.; Freytag, B.
2014AAS...22331507T Altcode:
We present the first grid of 3D model atmospheres for
hydrogen-atmosphere (DA) white dwarfs. These CO5BOLD
radiation-hydrodynamics simulations, unlike the previous 1D
calculations, do not rely on the mixing-length theory for the
treatment of convection. The simulations have been employed to compute
model spectra and we compared our improved Balmer line profiles to
spectroscopic data from the Sloan Digital Sky Survey and the White
Dwarf Catalog. The 3D surface gravities are found to be as much as
0.3 dex lower than the values derived from 1D models. The white dwarfs
with a radiative and a convective atmosphere have derived mean masses
that are the same within 0.01 Msun with our new models, in much better
agreement with our understanding of stellar evolution.
---------------------------------------------------------
Title: The first generations of stars
Authors: Caffau, E.; Gallagher, A.; Bonifacio, P.; Cayrel, R.;
Christlieb, N.; Clark, P. C.; Francois, P.; Glover, S.; Klessen,
R. S.; Koch, A.; Ludwig, H. G.; Monaco, L.; Plez, B.; Sbordone, L.;
Spite, M.; Spite, F.; Steffen, M.; Zaggia, S.
2014nic..confE..53C Altcode: 2014PoS...204E..53C
No abstract at ADS
---------------------------------------------------------
Title: TOPoS: chemical study of extremely metal-poor stars.
Authors: Caffau, E.; Sbordone, L.; Bonifacio, P.; Cayrel, R.;
Christlieb, N.; Clark, P.; François, P.; Glover, S.; Klessen, R.;
Koch, A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Spite, F.; Spite,
M.; Steffen, M.; Zaggia, S.
2014MmSAI..85..222C Altcode:
The extremely metal-poor (EMP) stars hold in their atmospheres
the fossil record of the chemical composition of the early phases
of the Galactic evolution. The chemical analysis of such objects
provides important constraints on these early phases. EMP stars
are very rare objects; to dig them out, large amounts of data have
to be processed. With an automatic procedure, we analysed objects
with colours of Turn-Off stars from the Sloan Digital Sky Survey to
select a sample of good candidate EMP stars. In the latest years,
we observed a sample of these candidates with X-Shooter and UVES,
and we have an ongoing ESO large programme to use these spectrographs
to observe EMP stars. I will report here the results on metallicity
and Strontium abundance. <P />Based on observations obtained at ESO
Paranal Observatory, programme 189.D-0165(A)
---------------------------------------------------------
Title: 6Li/7Li isotopic ratio in the most metal-poor binary
CS22876-032
Authors: Gonzalez-Hernandez, J.; Caffau, E.; Ludwig, H. G.; Bonifacio,
P.; Steffen, M.; Monaco, L.; Cayrel, R.
2014nic..confE..23G Altcode: 2014PoS...204E..23G
No abstract at ADS
---------------------------------------------------------
Title: Isotope spectroscopy
Authors: Caffau, E.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.;
Monaco, L.; Lo Curto, G.; Kamp, I.
2014AN....335...59C Altcode: 2013arXiv1310.6058C
The measurement of isotopic ratios provides a privileged insight
both into nucleosynthesis and into the mechanisms operating in
stellar envelopes, such as gravitational settling. In this article,
we give a few examples of how isotopic ratios can be determined from
high-resolution, high-quality stellar spectra. We consider examples of
the lightest elements, H and He, for which the isotopic shifts are very
large and easily measurable, and examples of heavier elements for which
the determination of isotopic ratios is more difficult. The presence
of <SUP>6</SUP>Li in the stellar atmospheres causes a subtle extra
depression in the red wing of the <SUP>7</SUP>Li 670.7 nm doublet which
can only be detected in spectra of the highest quality. But even with
the best spectra, the derived <SUP>6</SUP>Li abundance can only be as
good as the synthetic spectra used for their interpretation. It is now
known that 3D non-LTE modelling of the lithium spectral line profiles
is necessary to account properly for the intrinsic line asymmetry,
which is produced by convective flows in the atmospheres of cool stars,
and can mimic the presence of <SUP>6</SUP>Li. We also discuss briefly
the case of the carbon isotopic ratio in metal-poor stars, and provide a
new determination of the nickel isotopic ratios in the solar atmosphere.
---------------------------------------------------------
Title: X-shooter GTO: evidence for a population of extremely
metal-poor, alpha-poor stars
Authors: Caffau, E.; Bonifacio, P.; François, P.; Sbordone, L.;
Spite, M.; Monaco, L.; Plez, B.; Spite, F.; Zaggia, S.; Ludwig,
H. -G.; Cayrel, R.; Molaro, P.; Randich, S.; Hammer, F.; Hill, V.
2013A&A...560A..15C Altcode: 2013arXiv1309.4913C
Context. The extremely metal-poor stars are the direct descendants
of the first generation stars. They carry the chemical signature
of the pristine Universe at the time they formed, shortly after the
Big Bang. <BR /> Aims: We aim to derive information about extremely
metal-poor stars from their observed spectra. <BR /> Methods: Four
extremely metal-poor stars were selected from the Sloan Digital Sky
Survey (SDSS) and observed during the guaranteed observing time of
X-shooter. The X-shooter spectra were analysed using an automatic code,
MyGIsFOS, which is based on a traditional analysis method. It makes
use of a synthetic grid computed from one-dimensional, plane-parallel,
hydrostatic model atmospheres. <BR /> Results: The low metallicity
derived from the SDSS spectra is confirmed here. Two kinds of stars
are found. Two stars are confirmed to be extremely metal-poor, with no
evidence of any enhancement in carbon. The two other stars are strongly
enhanced in carbon. We could not derive iron abundance for one of them,
while [Ca/H] is below -4.5. Two of the stars are members of the rare
population of extremely metal-poor stars low in alpha elements. <P
/>Based on observations obtained at ESO Paranal Observatory, GTO
programme 089.D-0039.
---------------------------------------------------------
Title: On the Seismic Scaling Relations Δν — ρ and
ν<SUB>max</SUB> — ν<SUB>c</SUB>
Authors: Belkacem, K.; Samadi, R.; Mosser, B.; Goupil, M. -J.; Ludwig,
H. -G.
2013ASPC..479...61B Altcode: 2013arXiv1307.3132B
Scaling relations between asteroseismic quantities and stellar
parameters are essential tools for studying stellar structure
and evolution. We will address two of them, namely, the relation
between the large frequency separation (Δν) and the mean density
(ρ) as well as the relation between the frequency of the maximum
in the power spectrum of solar-like oscillations (ν<SUB>max</SUB>)
and the cut-off frequency (ν<SUB>c</SUB>). For the first relation,
we will consider the possible sources of uncertainties and explore them
with the help of a grid of stellar models. For the second one, we will
show that the basic physical picture is understood and that departure
from the observed relation arises from the complexity of non-adiabatic
processes involving the time-dependent treatment of convection. This
will be further discussed on the basis of a set of three-dimensional
(3D) hydrodynamical simulations of surface convection.
---------------------------------------------------------
Title: TOPoS. I. Survey design and analysis of the first sample
Authors: Caffau, E.; Bonifacio, P.; Sbordone, L.; François, P.;
Monaco, L.; Spite, M.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark,
P.; Glover, S.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Spite, F.;
Steffen, M.; Zaggia, S.
2013A&A...560A..71C Altcode: 2013arXiv1310.6963C
Context. The metal-weak tail of the metallicity distribution function
(MDF) of the Galactic Halo stars contains crucial information on the
formation mode of the first generation of stars. To determine this
observationally, it is necessary to observe large numbers of extremely
metal-poor stars. <BR /> Aims: We present here the Turn-Off Primordial
Stars survey (TOPoS) that is conducted as an ESO Large Programme at the
VLT. This project has four main goals: (i) to understand the formation
of low-mass stars in a low-metallicity gas: determine the metal-weak
tail of the halo MDF below [M/H] = -3.5; in particular, we aim at
determining the critical metallicity, that is the lowest metallicity
sufficient for the formation of low-mass stars; (ii) to determine in
extremely metal-poor stars the relative abundances of the elements that
are the signature of the massive first stars; (iii) to determine the
trend of the lithium abundance at the time when the Galaxy formed; and
(iv) to derive the fraction of C-enhanced extremely metal-poor stars
with respect to normal extremely metal-poor stars. The large number of
stars observed in the SDSS provides a good sample of candidate stars
at extremely low metallicity. <BR /> Methods: Candidates with turn-off
colours down to magnitude g = 20 were selected from the low-resolution
spectra of SDSS by means of an automated procedure. X-Shooter has the
potential of performing the necessary follow-up spectroscopy, providing
accurate metallicities and abundance ratios for several key elements
for these stars. <BR /> Results: We present here the stellar parameters
of the first set of stars. The nineteen stars range in iron abundance
between -4.1 and -2.9 dex relative to the Sun. Two stars have a high
radial velocity and, according to our estimate of their kinematics,
appear to be marginally bound to the Galaxy and are possibly accreted
from another galaxy. <P />Based on observations obtained at ESO Paranal
Observatory, GTO programme 189.D-0165(A).
---------------------------------------------------------
Title: Stellar granulation as seen in disk-integrated
intensity. II. Theoretical scaling relations compared with
observations
Authors: Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau, E.;
Campante, T. L.; Davies, G. R.; Kallinger, T.; Lund, M. N.; Mosser,
B.; Baglin, A.; Mathur, S.; Garcia, R. A.
2013A&A...559A..40S Altcode: 2013arXiv1309.1488S
Context. A large set of stars observed by CoRoT and Kepler shows clear
evidence for the presence of a stellar background, which is interpreted
to arise from surface convection, i.e., granulation. These observations
show that the characteristic time-scale (τ<SUB>eff</SUB>) and the
root-mean-square (rms) brightness fluctuations (σ) associated with the
granulation scale as a function of the peak frequency (ν<SUB>max</SUB>)
of the solar-like oscillations. <BR /> Aims: We aim at providing a
theoretical background to the observed scaling relations based on a
model developed in Paper I. <BR /> Methods: We computed for each 3D
model the theoretical power density spectrum (PDS) associated with
the granulation as seen in disk-integrated intensity on the basis of
the theoretical model published in Paper I. For each PDS we derived
the associated characteristic time (τ<SUB>eff</SUB>) and the rms
brightness fluctuations (σ) and compared these theoretical values with
the theoretical scaling relations derived from the theoretical model and
the measurements made on a large set of Kepler targets. <BR /> Results:
We derive theoretical scaling relations for τ<SUB>eff</SUB> and σ,
which show the same dependence on ν<SUB>max</SUB> as the observed
scaling relations. In addition, we show that these quantities also
scale as a function of the turbulent Mach number (ℳ<SUB>a</SUB>)
estimated at the photosphere. The theoretical scaling relations
for τ<SUB>eff</SUB> and σ match the observations well on a
global scale. Quantitatively, the remaining discrepancies with the
observations are found to be much smaller than previous theoretical
calculations made for red giants. <BR /> Conclusions: Our modelling
provides additional theoretical support for the observed variations
of σ and τ<SUB>eff</SUB> with ν<SUB>max</SUB>. It also highlights
the important role of ℳ<SUB>a</SUB> in controlling the properties
of the stellar granulation. However, the observations made with
Kepler on a wide variety of stars cannot confirm the dependence
of our scaling relations on ℳ<SUB>a</SUB>. Measurements of the
granulation background and detections of solar-like oscillations in a
statistically sufficient number of cool dwarf stars will be required
for confirming the dependence of the theoretical scaling relations with
ℳ<SUB>a</SUB>. <P />Appendices are available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Stellar granulation as seen in disk-integrated
intensity. I. Simplified theoretical modeling
Authors: Samadi, R.; Belkacem, K.; Ludwig, H. -G.
2013A&A...559A..39S Altcode: 2013arXiv1309.1620S
Context. Solar granulation has been known for a long time to be a
surface manifestation of convection. The space-borne missions CoRoT
and Kepler enable us to observe the signature of this phenomena
in disk-integrated intensity on a large number of stars. <BR />
Aims: The space-based photometric measurements show that the global
brightness fluctuations and the lifetime associated with granulation
obeys characteristic scaling relations. We thus aimed at providing
simple theoretical modeling to reproduce these scaling relations,
and subsequently at inferring the physical properties of granulation
across the Hertzsprung-Russell diagram. <BR /> Methods: We developed
a simple 1D theoretical model. The input parameters were extracted
from 3D hydrodynamical models of the surface layers of stars,
and the free parameters involved in the model were calibrated with
solar observations. Two different prescriptions for representing the
Fourier transform of the time-correlation of the eddy velocity were
compared: a Lorentzian and an exponential form. Finally, we compared our
theoretical prediction with 3D radiative hydrodynamical (RHD) numerical
modeling of stellar granulation (hereafter ab initio approach). <BR />
Results: Provided that the free parameters are appropriately adjusted,
our theoretical model reproduces the observed solar granulation
spectrum quite satisfactorily; the best agreement is obtained for
an exponential form. Furthermore, our model results in granulation
spectra that agree well with the ab initio approach using two 3D RHD
models that are representative of the surface layers of an F-dwarf and
a red-giant star. <BR /> Conclusions: We have developed a theoretical
model that satisfactory reproduces the solar granulation spectrum and
gives results consistent with the ab initio approach. The model is used
in a companion paper as theoretical framework for interpretating the
observed scaling relations. <P />Appendices are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Spectroscopic analysis of DA white dwarfs with 3D model
atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013A&A...559A.104T Altcode: 2013arXiv1309.0886T
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use
CO<SUP>5</SUP>BOLD radiation-hydrodynamics 3D simulations instead of the
mixing-length theory for the treatment of convection. The simulations
cover the effective temperature range of 6000 < T<SUB>eff</SUB>
(K) < 15 000 and the surface gravity range of 7 < log g <
9 where the large majority of DAs with a convective atmosphere
are located. We rely on horizontally averaged 3D structures (over
constant Rosseland optical depth) to compute ⟨3D⟩ spectra. It
is demonstrated that our ⟨3D⟩ spectra can be smoothly connected
to their 1D counterparts at higher and lower T<SUB>eff</SUB> where
the 3D effects are small. Analytical functions are provided in order
to convert spectroscopically determined 1D effective temperatures
and surface gravities to 3D atmospheric parameters. We apply our
improved models to well studied spectroscopic data sets from the
Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm
that the so-called high-log g problem is not present when employing
⟨3D⟩ spectra and that the issue was caused by inaccuracies in
the 1D mixing-length approach. The white dwarfs with a radiative
and a convective atmosphere have derived mean masses that are
the same within ~0.01 M<SUB>⊙</SUB>, in much better agreement
with our understanding of stellar evolution. Furthermore, the 3D
atmospheric parameters are in better agreement with independent
T<SUB>eff</SUB> and log g values from photometric and parallax
measurements. <P />Appendices are only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. III. Line formation in the atmospheres
of giants located close to the base of the red giant branch
Authors: Dobrovolskas, V.; Kučinskas, A.; Steffen, M.; Ludwig,
H. -G.; Prakapavičius, D.; Klevas, J.; Caffau, E.; Bonifacio, P.
2013A&A...559A.102D Altcode: 2013arXiv1310.7791D
<BR /> Aims: We utilize state-of-the-art three-dimensional (3D)
hydrodynamical and classical 1D stellar model atmospheres to study
the influence of convection on the formation properties of various
atomic and molecular spectral lines in the atmospheres of four red
giant stars, located close to the base of the red giant branch, RGB
(T<SUB>eff</SUB> ≈ 5000 K, log g = 2.5), and characterized by four
different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0. <BR /> Methods:
The role of convection in the spectral line formation is assessed with
the aid of abundance corrections, i.e., the differences in abundances
predicted for a given equivalent width of a particular spectral line
with the 3D and 1D model atmospheres. The 3D hydrodynamical and
classical 1D model atmospheres used in this study were calculated
with the CO<SUP>5</SUP>BOLD and 1D LHD codes, respectively. Identical
atmospheric parameters, chemical composition, equation of state, and
opacities were used with both codes, therefore allowing a strictly
differential analysis of the line formation properties in the 3D and 1D
models. <BR /> Results: We find that for lines of certain neutral atoms,
such as Mg i, Ti i, Fe i, and Ni i, the abundance corrections strongly
depend both on the metallicity of a given model atmosphere and the line
excitation potential, χ. While abundance corrections for all lines of
both neutral and ionized elements tend to be small at solar metallicity
(≤±0.1 dex), for lines of neutral elements with low ionization
potential and low-to-intermediate χ they quickly increase with
decreasing metallicity, reaching in their extremes -0.6 to -0.8 dex. In
all such cases the large abundance corrections are due to horizontal
temperature fluctuations in the 3D hydrodynamical models. Lines of
neutral elements with higher ionization potentials (E<SUB>ion</SUB>
≳ 10 eV) generally behave very similarly to lines of ionized elements
characterized by low ionization potentials (E<SUB>ion</SUB> ≲ 6
eV). In the latter case, the abundance corrections are small (generally,
≤±0.1 dex) and are caused by approximately equal contributions
from the horizontal temperature fluctuations and differences between
the temperature profiles in the 3D and 1D model atmospheres. Abundance
corrections of molecular lines are very sensitive to the metallicity of
the underlying model atmosphere and may be larger (in absolute value)
than ~-0.5 dex at [M/H] = -3.0 (~-1.5 dex in the case of CO). At fixed
metallicity and excitation potential, the abundance corrections show
little variation within the wavelength range studied here, 400-1600
nm. We also find that an approximate treatment of scattering in the
3D model calculations (i.e., ignoring the scattering opacity in the
outer, optically thin, atmosphere) leads to abundance corrections
that are altered by less than ~0.1 dex, both for atomic and molecular
(CO) lines, with respect to the model where scattering is treated as
true absorption throughout the entire atmosphere, with the largest
differences for the resonance and low-excitation lines. <P />Appendices
and Figs. 3, 5, 6, 8, 9, 11 are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Granulation properties of giants, dwarfs, and white dwarfs
from the CIFIST 3D model atmosphere grid
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.;
Caffau, E.
2013A&A...557A...7T Altcode: 2013arXiv1307.2810T
Three-dimensional model atmospheres for giants, dwarfs, and white
dwarfs, computed with the CO5BOLD code and part of the CIFIST grid,
have been used for spectroscopic and asteroseismic studies. Unlike
existing plane-parallel 1D structures, these simulations predict
the spatially and temporally resolved emergent intensity so that
granulation can be analysed, which provides insights on how convective
energy transfer operates in stars. The wide range of atmospheric
parameters of the CIFIST 3D simulations (3600 < T<SUB>eff</SUB>
(K) < 13 000 and 1 < log g < 9) allows the comparison of
convective processes in significantly different environments. We
show that the relative intensity contrast is correlated with both
the Mach and Péclet numbers in the photosphere. The horizontal size
of granules varies between 3 and 10 times the local pressure scale
height, with a tight correlation between the factor and the Mach
number of the flow. Given that convective giants, dwarfs, and white
dwarfs cover the same range of Mach and Péclet numbers, we conclude
that photospheric convection operates in a very similar way in those
objects. <P />Table 1 and Appendix A are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: The photospheric solar oxygen project. II. Non-concordance
of the oxygen abundance derived from two forbidden lines
Authors: Caffau, E.; Ludwig, H. -G.; Malherbe, J. -M.; Bonifacio,
P.; Steffen, M.; Monaco, L.
2013A&A...554A.126C Altcode: 2013arXiv1305.1763C
Context. In the Sun, the two forbidden [O i] lines at 630 and 636 nm
were previously found to provide discrepant oxygen abundances. <BR
/> Aims: We investigate whether this discrepancy is peculiar to the
Sun or whether it is also observed in other stars. <BR /> Methods:
We make use of high-resolution, high signal-to-noise ratio spectra of
four dwarf to turn-off stars, five giant stars, and one sub-giant star
observed with THEMIS, HARPS, and UVES to investigate the coherence of
the two lines. <BR /> Results: The two lines provide oxygen abundances
that are consistent, within observational errors, in all the giant
stars examined by us. On the other hand, for the two dwarf stars for
which a measurement was possible, for Procyon, and for the sub-giant
star Capella, the 636 nm line provides systematically higher oxygen
abundances, as already seen for the Sun. <BR /> Conclusions: The
only two possible reasons for the discrepancy are a serious error
in the oscillator strength of the Ni i line blending the 630 nm line
or the presence of an unknown blend in the 636 nm line, which makes
the feature stronger. The CN lines blending the 636 nm line cannot
be responsible for the discrepancy. The Ca i autoionisation line, on
the red wing of which the 636 nm line is formed, is not well modelled
by our synthetic spectra. However, a better reproduction of this line
would result in even higher abundances from the 636 nm, thus increasing
the discrepancy. <P />Based on observations collected at ESO Paranal
Observatory, Programme 182.D-5053(A).
---------------------------------------------------------
Title: Carbon-enhanced metal-poor stars: the most pristine objects?
Authors: Spite, M.; Caffau, E.; Bonifacio, P.; Spite, F.; Ludwig,
H. -G.; Plez, B.; Christlieb, N.
2013A&A...552A.107S Altcode: 2013arXiv1303.1791S
Context. Carbon-enhanced metal-poor stars (CEMP) form a significant
proportion of the metal-poor stars, their origin is not well understood,
and this carbon-enhancement appears in stars that exhibit different
abundance patterns. <BR /> Aims: Three very metal-poor C-rich turnoff
stars were selected from the SDSS survey, observed with the ESO VLT
(UVES) to precisely determine the element abundances. In turnoff
stars (unlike giants) the carbon abundance has not been affected by
mixing with deep layers and is therefore easier to interpret. <BR />
Methods: The analysis was performed with one dimensional (1D) local
thermodynamical equilibrium (LTE) static model atmospheres. When
available, non-LTE corrections were applied to the classical LTE
abundances. The 3D effects on the CH and CN molecular bands were
computed using hydrodynamical simulations of the stellar atmosphere
(CO<SUP>5</SUP>BOLD) and are found to be very important. <BR />
Results: To facilitate a comparison with previous results, only
1D abundances are used in the discussion. The abundances (or upper
limits) of the elements enable us to place these stars in different
CEMP classes. The carbon abundances confirm the existence of a plateau
at A(C)= 8.25 for [Fe/H] ≥ -3.4. The most metal-poor stars ([Fe/H]
< -3.4) have significantly lower carbon abundances, suggesting a
lower plateau at A(C) ≈ 6.5. Detailed analyses of a larger sample
of very low metallicity carbon-rich stars are required to confirm
(or refute) this possible second plateau and specify the behavior of
the CEMP stars at very low metallicity. <P />Based on observations
obtained with the ESO Very Large Telescope at Paranal Observatory,
Chile (ID 087.D-0123(A).Table 5 is available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Pure-hydrogen 3D model atmospheres of cool white dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013A&A...552A..13T Altcode: 2013arXiv1302.2013T
A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white
dwarfs is presented for a surface gravity of log g = 8 and effective
temperatures from 6000 to 13 000 K. We show that convective properties,
such as flow velocities, characteristic granulation size and intensity
contrast of the granulation patterns, change significantly over this
range. We demonstrate that these 3D simulations are not sensitive to
numerical parameters unlike the 1D structures that considerably depend
on the mixing-length parameters. We conclude that 3D spectra can be used
directly in the spectroscopic analyses of DA white dwarfs. We confirm
the result of an earlier preliminary study that 3D model spectra provide
a much better characterization of the mass distribution of white dwarfs
and that shortcomings of the 1D mixing-length theory are responsible
for the spurious high-log g determinations of cool white dwarfs. In
particular, the 1D theory is unable to account for the cooling effect
of the convective overshoot in the upper atmospheres.
---------------------------------------------------------
Title: Fluorine Abundances of Galactic Low-metallicity Giants
Authors: Li, H. N.; Ludwig, H. -G.; Caffau, E.; Christlieb, N.;
Zhao, G.
2013ApJ...765...51L Altcode: 2013arXiv1302.3928L
With abundances and 2σ upper limits of fluorine (F) in seven metal-poor
field giants, nucleosynthesis of stellar F at low metallicity is
discussed. The measurements are derived from the HF(1-0) R9 line at
23358 Å using near-infrared K-band high-resolution spectra obtained
with CRIRES at the Very Large Telescope. The sample reaches lower
metallicities than previous studies on F of field giants, ranging from
[Fe/H] = -1.56 down to -2.13. Effects of three-dimensional model
atmospheres on the derived F and O abundances are quantitatively
estimated and shown to be insignificant for the program stars. The
observed F yield in the form of [F/O] is compared with two sets of
Galactic chemical evolution models, which quantitatively demonstrate
the contribution of Type II supernova (SN II) ν-process and asymptotic
giant branch/Wolf-Rayet stars. It is found that at this low-metallicity
region, models cannot well predict the observed distribution of
[F/O], while the observations are better fit by models considering
an SN II ν-process with a neutrino energy of E <SUB>ν</SUB> = 3
× 10<SUP>53</SUP> erg. Our sample contains HD 110281, a retrograde
orbiting low-α halo star, showing a similar F evolution as globular
clusters. This supports the theory that such halo stars are possibly
accreted from dwarf galaxy progenitors of globular clusters in the halo.
---------------------------------------------------------
Title: Is the Sun Lighter than the Earth? Isotopic CO in the
Photosphere, Viewed through the Lens of Three-dimensional Spectrum
Synthesis
Authors: Ayres, Thomas R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.;
Wedemeyer-Böhm, S.
2013ApJ...765...46A Altcode: 2013arXiv1301.5281A
We consider the formation of solar infrared (2-6 μm) rovibrational
bands of carbon monoxide (CO) in CO5BOLD 3D convection models,
with the aim of refining abundances of the heavy isotopes of carbon
(<SUP>13</SUP>C) and oxygen (<SUP>18</SUP>O, <SUP>17</SUP>O), to
compare with direct capture measurements of solar wind light ions
by the Genesis Discovery Mission. We find that previous, mainly 1D,
analyses were systematically biased toward lower isotopic ratios (e.g.,
R <SUB>23</SUB> ≡ <SUP>12</SUP>C/<SUP>13</SUP>C), suggesting an
isotopically "heavy" Sun contrary to accepted fractionation processes
that were thought to have operated in the primitive solar nebula. The
new 3D ratios for <SUP>13</SUP>C and <SUP>18</SUP>O are R <SUB>23</SUB>
= 91.4 ± 1.3 (R <SUB>⊕</SUB> = 89.2) and R <SUB>68</SUB> =
511 ± 10 (R <SUB>⊕</SUB> = 499), where the uncertainties are
1σ and "optimistic." We also obtained R <SUB>67</SUB> = 2738 ±
118 (R <SUB>⊕</SUB> = 2632), but we caution that the observed
<SUP>12</SUP>C<SUP>17</SUP>O features are extremely weak. The new solar
ratios for the oxygen isotopes fall between the terrestrial values and
those reported by Genesis (R <SUB>68</SUB> = 530, R <SUB>67</SUB> =
2798), although including both within 2σ error flags, and go in the
direction favoring recent theories for the oxygen isotope composition
of Ca-Al inclusions in primitive meteorites. While not a major focus
of this work, we derive an oxygen abundance, epsilon<SUB>O</SUB> ~
603 ± 9 ppm (relative to hydrogen; log epsilon ~ 8.78 on the H =
12 scale). The fact that the Sun is likely lighter than the Earth,
isotopically speaking, removes the necessity of invoking exotic
fractionation processes during the early construction of the inner
solar system.
---------------------------------------------------------
Title: The potential of 3D radiation-hydrodynamics models for white
dwarf asteroseismology
Authors: Tremblay, P. -E.; Ludwig, H. -G.
2013EPJWC..4305008T Altcode:
White dwarfs with hydrogen-rich atmospheres (DA) are the most abundant
of all degenerate objects. In recent years work has been dedicated
to increase the accuracy of their model atmospheres. Most notably,
convective motions are now treated with 3D radiation-hydrodynamics
instead of the standard mixing-length theory. We present and describe
selected 3D model atmospheres close and within the instability strip
of the pulsating ZZ Ceti white dwarfs. Our 3D simulations depend only
weakly on numerical parameters and compared to 1D models, they provide
more realistic determinations of the depth of the convective zone. The
3D structures can then be adopted as input for asteroseismology.
---------------------------------------------------------
Title: Velocity and abundance precisions for future high-resolution
spectroscopic surveys: A study for 4MOST
Authors: Caffau, E.; Koch, A.; Sbordone, L.; Sartoretti, P.; Hansen,
C. J.; Royer, F.; Leclerc, N.; Bonifacio, P.; Christlieb, N.; Ludwig,
H. -G.; Grebel, E. K.; de Jong, R. S.; Chiappini, C.; Walcher, J.;
Mignot, S.; Feltzing, S.; Cohen, M.; Minchev, I.; Helmi, A.; Piffl,
T.; Depagne, E.; Schnurr, O.
2013AN....334..197C Altcode: 2012arXiv1211.1406C
In preparation for future, large-scale, multi-object, high-resolution
spectroscopic surveys of the Galaxy, we present a series of tests
of the precision in radial velocity and chemical abundances that any
such project can achieve at a 4 m class telescope. We briefly discuss
a number of science cases that aim at studying the chemo-dynamical
history of the major Galactic components (bulge, thin and thick disks,
and halo) - either as a follow-up to the Gaia mission or on their own
merits. Based on a large grid of synthetic spectra that cover the full
range in stellar parameters of typical survey targets, we devise an
optimal wavelength range and argue for a moderately high-resolution
spectrograph. As a result, the kinematic precision is not limited by
any of these factors, but will practically only suffer from systematic
effects, easily reaching uncertainties <1 km s<SUP>-1</SUP>. Under
realistic survey conditions (namely, considering stars brighter than
r=16 mag with reasonable exposure times) we prefer an ideal resolving
power of R∼20 000 on average, for an overall wavelength range (with
a common two-arm spectrograph design) of [395;456.5] nm and [587;673]
nm. We show for the first time on a general basis that it is possible
to measure chemical abundance ratios to better than 0.1 dex for many
species (Fe, Mg, Si, Ca, Ti, Na, Al, V, Cr, Mn, Co, Ni, Y, Ba, Nd, Eu)
and to an accuracy of about 0.2 dex for other species such as Zr, La,
and Sr. While our feasibility study was explicitly carried out for
the 4MOST facility, the results can be readily applied to and used
for any other conceptual design study for high-resolution spectrographs.
---------------------------------------------------------
Title: Amplitudes of solar-like oscillations in red giants: Departures
from the quasi-adiabatic approximation
Authors: Samadi, R.; Belkacem, K.; Dupret, M. -A.; Goupil, M. J.;
Ludwig, H. -G.; Barban, C.; Baudin, F.; Caffau, E.
2013EPJWC..4303008S Altcode:
CoRoT and Kepler measurements reveal us that the amplitudes of
solar-like oscillations detected in red giant stars scale from stars
to stars in a characteristic way. This observed scaling relation
is not yet fully understood but constitutes potentially a powerful
diagnostic about mode physics. Quasi-adiabatic theoretical scaling
relations in terms of mode amplitudes result in systematic and large
differences with the measurements performed for red giant stars. The
use of a non-adiabatic intensity-velocity relation derived from a
non-adiabatic pulsation code significantly reduces the discrepancy
with the CoRoT measurements. The origin of the remaining difference
is still unknown. Departure from adiabatic eigenfunction is a very
likely explanation that is investigated in the present work using a
3D hydrodynamical model of the surface layers of a representative red
giant star.
---------------------------------------------------------
Title: Isotopic CO in the Solar Photosphere, Viewed Through the Lens
of 3D Spectrum Synthesis
Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.;
Wedemeyer-Bohm, S.
2013LPI....44.3038A Altcode: 2013LPICo1719.3038A
New analyses of CO isotopologue abundances in the solar photosphere
are now consistent with Genesis solar wind results, although ^17O
error bars are still large.
---------------------------------------------------------
Title: Three-dimensional magnetohydrodynamic simulations of M-dwarf
chromospheres
Authors: Wedemeyer, S.; Ludwig, H. -G.; Steiner, O.
2013AN....334..137W Altcode: 2013csss...17..137W; 2012arXiv1207.2342W
We present first results from three-dimensional radiation
magnetohydrodynamic simulations of M-type dwarf stars with
CO<SUP>5</SUP>BOLD. The local models include the top of the convection
zone, the photosphere, and the chromosphere. The results are illustrated
for models with an effective temperature of 3240 K and a gravitational
acceleration of {log g = 4.5}, which represent analogues of AD Leo. The
models have different initial magnetic field strengths and field
topologies. This first generation of models demonstrates that the
atmospheres of M dwarfs are highly dynamic and intermittent. Magnetic
fields and propagating shock waves produce a complicated fine-structure,
which is clearly visible in synthetic intensity maps in the core of the
Ca II K spectral line and also at millimeter wavelengths. The dynamic
small-scale pattern cannot be described by means of one-dimensional
models, which has important implications for the construction of
semi-empirical model atmospheres and thus for the interpretation
of observations in general. Detailed three-dimensional numerical
simulations are valuable in this respect. Furthermore, such models
facilitate the analysis of small-scale processes, which cannot be
observed on stars but nevertheless might be essential for understanding
M-dwarf atmospheres and their activity. An example are so-called
“magnetic tornadoes”, which have recently been found on the Sun and
are presented here in M-dwarf models for the first time.
---------------------------------------------------------
Title: Convective line shifts for the Gaia RVS from the CIFIST 3D
model atmosphere grid
Authors: Allende Prieto, C.; Koesterke, L.; Ludwig, H. -G.; Freytag,
B.; Caffau, E.
2013A&A...550A.103A Altcode: 2013arXiv1301.3703A
Context. To derive space velocities of stars along the line of sight
from wavelength shifts in stellar spectra requires accounting for a
number of second-order effects. For most stars, gravitational redshifts,
convective blueshifts, and transverse stellar motion are the dominant
contributors. <BR /> Aims: We provide theoretical corrections for the
net velocity shifts due to convection expected for the measurements
from the Gaia Radial Velocity Spectrometer (RVS). <BR /> Methods: We
used a set of three-dimensional time-dependent simulations of stellar
surface convection computed with CO5BOLD to calculate spectra of
late-type stars in the Gaia RVS range and to infer the net velocity
offset that convective motions will induce in radial velocities
derived by cross-correlation. <BR /> Results: The net velocity shifts
derived by cross-correlation depend both on the wavelength range and
spectral resolution of the observations. Convective shifts for Gaia
RVS observations are less than 0.1 km s<SUP>-1</SUP> for late-K-type
stars, and they increase with stellar mass, reaching about 0.3 km
s<SUP>-1</SUP> or more for early F-type dwarfs. This tendency is the
result of an increase with effective temperature in both temperature
and velocity fluctuations in the line-forming region. Our simulations
also indicate that the net RVS convective shifts can be positive
(i.e. redshifts) in some cases. Overall, the blueshifts weaken
slightly with increasing surface gravity, and are enhanced at low
metallicity. Gravitational redshifts amount to 0.7 km s<SUP>-1</SUP>
and dominate convective blueshifts for dwarfs, but become much
weaker for giants. <P />Appendix A is available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>Model
spectra from the 1D and 3D calculations are only available
in electronic form at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A><A
href="http://130.79.128.5">130.79.128.5</A> or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A103">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A103</A>
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. II. Spectral line formation in the
atmosphere of a giant located near the RGB tip
Authors: Kučinskas, A.; Steffen, M.; Ludwig, H. -G.; Dobrovolskas,
V.; Ivanauskas, A.; Klevas, J.; Prakapavičius, D.; Caffau, E.;
Bonifacio, P.
2013A&A...549A..14K Altcode: 2012arXiv1211.7313K
<BR /> Aims: We investigate the role of convection in the formation of
atomic and molecular lines in the atmosphere of a red giant star. For
this purpose we study the formation properties of spectral lines that
belong to a number of astrophysically important tracer elements,
including neutral and singly ionized atoms (Li I, N I, O I, Na I,
Mg I, Al I, Si I, Si II, S I, K I, Ca I, Ca II, Ti I, Ti II, Cr I,
Cr II, Mn I, Fe I, Fe II, Co I, Ni I, Zn I, Sr II, Ba II, and Eu II),
and molecules (CH, CO, C<SUB>2</SUB>, NH, CN, and OH). <BR /> Methods:
We focus our investigation on a prototypical red giant located close to
the red giant branch (RGB) tip (T<SUB>eff</SUB> = 3660 K, log g = 1.0,
[M/H] = 0.0). We used two types of model atmospheres, 3D hydrodynamical
and classical 1D, calculated with the CO<SUP>5</SUP>BOLD and LHD
stellar atmosphere codes, respectively. Both codes share the same
atmospheric parameters, chemical composition, equation of state,
and opacities, which allowed us to make a strictly differential
comparison between the line formation properties predicted in 3D and
1D. The influence of convection on the spectral line formation was
assessed with the aid of 3D-1D abundance corrections, which measure
the difference between the abundances of chemical species derived
with the 3D hydrodynamical and 1D classical model atmospheres. <BR
/> Results: We find that convection plays a significant role in
the spectral line formation in this particular red giant. The
derived 3D-1D abundance corrections rarely exceed ± 0.1 dex when
lines of neutral atoms and molecules are considered, which is in
line with the previous findings for solar-metallicity red giants
located on the lower RGB. The situation is different with lines that
belong to ionized atoms, or to neutral atoms with high ionization
potential. In both cases, the corrections for high-excitation lines
(χ > 8 eV) may amount to Δ<SUB>3D-1D</SUB> ~ -0.4 dex. The
3D-1D abundance corrections generally show a significant wavelength
dependence; in most cases they are smaller in the near-infrared, at
1600-2500 nm. <P />Appendices are available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Granulation in DA white dwarfs from CO5BOLD 3D model
atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.
2013MSAIS..24...61T Altcode:
Time-dependent 3D simulations of pure-hydrogen DA white dwarf
atmospheres have been computed in recent years. Synthetic Balmer lines
spectra drawn from these radiation-hydrodynamics (RHD) simulations
have been shown to predict surface gravities significantly lower than
the standard 1D models, in much better agreement with the expectation
that white dwarfs cool at constant mass. We have now computed a grid
of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities
from log g = 7 to log g = 8.5 and effective temperatures from 6000 to
13,000 K. Over this range, we observe a significant variation of the
intensity contrast of the surface granulation patterns, which indicates
the strength of the 3D effects. Furthermore, the size and appearance
of granules are also varying considerably. An explanation of these
behaviours can lead to a better understanding of the physical processes
responsible for the energy transfer in white dwarf atmospheres.
---------------------------------------------------------
Title: Micro- and macroturbulence predictions from CO5BOLD 3D stellar
atmospheres .
Authors: Steffen, M.; Caffau, E.; Ludwig, H. -G.
2013MSAIS..24...37S Altcode: 2013arXiv1306.4307S
We present an overview of the current status of our efforts to derive
the microturbulence and macroturbulence parameters (xi_mic and xi_mac)
from the CIFIST grid of CO5BOLD 3D model atmospheres as a function
of the basic stellar parameters T_{eff}, log g, and [M/H]. The latest
results for the Sun and Procyon show that the derived microturbulence
parameter depends significantly on the numerical resolution of the
underlying 3D simulation, confirming that `low-resolution' models tend
to underestimate the true value of xi_mic . Extending the investigation
to 12 further simulations with different T_{eff}, log g, and [M/H],
we obtain a first impression of the predicted trend of xi_mic over the
Hertzsprung-Russell diagram: in agreement with empirical evidence,
microturbulence increases towards higher effective temperature and
lower gravity. The metallicity dependence of xi_mic must be interpreted
with care, since it also reflects the deviation between the 1D and 3D
photospheric temperature stratifications that increases systematically
towards lower [M/H].
---------------------------------------------------------
Title: The influence of convection on the atmospheric structures
and observable properties of red giant stars.
Authors: Kučinskas, A.; Ludwig, H. -G.; Steffen, M.; Dobrovolskas,
V.; Klevas, J.; Prakapavičius, D.; Caffau, E.; Bonifacio, P.
2013MSAIS..24...68K Altcode: 2013arXiv1305.3441K
During the recent years significant progress has been made in the
modeling of red giant atmospheres with the aid of 3D hydrodynamical
model atmosphere codes. In this contribution we provide an overview of
selected results obtained in this context by utilizing 3D hydrodynamical
CO<SUP>5</SUP>BOLD stellar model atmospheres. Hydrodynamical simulations
show that convective motions lead to significant differences in the
atmospheric structures of red giants with respect to those predicted by
the classical 1D model atmospheres. Results of these simulations also
show that in certain cases 1D models fail to reproduce even the average
properties of the 3D hydrodynamical models, such as P-T profiles. Large
horizontal temperature fluctuations in the 3D model atmospheres, as well
as differences between the temperature profiles of the average xtmean
{3D} and 1D models, lead to large discrepancies in the strengths of
spectral lines predicted by the 3D and 1D model atmospheres. This is
especially important in models at lowest metallicities ([M/H]<-2.0)
where the 3D-1D abundance differences may reach (or even exceed) -0.6
dex for lines of neutral atoms and molecules. We also discuss several
simplifications and numerical aspects involved in the present 3D
hydrodynamical modeling of red giant atmospheres, and briefly address
several issues where urgent progress may be needed.
---------------------------------------------------------
Title: Molecular bands in extremely metal-poor stars: Granulation
effects
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Spite, M.; Plez,
B.; Steffen, M.; Spite, F.
2013MSAIS..24..138B Altcode: 2013arXiv1305.2065B
The bands of diatomic molecules are important abundance indicators,
especially in metal-poor stars, where they are still measurable in
metallicity regimes where the atomic lines of their constituting
metallic elements have become vanishingly small. In order to use
them for abundance determinations it is imperative to understand the
formation of these bands. In this contribution we report on our results
obtained using CO^5{BOLD} hydrodynamical simulations. Some effects
that are qualitatively different from what found in 1D computations
are highlighted. Due to the large number of lines that form the bands,
their spectrum synthesis is computationally challenging. We discuss
some of the computational strategies we employed to parallelise the
computation and possible future developments.
---------------------------------------------------------
Title: Signs of atmospheric inhomogeneities in cool stars from
1D-NLTE analysis of iron lines
Authors: Mashonkina, L.; Ludwig, H. -G.; Korn, A.; Sitnova, T.;
Caffau, E.
2013MSAIS..24..120M Altcode: 2013arXiv1303.0357M
For the well studied halo star HD 122563 and the four stars in the
globular cluster NGC 6397, we determine NLTE abundances of iron
using classical plane-parallel model atmospheres. Each star reveals
a discrepancy in abundances between the Fe I lines arising from the
ground state and the other Fe I lines, in qualitative agreement with
the 3D-LTE line formation predictions, however, the magnitude of the
observed effect is a factor of 2 smaller compared with the predicted
one. When ignoring the Fe I low-excitation lines, the NLTE abundances
from the two ionization stages, Fe I and Fe II are consistent in each
investigated star. For the subgiants in NGC 6397, this is only true when
using the cooler effective temperature scale of \citet{Alonso1999}. We
also present full 3D-LTE line formation calculations for some selected
iron lines in the solar and metal-poor 4480/2/-3 models and NLTE
calculations with the corresponding spatial and temporal average
<{3D}> models. The use of the <{3D}> models is justified
only for particular Fe I lines in particular physical conditions. Our
NLTE calculations reproduce well the centre-to-limb variation of
the solar Fe I 7780 Å line, but they are unsuccessful for Fe I 6151
Å. The metal-poor <{3D}> model was found to be adequite for
the strong Fe I 5166 Å (E_exc = 0) line, but inadequite in all other
investigated cases.
---------------------------------------------------------
Title: Solar carbon monoxide: poster child for 3D effects .
Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.;
Wedemeyer-Böhm, S.
2013MSAIS..24...85A Altcode:
Photospheric infrared (2-6 mu m) rovibrational bands of carbon
monoxide (CO) provide a tough test for 3D convection models such as
those calculated using CO5BOLD. The molecular formation is highly
temperature-sensitive, and thus responds in an exaggerated way to
thermal fluctuations in the dynamic atmosphere. CO, itself, is an
important tracer of the oxygen abundance, a still controversial
issue in solar physics; as well as the heavy isotopes of carbon
(<SUP>13</SUP>C) and oxygen (<SUP>18</SUP>O, <SUP>17</SUP>O), which,
relative to terrestrial values, are fingerprints of fractionation
processes that operated in the primitive solar nebula. We show how 3D
models impact the CO line formation, and add in a second constraint
involving the near-UV Ca RIPTSIZE II line wings, which also are highly
temperature sensitive, but in the opposite sense to the molecules. We
find that our reference CO5BOLD snapshots appear to be slightly too
cool on average in the outer layers of the photosphere where the CO
absorptions and Ca RIPTSIZE II wing emissions arise. We show, further,
that previous 1D modeling was systematically biased toward higher
oxygen abundances and lower isotopic ratios (e.g., R<SUB>23</SUB>equiv
<SUP>12</SUP>C/<SUP>13</SUP>C), suggesting an isotopically “heavy”
Sun contrary to direct capture measurements of solar wind light ions
by the Genesis Discovery Mission. New 3D ratios for the oxygen isotopes
are much closer to those reported by Genesis, and the associated oxygen
abundance from CO now is consistent with the recent Caffau et al. study
of atomic oxygen. Some lingering discrepancies perhaps can be explained
by magnetic bright points. Solar CO demonstrates graphically the wide
gulf that can occur between a 3D analysis and 1D.
---------------------------------------------------------
Title: White Dwarfs Escaping From the Hyades
Authors: Tremblay, P. -E.; Schilbach, E.; Röser, S.; Jordan, S.;
Ludwig, H. -G.; Goldman, B.
2013ASPC..469..105T Altcode:
Ten white dwarfs are currently known to be associated with the
Hyades cluster. This number of degenerate objects is too low when
compared to the predictions of numerical simulations. Using the PPMXL
catalog of proper motions and positions, 17 new Hyades white dwarf
candidates have recently been proposed, most of them outside of the
tidal radius. They could potentially explain the discrepancy between
the observed and predicted number of associated objects. We review
the Hyades membership of these candidates by using spectroscopic and
photometric observations, as well as by simulating the contamination
from field white dwarfs. We find that six objects remain candidates,
and three more objects have an uncertain membership status due to
their unknown or imprecise atmospheric parameters.
---------------------------------------------------------
Title: Opacities in CO<SUP>5</SUP>BOLD
Authors: Ludwig, H. -G.; Steffen, M.
2013MSAIS..24...53L Altcode:
We describe the present treatment of the frequency-dependence of
the radiative transfer in CO<SUP>5</SUP>BOLD. This mostly refers to
the way opacities are binned into groups. We discuss the basic ideas
behind the Opacity Binning Method, give some details of the practical
implementation in CO<SUP>5</SUP>BOLD, and point to some issues where
improvements are needed.
---------------------------------------------------------
Title: Oxygen spectral line synthesis: 3D non-LTE with
CO<SUP>5</SUP>BOLD hydrodynamical model atmospheres.
Authors: Prakapavičius, D.; Steffen, M.; Kučinskas, A.; Ludwig,
H. -G.; Freytag, B.; Caffau, E.; Cayrel, R.
2013MSAIS..24..111P Altcode: 2013arXiv1303.2016P
In this work we present first results of our current project aimed at
combining the 3D hydrodynamical stellar atmosphere approach with non-LTE
(NLTE) spectral line synthesis for a number of key chemical species. We
carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm
triplet, using a modified and improved version of our NLTE3D package to
calculate departure coefficients for the atomic levels of oxygen in a
CO<SUP>5</SUP>BOLD 3D hydrodynamical solar model atmosphere. Spectral
line synthesis was subsequently performed with the Linfor3D code. In
agreement with previous studies, we find that the lines of the
oxygen triplet produce deeper cores under NLTE conditions, due to
the diminished line source function in the line forming region. This
means that the solar oxygen IR 777 nm lines should be stronger in NLTE,
leading to negative 3D NLTE-LTE abundance corrections. Qualitatively
this result would support previous claims for a relatively low solar
oxygen abundance. Finally, we outline several further steps that need
to be taken in order to improve the physical realism and numerical
accuracy of our current 3D-NLTE calculations.
---------------------------------------------------------
Title: 3D Model Atmospheres of DA White Dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013ASPC..469..155T Altcode:
The spectroscopically determined surface gravities of cool
hydrogen-atmosphere DA white dwarfs are significantly higher than
the mean value of log g ∼ 8 found for hotter objects with radiative
atmospheres (T<SUB>eff</SUB> > 13000 K). It was recently suggested
that a problem with the treatment of convective energy transport in
the 1D mixing-length theory was the explanation for this high-log
g problem. We have now computed a grid of pure-hydrogen 3D model
atmospheres with the CO<SUP>5</SUP>BOLD code for surface gravities
from log g = 7 to 8.5 and effective temperatures from 6000 to 13000
K. Over this range, the intensity contrast of the surface granulation
pattern, which describes the strength of the 3D effects, is varying
significantly. We confirm the result of an earlier investigation that
3D model spectra provide a much better characterization of the mass
distribution of cool white dwarfs.
---------------------------------------------------------
Title: Three-dimensional hydrodynamical CO<SUP>5</SUP>BOLD model
atmospheres of red giant stars. I. Atmospheric structure of a giant
located near the RGB tip
Authors: Ludwig, H. -G.; Kučinskas, A.
2012A&A...547A.118L Altcode: 2012arXiv1211.7304L
Context. Red giant stars are important tracers of stellar populations
in the Galaxy and beyond, thus accurate modeling of their structure and
related observable properties is of great importance. Three-dimensional
(3D) hydrodynamical stellar atmosphere models offer a new level of
realism in the modeling of red giant atmospheres but still need to be
established as standard tools. <BR /> Aims: We investigate the character
and role of convection in the atmosphere of a prototypical red giant
located close to the red giant branch (RGB) tip with atmospheric
parameters, T<SUB>eff</SUB> = 3660 K, log g = 1.0, [M/H] = 0.0. <BR
/> Methods: Differential analysis of the atmospheric structures is
performed using the 3D hydrodynamical and 1D classical atmosphere models
calculated with the CO<SUP>5</SUP>BOLD and LHD codes, respectively. All
models share identical atmospheric parameters, elemental composition,
opacities and equation-of-state. <BR /> Results: We find that the
atmosphere of this particular red giant consists of two rather distinct
regions: the lower atmosphere dominated by convective motions and the
upper atmosphere dominated by wave activity. Convective motions form a
prominent granulation pattern with an intensity contrast (~18%) which
is larger than in the solar models (~15%). The upper atmosphere is
frequently traversed by fast shock waves, with vertical and horizontal
velocities of up to Mach ~2.5 and ~6.0, respectively. The typical
diameter of the granules amounts to ~5 Gm which translates into ~400
granules covering the whole stellar surface. The turbulent pressure
in the giant model contributes up to ~35% to the total (i.e., gas
plus turbulent) pressure which shows that it cannot be neglected in
stellar atmosphere and evolutionary modeling. However, there exists
no combination of the mixing-length parameter, α<SUB>MLT</SUB>, and
turbulent pressure, P<SUB>turb</SUB>, that would allow to satisfactorily
reproduce the 3D temperature-pressure profile with 1D atmosphere models
based on a standard formulation of mixing-length theory.
---------------------------------------------------------
Title: Spectroscopic and photometric studies of white dwarfs in
the Hyades
Authors: Tremblay, P. -E.; Schilbach, E.; Röser, S.; Jordan, S.;
Ludwig, H. -G.; Goldman, B.
2012A&A...547A..99T Altcode: 2012arXiv1209.4309T
The Hyades cluster is known to harbour ten so-called classical white
dwarf members. Numerous studies through the years have predicted
that more than twice this amount of degenerate stars should be
associated with the cluster. Using the PPMXL Catalog of proper
motions and positions, a recent study proposed 17 new white dwarf
candidates. We review the membership of these candidates by using
published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to
the ten classical Hyades white dwarfs, we find six white dwarfs that
may be of Hyades origin and three more objects that have an uncertain
membership status due to their unknown or imprecise atmospheric
parameters. Among those, two to three are expected as field stars
contamination. Accurate radial velocity measurements will confirm or
reject the candidates. One consequence is that the longstanding problem
that no white dwarf older than ~340 Myr appears to be associated with
the cluster remains unsolved.
---------------------------------------------------------
Title: 4MOST: 4-metre multi-object spectroscopic telescope
Authors: de Jong, Roelof S.; Bellido-Tirado, Olga; Chiappini,
Cristina; Depagne, Éric; Haynes, Roger; Johl, Diana; Schnurr,
Olivier; Schwope, Axel; Walcher, Jakob; Dionies, Frank; Haynes,
Dionne; Kelz, Andreas; Kitaura, Francisco S.; Lamer, Georg; Minchev,
Ivan; Müller, Volker; Nuza, Sebastián. E.; Olaya, Jean-Christophe;
Piffl, Tilmann; Popow, Emil; Steinmetz, Matthias; Ural, Ugur; Williams,
Mary; Winkler, Roland; Wisotzki, Lutz; Ansorge, Wolfgang R.; Banerji,
Manda; Gonzalez Solares, Eduardo; Irwin, Mike; Kennicutt, Robert C.;
King, Dave; McMahon, Richard G.; Koposov, Sergey; Parry, Ian R.; Sun,
David; Walton, Nicholas A.; Finger, Gert; Iwert, Olaf; Krumpe, Mirko;
Lizon, Jean-Louis; Vincenzo, Mainieri; Amans, Jean-Philippe; Bonifacio,
Piercarlo; Cohen, Mathieu; Francois, Patrick; Jagourel, Pascal; Mignot,
Shan B.; Royer, Frédéric; Sartoretti, Paola; Bender, Ralf; Grupp,
Frank; Hess, Hans-Joachim; Lang-Bardl, Florian; Muschielok, Bernard;
Böhringer, Hans; Boller, Thomas; Bongiorno, Angela; Brusa, Marcella;
Dwelly, Tom; Merloni, Andrea; Nandra, Kirpal; Salvato, Mara; Pragt,
Johannes H.; Navarro, Ramón; Gerlofsma, Gerrit; Roelfsema, Ronald;
Dalton, Gavin B.; Middleton, Kevin F.; Tosh, Ian A.; Boeche, Corrado;
Caffau, Elisabetta; Christlieb, Norbert; Grebel, Eva K.; Hansen,
Camilla; Koch, Andreas; Ludwig, Hans-G.; Quirrenbach, Andreas;
Sbordone, Luca; Seifert, Walter; Thimm, Guido; Trifonov, Trifon;
Helmi, Amina; Trager, Scott C.; Feltzing, Sofia; Korn, Andreas;
Boland, Wilfried
2012SPIE.8446E..0TD Altcode: 2012arXiv1206.6885D
The 4MOST consortium is currently halfway through a Conceptual
Design study for ESO with the aim to develop a wide-field ( <
3 square degree, goal < 5 square degree), high-multiplex ( <
1500 fibres, goal 3000 fibres) spectroscopic survey facility for
an ESO 4m-class telescope (VISTA). 4MOST will run permanently on
the telescope to perform a 5 year public survey yielding more than
20 million spectra at resolution R∼5000 (λ=390-1000 nm) and more
than 2 million spectra at R~20,000 (395-456.5 nm and 587-673 nm). The
4MOST design is especially intended to complement three key all-sky,
space-based observatories of prime European interest: Gaia, eROSITA and
Euclid. Initial design and performance estimates for the wide-field
corrector concepts are presented. Two fibre positioner concepts are
being considered for 4MOST. The first one is a Phi-Theta system similar
to ones used on existing and planned facilities. The second one is a
new R-Theta concept with large patrol area. Both positioner concepts
effectively address the issues of fibre focus and pupil pointing. The
4MOST spectrographs are fixed configuration two-arm spectrographs,
with dedicated spectrographs for the high- and low-resolution fibres. A
full facility simulator is being developed to guide trade-off decisions
regarding the optimal field-of-view, number of fibres needed, and the
relative fraction of high-to-low resolution fibres. The simulator takes
mock catalogues with template spectra from Design Reference Surveys
as starting point, calculates the output spectra based on a throughput
simulator, assigns targets to fibres based on the capabilities of the
fibre positioner designs, and calculates the required survey time by
tiling the fields on the sky. The 4MOST consortium aims to deliver the
full 4MOST facility by the end of 2018 and start delivering high-level
data products for both consortium and ESO community targets a year
later with yearly increments.
---------------------------------------------------------
Title: Detailed Abundances in Extremely Metal Poor Dwarf Stars
Extracted from SDSS
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.
2012ASPC..458...69S Altcode: 2012arXiv1201.1044S
We report on the result of an ongoing campaign to determine chemical
abundances in extremely metal poor (EMP) turn-off (TO) stars selected
from the Sloan Digital Sky Survey (SDSS) low resolution spectra. This
contribution focuses principally on the largest part of the sample
(18 stars out of 29), observed with UVES@VLT and analyzed by means of
the automatic abundance analysis code MyGIsFOS to derive atmosphere
parameters and detailed compositions. The most significant findings
include i) the detection of a C-rich, strongly Mg-enhanced star
([Mg/Fe]=1.45); ii) a group of Mn-rich stars ([Mn/Fe]>-0.4); iii)
a group of Ni-rich stars ([Ni/Fe]>0.2). Li is measured in twelve
stars, while for three upper limits are derived.
---------------------------------------------------------
Title: Amplitudes of solar-like oscillations in red giant
stars. Evidence for non-adiabatic effects using CoRoT observations
Authors: Samadi, R.; Belkacem, K.; Dupret, M. -A.; Ludwig, H. -G.;
Baudin, F.; Caffau, E.; Goupil, M. -J.; Barban, C.
2012A&A...543A.120S Altcode: 2012arXiv1205.4846S
Context. A growing number of solar-like oscillations has been detected
in red giant stars thanks to the CoRoT and Kepler space-crafts. In
the same way as for main-sequence stars, mode driving is attributed
to turbulent convection in the uppermost convective layers of those
stars. <BR /> Aims: The seismic data gathered by CoRoT on red giant
stars allow us to test the mode driving theory in physical conditions
different from main-sequence stars. <BR /> Methods: Using a set of
3D hydrodynamical models representative of the upper layers of sub-
and red giant stars, we computed the acoustic mode energy supply rate
({p_max}). Assuming adiabatic pulsations and using global stellar
models that assume that the surface stratification comes from the
3D hydrodynamical models, we computed the mode amplitude in terms
of surface velocity. This was converted into intensity fluctuations
using either a simplified adiabatic scaling relation or a non-adiabatic
one. <BR /> Results: From L and M (the luminosity and mass), the energy
supply rate {p_max} is found to scale as (L/M)<SUP>2.6</SUP> for both
main-sequence and red giant stars, extending previous results. The
theoretical amplitudes in velocity under-estimate the Doppler velocity
measurements obtained so far from the ground for red giant stars by
about 30%. In terms of intensity, the theoretical scaling law based
on the adiabatic intensity-velocity scaling relation results in an
under-estimation by a factor of about 2.5 with respect to the CoRoT
seismic measurements. On the other hand, using the non-adiabatic
intensity-velocity relation significantly reduces the discrepancy with
the CoRoT data. The theoretical amplitudes remain 40% below, however,
the CoRoT measurements. <BR /> Conclusions: Our results show that
scaling relations of mode amplitudes cannot be simply extended from
main-sequence to red giant stars in terms of intensity on the basis of
adiabatic relations because non-adiabatic effects for red giant stars
are important and cannot be neglected. We discuss possible reasons
for the remaining differences.
---------------------------------------------------------
Title: Chemical abundances of distant extremely metal-poor unevolved
stars
Authors: Bonifacio, P.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.;
Spite, M.; González Hernández, J. I.; Behara, N. T.
2012A&A...542A..87B Altcode: 2012arXiv1204.1641B
Context. The old Galactic halo stars hold the fossil record of
the interstellar medium chemical composition at the time of their
formation. Most of the stars studied so far are relatively near to the
Sun, this prompts the study of more distant stars, both to increase the
size of the sample and to search for possible variations of abundance
patterns at greater distances. <BR /> Aims: The purpose of our study
is to determine the chemical composition of a sample of 16 candidate
extremely metal-poor (EMP) dwarf stars, extracted from the Sloan
Digital Sky Survey (SDSS). There are two main purposes: in the first
place to verify the reliability of the metallicity estimates derived
from the SDSS spectra; in the second place to see if the abundance
trends found for the brighter nearer stars studied previously also
hold for this sample of fainter, more distant stars. <BR /> Methods:
We used the UVES at the VLT to obtain high-resolution spectra of the
programme stars. The abundances were determined by an automatic analysis
with the MyGIsFOS code, with the exception of lithium, for which the
abundances were determined from the measured equivalent widths of the
Li i resonance doublet. <BR /> Results: All candidates are confirmed
to be EMP stars, with [Fe/H] ≤ -3.0. The chemical composition of the
sample of stars is similar to that of brighter and nearer samples. We
measured the lithium abundance for 12 stars and provide stringent upper
limits for three other stars, for a fourth star the upper limit is not
significant, owing to the low signal-to noise ratio of the spectrum. The
"meltdown" of the Spite plateau is confirmed, but some of the lowest
metallicity stars of the sample lie on the plateau. <BR /> Conclusions:
The concordance of the metallicities derived from high-resolution
spectra and those estimated from the SDSS spectra suggests that
the latter may be used to study the metallicity distribution of the
halo. The abundance pattern suggests that the halo was well mixed for
all probed metallicities and distances. The fact that at the lowest
metallicities we find stars on the Spite plateau suggests that the
meltdown depends on at least another parameter, besides metallicity. <P
/>Based on spectra obtained with UVES at the 8.2 m Kueyen ESO telescope,
programmes 078.D-0217 and 081.D.0373.Table 1 is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: A primordial star in the heart of the Lion
Authors: Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite,
F.; Zaggia, S.; Ludwig, H. -G.; Steffen, M.; Mashonkina, L.; Monaco,
L.; Sbordone, L.; Molaro, P.; Cayrel, R.; Plez, B.; Hill, V.; Hammer,
F.; Randich, S.
2012A&A...542A..51C Altcode: 2012arXiv1203.2607C
Context. The discovery and chemical analysis of extremely metal-poor
stars permit a better understanding of the star formation of the first
generation of stars and of the Universe emerging from the Big Bang. <BR
/> Aims: We report the study of a primordial star situated in the centre
of the constellation Leo (SDSS J102915+172927). <BR /> Methods: The
star, selected from the low-resolution spectrum of the Sloan Digital
Sky Survey, was observed at intermediate (with X-Shooter at VLT) and
at high spectral resolution (with UVES at VLT). The stellar parameters
were derived from the photometry. The standard spectroscopic analysis
based on 1D ATLAS models was completed by applying 3D and non-LTE
corrections. <BR /> Results: An iron abundance of [Fe/H ] = -4.89 makes
SDSS J102915+172927 one of the lowest [Fe/H] stars known. However,
the absence of measurable C and N enhancements indicates that it has
the lowest metallicity, Z ≤ 7.40 × 10<SUP>-7</SUP> (metal-mass
fraction), ever detected. No oxygen measurement was possible. <BR />
Conclusions: The discovery of SDSS J102915+172927 highlights that
low-mass star formation occurred at metallicities lower than previously
assumed. Even lower metallicity stars may yet be discovered, with a
chemical composition closer to the composition of the primordial gas
and of the first supernovae. <P />Based on observations obtained at ESO
Paranal Observatory, GTO programme 086.D-0094 and programme 286.D-5045.
---------------------------------------------------------
Title: NLTE determination of the calcium abundance and 3D corrections
in extremely metal-poor stars
Authors: Spite, M.; Andrievsky, S. M.; Spite, F.; Caffau, E.; Korotin,
S. A.; Bonifacio, P.; Ludwig, H. -G.; François, P.; Cayrel, R.
2012A&A...541A.143S Altcode: 2012arXiv1204.1139S
Context. Calcium is a key element for constraining the models of
chemical enrichment of the Galaxy. <BR /> Aims: Extremely metal-poor
stars contain the fossil records of the chemical composition of the
early Galaxy and it is important to compare Ca abundance with abundances
of other light elements, that are supposed to be synthesized in the
same stellar evolution phases. <BR /> Methods: The NLTE profiles of the
calcium lines were computed in a sample of 53 extremely metal-poor stars
with a modified version of the program MULTI, which allows a very good
description of the radiation field. <BR /> Results: With our new model
atom we are able to reconcile the abundance of Ca deduced from the Ca
I and Ca II lines in Procyon. This abundance is found to be solar. We
find that [Ca/Fe] = 0.50±0.09 in the early Galaxy, a value slightly
higher than the previous LTE estimations. The scatter of the ratios
[X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where
X is a "light metal" (O, Na, Mg, Al, S, and K) with the exception of
Al. These scatters cannot be explained by error of measurements, except
for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or
even smaller than, the scatter around the mean value of [X/Ca]. We note
that at low metallicity, the wavelength of the Ca I resonance line is
shifted relative to the (weaker) subordinate lines, a signature of the
effect of convection. The Ca abundance deduced from the Ca I resonance
line (422.7 nm) is found to be systematically smaller at very low
metallicity than the abundance deduced from the subordinate lines. Our
computations of the effects of convection (3D effects) are not able to
explain this difference. A fully consistent 3D NLTE model atmosphere
and line formation scheme would be necessary to fully capture the
physics of the stellar atmosphere. <P />Based on observations obtained
with the ESO Very Large Telescope at Paranal Observatory, Chile (Large
Programme "First Stars", ID 165.N-0276(A); P.I.: R. Cayrel).The NLTE
corrections of the Ca lines are available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A143">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A143</A>
---------------------------------------------------------
Title: VizieR Online Data Catalog: Li and Na in globular cluster M4
(Monaco+, 2012)
Authors: Monaco, L.; Villanova, S.; Bonifacio, P.; Caffau, E.; Geisler,
D.; Marconi, G.; Momany, Y.; Ludwig, H. -G.
2012yCat..35390157M Altcode: 2012yCat..35399157M
We observed stars along the M4 MS and SGB using the FLAMES/GIRAFFE
spectrograph at ESO Paranal. Observations were conducted in service
mode between April and July 2010 using the HR12 and HR15N settings. <P
/>(1 data file).
---------------------------------------------------------
Title: Barium abundance in red giants of NGC 6752. Non-local
thermodynamic equilibrium and three-dimensional effects
Authors: Dobrovolskas, V.; Kučinskas, A.; Andrievsky, S. M.; Korotin,
S. A.; Mishenina, T. V.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.
2012A&A...540A.128D Altcode: 2012arXiv1203.3124D
<BR /> Aims: We study the effects related to departures from non-local
thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres
of red giant stars, to assess their influence on the formation of
Ba II lines. We estimate the impact of these effects on the barium
abundance determinations for 20 red giants in Galactic globular cluster
NGC 6752. <BR /> Methods: One-dimensional (1D) local thermodynamic
equilibrium (LTE) and 1D NLTE barium abundances were derived using
classical 1D ATLAS9 stellar model atmospheres. The three-dimensional
(3D) LTE abundances were obtained for 8 red giants on the lower RGB,
by adjusting their 1D LTE abundances using 3D-1D abundance corrections,
i.e., the differences between the abundances obtained from the same
spectral line using the 3D hydrodynamical and classical 1D stellar
model atmospheres. The 3D-1D abundance corrections were obtained
in a strictly differential way using the 3D hydrodynamical and
classical 1D codes CO<SUP>5</SUP>BOLD and LHD. Both codes utilized
identical stellar atmospheric parameters, opacities, and equation
of state. <BR /> Results: The mean 1D barium-to-iron abundance
ratios derived for 20 giants are ⟨[Ba/Fe]⟩<SUB>1D LTE</SUB> =
0.24 ± 0.05(stat.) ± 0.08(sys.) and ⟨[Ba/Fe]⟩<SUB>1D NLTE</SUB>
= 0.05 ± 0.06(stat.) ± 0.08(sys.). The 3D-1D abundance correction
obtained for 8 giants is small (~+0.05 dex), thus leads to only minor
adjustment when applied to the mean 1D NLTE barium-to-iron abundance
ratio for the 20 giants, ⟨[Ba/Fe]⟩<SUB>3D + NLTE</SUB> = 0.10
± 0.06(stat.) ± 0.10(sys.). The intrinsic abundance spread between
the individual cluster stars is small and can be explained in terms
of uncertainties in the abundance determinations. <BR /> Conclusions:
Deviations from LTE play an important role in the formation of barium
lines in the atmospheres of red giants studied here. The role of
3D hydrodynamical effects should not be dismissed either, even if
the obtained 3D-1D abundance corrections are small. This result is
a consequence of subtle fine-tuning of individual contributions from
horizontal temperature fluctuations and differences between the average
temperature profiles in the 3D and 1D model atmospheres: owing to the
comparable size and opposite sign, their contributions nearly cancel
each other. This fine-tuning is characteristic of the particular set
of atmospheric parameters and the element investigated, hence should
not necessarily be a general property of spectral line formation in
the atmospheres of red giant stars.
---------------------------------------------------------
Title: LTE Model Atmospheres: MARCS, ATLAS and CO5BOLD
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.
2012IAUS..282..213B Altcode:
In this talk, we review the basic assumptions and physics covered
by classical 1D LTE model atmospheres. We will focus on ATLAS and
MARCS models of F-G-K stars and describe what resources are available
through the web, both in terms of codes and model-atmosphere grids. We
describe the advances made in hydrodynamical simulations of convective
stellar atmospheres with the CO<SUP>5</SUP>BOLD code and what grids
and resources are available, with a prospect of what will be available
in the near future.
---------------------------------------------------------
Title: Chapter 1 : How to Derive Oxygen Abundances
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54....3S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VizieR Online Data Catalog: NLTE Corrections of the Ca lines
(Spite+, 2012)
Authors: Spite, M.; Andrievsky, S. M.; Spite, F.; Caffau, E.; Korotin,
S. A.; Bonifacio, P.; Ludwig, H. -G.; Francois, P.; Cayrel, R.
2012yCat..35410143S Altcode: 2012yCat..35419143S
The NLTE corrections were computed for 51 CaI lines and 16 CaII lines
for a grid of models with different metallicities from [Fe/H]=0.0
to [Fe/H]=-3. These corrections must be added to the LTE value of
[Ca/H], they were computed only if the equivalent width of the Ca line
was stronger than 3mÅ. In the tables the model is given in the form
(Teff, logg, [Fe/H], [Ca/Fe]) where Teff is the effective temperature,
and logg the logarithm of the surface gravity) <P />(5 data files).
---------------------------------------------------------
Title: Chapter 2 : A Panorama of Oxygen in the Universe
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54...65S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Gaia-ESO Public Spectroscopic Survey
Authors: Gilmore, G.; Randich, S.; Asplund, M.; Binney, J.; Bonifacio,
P.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.; Micela, G.;
Negueruela, I.; Prusti, T.; Rix, H. -W.; Vallenari, A.; Alfaro, E.;
Allende-Prieto, C.; Babusiaux, C.; Bensby, T.; Blomme, R.; Bragaglia,
A.; Flaccomio, E.; François, P.; Irwin, M.; Koposov, S.; Korn, A.;
Lanzafame, A.; Pancino, E.; Paunzen, E.; Recio-Blanco, A.; Sacco,
G.; Smiljanic, R.; Van Eck, S.; Walton, N.; Aden, D.; Aerts, C.;
Affer, L.; Alcala, J. -M.; Altavilla, G.; Alves, J.; Antoja, T.;
Arenou, F.; Argiroffi, C.; Asensio Ramos, A.; Bailer-Jones, C.;
Balaguer-Nunez, L.; Bayo, A.; Barbuy, B.; Barisevicius, G.; Barrado
y Navascues, D.; Battistini, C.; Bellas Velidis, I.; Bellazzini, M.;
Belokurov, V.; Bergemann, M.; Bertelli, G.; Biazzo, K.; Bienayme, O.;
Bland-Hawthorn, J.; Boeche, C.; Bonito, S.; Boudreault, S.; Bouvier,
J.; Brandao, I.; Brown, A.; de Bruijne, J.; Burleigh, M.; Caballero,
J.; Caffau, E.; Calura, F.; Capuzzo-Dolcetta, R.; Caramazza, M.;
Carraro, G.; Casagrande, L.; Casewell, S.; Chapman, S.; Chiappini,
C.; Chorniy, Y.; Christlieb, N.; Cignoni, M.; Cocozza, G.; Colless,
M.; Collet, R.; Collins, M.; Correnti, M.; Covino, E.; Crnojevic,
D.; Cropper, M.; Cunha, M.; Damiani, F.; David, M.; Delgado, A.;
Duffau, S.; Edvardsson, B.; Eldridge, J.; Enke, H.; Eriksson, K.;
Evans, N. W.; Eyer, L.; Famaey, B.; Fellhauer, M.; Ferreras, I.;
Figueras, F.; Fiorentino, G.; Flynn, C.; Folha, D.; Franciosini,
E.; Frasca, A.; Freeman, K.; Fremat, Y.; Friel, E.; Gaensicke, B.;
Gameiro, J.; Garzon, F.; Geier, S.; Geisler, D.; Gerhard, O.; Gibson,
B.; Gomboc, A.; Gomez, A.; Gonzalez-Fernandez, C.; Gonzalez Hernandez,
J.; Gosset, E.; Grebel, E.; Greimel, R.; Groenewegen, M.; Grundahl,
F.; Guarcello, M.; Gustafsson, B.; Hadrava, P.; Hatzidimitriou, D.;
Hambly, N.; Hammersley, P.; Hansen, C.; Haywood, M.; Heber, U.; Heiter,
U.; Held, E.; Helmi, A.; Hensler, G.; Herrero, A.; Hill, V.; Hodgkin,
S.; Huelamo, N.; Huxor, A.; Ibata, R.; Jackson, R.; de Jong, R.;
Jonker, P.; Jordan, S.; Jordi, C.; Jorissen, A.; Katz, D.; Kawata,
D.; Keller, S.; Kharchenko, N.; Klement, R.; Klutsch, A.; Knude,
J.; Koch, A.; Kochukhov, O.; Kontizas, M.; Koubsky, P.; Lallement,
R.; de Laverny, P.; van Leeuwen, F.; Lemasle, B.; Lewis, G.; Lind,
K.; Lindstrom, H. P. E.; Lobel, A.; Lopez Santiago, J.; Lucas, P.;
Ludwig, H.; Lueftinger, T.; Magrini, L.; Maiz Apellaniz, J.; Maldonado,
J.; Marconi, G.; Marino, A.; Martayan, C.; Martinez-Valpuesta, I.;
Matijevic, G.; McMahon, R.; Messina, S.; Meyer, M.; Miglio, A.;
Mikolaitis, S.; Minchev, I.; Minniti, D.; Moitinho, A.; Momany, Y.;
Monaco, L.; Montalto, M.; Monteiro, M. J.; Monier, R.; Montes, D.;
Mora, A.; Moraux, E.; Morel, T.; Mowlavi, N.; Mucciarelli, A.; Munari,
U.; Napiwotzki, R.; Nardetto, N.; Naylor, T.; Naze, Y.; Nelemans, G.;
Okamoto, S.; Ortolani, S.; Pace, G.; Palla, F.; Palous, J.; Parker, R.;
Penarrubia, J.; Pillitteri, I.; Piotto, G.; Posbic, H.; Prisinzano,
L.; Puzeras, E.; Quirrenbach, A.; Ragaini, S.; Read, J.; Read, M.;
Reyle, C.; De Ridder, J.; Robichon, N.; Robin, A.; Roeser, S.; Romano,
D.; Royer, F.; Ruchti, G.; Ruzicka, A.; Ryan, S.; Ryde, N.; Santos,
N.; Sanz Forcada, J.; Sarro Baro, L. M.; Sbordone, L.; Schilbach, E.;
Schmeja, S.; Schnurr, O.; Schoenrich, R.; Scholz, R. -D.; Seabroke, G.;
Sharma, S.; De Silva, G.; Smith, M.; Solano, E.; Sordo, R.; Soubiran,
C.; Sousa, S.; Spagna, A.; Steffen, M.; Steinmetz, M.; Stelzer, B.;
Stempels, E.; Tabernero, H.; Tautvaisiene, G.; Thevenin, F.; Torra,
J.; Tosi, M.; Tolstoy, E.; Turon, C.; Walker, M.; Wambsganss, J.;
Worley, C.; Venn, K.; Vink, J.; Wyse, R.; Zaggia, S.; Zeilinger, W.;
Zoccali, M.; Zorec, J.; Zucker, D.; Zwitter, T.; Gaia-ESO Survey Team
2012Msngr.147...25G Altcode:
The Gaia-ESO Public Spectroscopic Survey has begun and will obtain high
quality spectroscopy of some 100000 Milky Way stars, in the field and
in open clusters, down to magnitude 19, systematically covering all the
major components of the Milky Way. This survey will provide the first
homogeneous overview of the distributions of kinematics and chemical
element abundances in the Galaxy. The motivation, organisation and
implementation of the Gaia-ESO Survey are described, emphasising the
complementarity with the ESA Gaia mission. Spectra from the very first
observing run of the survey are presented.
---------------------------------------------------------
Title: Lithium and sodium in the globular cluster <ASTROBJ>M
4</ASTROBJ>. Detection of a Li-rich dwarf star: preservation or
pollution?
Authors: Monaco, L.; Villanova, S.; Bonifacio, P.; Caffau, E.; Geisler,
D.; Marconi, G.; Momany, Y.; Ludwig, H. -G.
2012A&A...539A.157M Altcode: 2011arXiv1108.0138M
Context. The abundance inhomogeneities of light elements observed
in globular clusters (GCs), and notably the ubiquitous Na-O
anti-correlation, are generally interpreted as evidence that GCs
comprise several generations of stars. There is an on-going debate
as to the nature of the stars, which produce the inhomogeneous
elements, and investigating the behavior of several elements is
a way to shed new light on this problem. <BR /> Aims: We aim at
investigating the Li and Na content of the GC M 4, that is known to
have a well defined Na-O anti-correlation. <BR /> Methods: We obtained
moderate resolution (R = 17 000-18 700) spectra for 91 main sequence
(MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at
the FLAMES/VLT ESO facility. Using model atmospheres analysis we
measured lithium and sodium abundances. <BR /> Results: We detect
a weak Li-Na anti-correlation among un-evolved MS stars. One star
in the sample, # 37934, shows the remarkably high lithium abundance
A(Li) = 2.87, compatible with current estimates of the primordial
lithium abundance. <BR /> Conclusions: The shallow slope found
for the Li-Na anti-correlation suggests that lithium is produced
in parallel to sodium. This evidence, coupled with its sodium-rich
nature, suggests that the high lithium abundance of star # 37934 may
originate by pollution from a previous generations of stars. The
recent detection of a Li-rich dwarf of pollution origin in the
globular cluster NGC 6397 may also point in this direction. Still,
no clear cut evidence is available against a possible preservation
of the primordial lithium abundance for star # 37934. <P />Based on
observations taken at ESO VLT Kueyen telescope (Cerro Paranal, Chile,
program: 085.D-0537A).Table A.1 is available in electronic form at
<A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Oxygen in the Universe
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Foreword
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54....1S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Appendix A : The atomic physics of oxygen
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54..319S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of the solar near-surface layers with the CO5BOLD,
MURaM, and Stagger codes
Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron,
R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M.
2012A&A...539A.121B Altcode: 2012arXiv1201.1103B
Context. Radiative hydrodynamic simulations of solar and stellar surface
convection have become an important tool for exploring the structure and
gas dynamics in the envelopes and atmospheres of late-type stars and for
improving our understanding of the formation of stellar spectra. <BR
/> Aims: We quantitatively compare results from three-dimensional,
radiative hydrodynamic simulations of convection near the solar surface
generated with three numerical codes (CO<SUP>5</SUP>BOLD, MURaM,
and Stagger) and different simulation setups in order to investigate
the level of similarity and to cross-validate the simulations. <BR
/> Methods: For all three simulations, we considered the average
stratifications of various quantities (temperature, pressure, flow
velocity, etc.) on surfaces of constant geometrical or optical depth,
as well as their temporal and spatial fluctuations. We also compared
observables, such as the spatially resolved patterns of the emerging
intensity and of the vertical velocity at the solar optical surface
as well as the center-to-limb variation of the continuum intensity
at various wavelengths. <BR /> Results: The depth profiles of the
thermodynamical quantities and of the convective velocities as well as
their spatial fluctuations agree quite well. Slight deviations can be
understood in terms of differences in box size, spatial resolution
and in the treatment of non-gray radiative transfer between the
simulations. <BR /> Conclusions: The results give confidence in the
reliability of the results from comprehensive radiative hydrodynamic
simulations.
---------------------------------------------------------
Title: Chapter 4 : The Evolution of Oxygen in Galaxies
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54..255S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chapter 3 : Oxygen Production and Destruction
Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz,
S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig,
H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton,
Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret,
F.; Katsuma, M.; Zeippen, C. J.
2012EAS....54..187S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simulations of stellar convection with CO5BOLD
Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm,
S.; Schaffenberger, W.; Steiner, O.
2012JCoPh.231..919F Altcode: 2011arXiv1110.6844F
High-resolution images of the solar surface show a granulation
pattern of hot rising and cooler downward-sinking material - the
top of the deep-reaching solar convection zone. Convection plays a
role for the thermal structure of the solar interior and the dynamo
acting there, for the stratification of the photosphere, where most
of the visible light is emitted, as well as for the energy budget of
the spectacular processes in the chromosphere and corona. Convective
stellar atmospheres can be modeled by numerically solving the coupled
equations of (magneto)hydrodynamics and non-local radiation transport
in the presence of a gravity field. The CO5BOLD code described in this
article is designed for so-called "realistic" simulations that take
into account the detailed microphysics under the conditions in solar
or stellar surface layers (equation-of-state and optical properties of
the matter). These simulations indeed deserve the label "realistic"
because they reproduce the various observables very well - with only
minor differences between different implementations. The agreement
with observations has improved over time and the simulations are now
well-established and have been performed for a number of stars. Still,
severe challenges are encountered when it comes to extending these
simulations to include ideally the entire star or substellar object:
the strong stratification leads to completely different conditions in
the interior, the photosphere, and the corona. Simulations have to cover
spatial scales from the sub-granular level to the stellar diameter and
time scales from photospheric wave travel times to stellar rotation
or dynamo cycle periods. Various non-equilibrium processes have to be
taken into account. Last but not least, realistic simulations are based
on detailed microphysics and depend on the quality of the input data,
which can be the actual accuracy limiter. This article provides an
overview of the physical problem and the numerical solution and the
capabilities of CO5BOLD, illustrated with a number of applications.
---------------------------------------------------------
Title: <SUP>6</SUP>Li detection in metal-poor stars: can 3D model
atmospheres solve the second lithium problem?
Authors: Steffen, M.; Cayrel, R.; Caffau, E.; Bonifacio, P.; Ludwig,
H. -G.; Spite, M.
2012MSAIS..22..152S Altcode: 2012arXiv1206.2239S
The presence of <SUP>6</SUP>Li in the atmospheres of metal-poor
halo stars is usually inferred from the detection of a subtle extra
depression in the red wing of the <SUP>7</SUP>Li doublet line at
670.8 nm. However, as pointed out recently by \cite{Cayrel2007},
the intrinsic line asymmetry caused by convective flows in the
photospheres of cool stars is almost indistinguishable from the
asymmetry produced by a weak <SUP>6</SUP>Li blend on a (presumed)
symmetric <SUP>7</SUP>Li profile. Previous determinations of the
<SUP>6</SUP>Li/ <SUP>7</SUP>Li isotopic ratio based on 1D model
atmospheres, ignoring the convection-induced line asymmetry, must
therefore be considered as upper limits. By comparing synthetic
1D LTE and 3D non-LTE line profiles of the <SUP>i</SUP>Li 670.8 nm
feature, we quantify the differential effect of the convective line
asymmetry on the derived <SUP>6</SUP>Li abundance as a function of
effective temperature, gravity, and metallicity. As expected, we
find that the asymmetry effect systematically reduces the resulting
<SUP>6</SUP>Li/<SUP>7</SUP>Li ratios. Depending on the stellar
parameters, the 3D-1D offset in <SUP>6</SUP>Li/<SUP>7</SUP>Li ranges
between -0.005 and -0.020. When this purely theoretical correction is
taken into account for the \cite{A2006} sample of stars, the number of
significant <SUP>6</SUP>Li detections decreases from 9 to 5 (2sigma
criterion), or from 5 to 2 (3sigma criterion). <P />We also present
preliminary results of a re-analysis of high-resolution, high S/N
spectra of individual metal-poor turn-off stars, to see whether the
second Lithium problem actually disappears when accounting properly for
convection and non-LTE line formation in 3D stellar atmospheres. Out
of 8 stars, HD 84937 seems to be the only significant (2sigma )
detection of <SUP>6</SUP>Li. In view of our results, the existence of
a <SUP>6</SUP>Li plateau appears questionable.
---------------------------------------------------------
Title: 3D Model Atmospheres of Red Giant Stars
Authors: Ludwig, Hans-Günter; Steffen, Matthias
2012ASSP...26..125L Altcode: 2011arXiv1108.4554L; 2012rgps.book..125L
We provide a brief overview of the modelling of the atmospheres of
red giant stars with the 3D radiation-hydrodynamics code CO5BOLD. We
emphasize aspects where 3D modelling provides additional insight
beyond standard hydrostatic 1D models, and comment on present modelling
challenges.
---------------------------------------------------------
Title: Radiation-Hydrodynamics Simulations of Cool Stellar and
Substellar Atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2011ASPC..448..855F Altcode: 2011csss...16..855F
In the atmospheres of brown dwarfs, not only molecules but much
larger and heavier "dust" particles can form. The latter should sink
under the influence of gravity into deeper layers and vanish from the
atmosphere, clearing it from condensable material. However, observed
spectra can only be reproduced by models assuming the presence of dust
and its resulting greenhouse effect in the visible layers. Apparently,
hydrodynamical mixing can counteract the gravitational settling. We
present new 2D and 3D radiation-hydrodynamics simulations with CO5BOLD
of the upper part of the convection zone and the atmosphere of cool
stars and brown dwarfs in a range of temperatures and gravities
that enable the formation of dust clouds in the visible layers. We
find that the differences between 2D and 3D models are remarkably
small. Lowering the gravity has a somewhat similar effect on the surface
intensity contrast as increasing the effective temperature. The biggest
uncertainties of the simulations come from approximations made in the
description of the dust chemistry. Global circulation and rotation
likely play an important role.
---------------------------------------------------------
Title: Radiation Hydrodynamics Simulations of Dust Clouds in the
Atmospheres of Substellar Objects
Authors: Freytag, B.; Allard, F.; Homeier, D.; Ludwig, H.; Steffen, M.
2011ASPC..450..125F Altcode:
The temperature structure and the motions in the atmospheres of cool
stars are affected by the underlying convection zone. The radiation
hydrodynamics code CO5BOLD has been developed to simulate (small patches
of the) convective surface layers of these stars. Updated opacity tables
based on PHOENIX data and a description for the formation, destruction,
advective transport, and settling of dust have made the code fit to
handle the conditions in brown dwarf atmospheres. Currently, objects
from 8500 K down to about 900 K have been simulated. Recently, incident
radiation has been included, allowing simulations with conditions
found on hot planets. In non-irradiated brown dwarf models we encounter
mixing by gravity waves and in the cooler models convection within the
clouds. The qualitative effects of incident radiation are surprisingly
small, as long as the effective temperature of the object stays well
below the dust condensation temperature. Beyond that point, there are
no layers where dust could form, anymore.
---------------------------------------------------------
Title: X-shooter Finds an Extremely Primitive Star
Authors: Caffau, E.; Bonifacio, P.; François, P.; Sbordone, L.;
Monaco, L.; Spite, M.; Spite, F.; Ludwig, H. -G.; Cayrel, R.; Zaggia,
S.; Hammer, F.; Randich, S.; Molaro, P.; Hill, V.
2011Msngr.146...28C Altcode:
Low-mass extremely metal-poor (EMP) stars hold the fossil record of
the chemical composition of the early phases of the Universe in their
atmospheres. Chemical analysis of such objects provides important
constraints on these early phases. EMP stars are rather rare objects:
to dig them out, large amounts of data have to be considered. We have
analysed stars from the Sloan Digital Sky Survey using an automatic
procedure and selected a sample of good candidate EMP stars, which we
observed with the spectrographs X-shooter and UVES. We could confirm
the low metallicity of our sample of stars, and we succeeded in finding
a record metal-poor star.
---------------------------------------------------------
Title: X-Shooter GTO: chemical analysis of a sample of EMP candidates
Authors: Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite,
F.; Zaggia, S.; Ludwig, H. -G.; Monaco, L.; Sbordone, L.; Cayrel,
R.; Hammer, F.; Randich, S.; Hill, V.; Molaro, P.
2011A&A...534A...4C Altcode: 2011arXiv1109.0992C
Context. Extremely metal-poor stars (EMP) are very rare objects that
hold in their atmospheres the fossil record of the chemical composition
of the early phases of Galactic evolution. Finding these objects and
determining their chemical composition provides important constraints
on these early phases. <BR /> Aims: Using a carefully designed selection
method, we chose a sample of candidate EMP stars from the low resolution
spectra of the Sloan Digital Sky Survey and observed them with X-Shooter
at the VLT to confirm their metallicities and determine abundances
for as many elements as possible. <BR /> Methods: The X-Shooter
spectra are analysed by means of one-dimensional, plane-parallel,
hydrostatic model atmospheres. Corrections for the granulation effects
are computed using CO5BOLD hydrodynamical simulations. <BR /> Results:
All the candidates are confirmed to be EMP stars, proving the efficiency
of our selection method within about 0.5 dex. The chemical composition
of this sample is compatible with those of brighter samples, suggesting
that the stars in the Galactic halo are well mixed. <BR /> Conclusions:
These observations show that it is feasible to observe, in a limited
amount of time, a large sample of about one hundred stars among EMP
candidates selected from the SDSS. Such a size of sample will allow us,
in particular, to confirm or refute the existence of a vertical drop
in the Galactic halo metallicity distribution function around [Fe/H] ~
-3.5. <P />Based on observations obtained at ESO Paranal Observatory,
GTO programme 086.D-0094.
---------------------------------------------------------
Title: An extremely primitive star in the Galactic halo
Authors: Caffau, Elisabetta; Bonifacio, Piercarlo; François, Patrick;
Sbordone, Luca; Monaco, Lorenzo; Spite, Monique; Spite, François;
Ludwig, Hans-G.; Cayrel, Roger; Zaggia, Simone; Hammer, François;
Randich, Sofia; Molaro, Paolo; Hill, Vanessa
2011Natur.477...67C Altcode: 2012arXiv1203.2612C
The early Universe had a chemical composition consisting of
hydrogen, helium and traces of lithium; almost all other elements
were subsequently created in stars and supernovae. The mass fraction
of elements more massive than helium, Z, is known as `metallicity'. A
number of very metal-poor stars has been found, some of which have a
low iron abundance but are rich in carbon, nitrogen and oxygen. For
theoretical reasons and because of an observed absence of stars
with Z<1.5×10<SUP>-5</SUP>, it has been suggested that low-mass
stars cannot form from the primitive interstellar medium until it
has been enriched above a critical value of Z, estimated to lie in
the range 1.5×10<SUP>-8</SUP> to 1.5×10<SUP>-6</SUP> (ref. 8),
although competing theories claiming the contrary do exist. (We
use `low-mass' here to mean a stellar mass of less than 0.8 solar
masses, the stars that survive to the present day.) Here we report the
chemical composition of a star in the Galactic halo with a very low Z
(<=6.9×10<SUP>-7</SUP>, which is 4.5×10<SUP>-5</SUP> times that
of the Sun) and a chemical pattern typical of classical extremely
metal-poor stars--that is, without enrichment of carbon, nitrogen
and oxygen. This shows that low-mass stars can be formed at very low
metallicity, that is, below the critical value of Z. Lithium is not
detected, suggesting a low-metallicity extension of the previously
observed trend in lithium depletion. Such lithium depletion implies
that the stellar material must have experienced temperatures above
two million kelvin in its history, given that this is necessary to
destroy lithium.
---------------------------------------------------------
Title: LTE model atmopsheres MARCS, ATLAS and CO5BOLD
Authors: Bonifacio, Piercarlo; Caffau, Elisabetta; Ludwig,
Hans-Guenter; Steffen, Matthias
2011arXiv1109.0717B Altcode:
In this talk we review the basic assumptions and physics covered by
classical 1D LTE model atmospheres. We will focus on ATLAS and MARCS
models of F-G-K stars and describe what resources are available through
the web, both in terms of codes and model-atmosphere grids. We describe
the advances made in hydrodynamical simulations of convective stellar
atmospheres with the CO5BOLD code and what grids and resources are
available, with a prospect of what will be available in the near future.
---------------------------------------------------------
Title: Photocentric and Photometric Variability of Red Supergiant
Stars
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
Y.; Spang, A.; Chesneau, O.
2011ASPC..445..169C Altcode:
Red supergiant stars are characterized by large convection-related
surface structures that cause surface inhomogeneities and shock
waves. We explore the impact of granulation on photocentric and
photometric variability using 3D simulations of convection with Co5BOLD
and the post-processing radiative transfer code OPTIM 3D to compute
spectra and intensity maps in the Gaia G band (325 - 1030 nm).
---------------------------------------------------------
Title: Solution to the problem of the surface gravity distribution
of cool DA white dwarfs from improved 3D model atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Bergeron,
P.; Freytag, B.
2011A&A...531L..19T Altcode: 2011arXiv1106.6007T
The surface gravities of cool (T<SUB>eff</SUB> < 13 000 K)
hydrogen-atmosphere DA white dwarfs, determined from spectroscopic
analyses, are found to be significantly higher than the canonical value
of log g ~ 8 expected for these stars. It was recently concluded that
a problem with the treatment of convective energy transport within
the framework of the mixing-length theory was the most plausible
explanation for this high-log g problem. We pursue the investigation
of this discrepancy by computing model spectra of cool convective
white dwarfs from a small sequence (11 300 K < T<SUB>eff</SUB>
< 12 800 K) of 3D hydrodynamical model atmospheres, which feature
a sophisticated treatment of convection and radiative transfer. Our
approach is to proceed with a differential analysis between 3D and
standard 1D models. We find that the 3D spectra predict significantly
lower surface gravities, with corrections of the right amplitude
as a function of effective temperature to obtain values of log g ~
8 on average. We conclude that the surface gravity distribution of
cool convective DA white dwarfs is much closer to that of hotter
radiative objects when using, for the treatment of the convection,
3D models instead of the mixing-length framework. <P />Figure 2 is
available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Radiative hydrodynamic simulations of red supergiant
stars. III. Spectro-photocentric variability, photometric variability,
and consequences on Gaia measurements
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
Y.; Spang, A.; Chesneau, O.
2011A&A...528A.120C Altcode: 2010arXiv1012.5234C
Context. It has been shown that convection in red supergiant stars (RSG)
gives rise to large granules that cause surface inhomogeneities and
shock waves in the photosphere. The resulting motion of the photocentre
(on time scales ranging from months to years) could possibly have
adverse effects on the parallax determination with Gaia. <BR /> Aims:
We explore the impact of the granulation on the photocentric and
photometric variability. We quantify these effects in order to better
characterise the error that could possibly alter the parallax. <BR
/> Methods: We use 3D radiative-hydrodynamics (RHD) simulations of
convection with CO5BOLD and the post-processing radiative transfer
code Optim3D to compute intensity maps and spectra in the Gaia G band
[325-1030 nm]. <BR /> Results: We provide astrometric and photometric
predictions from 3D simulations of RSGs that are used to evaluate
the possible degradation of the astrometric parameters of evolved
stars derived by Gaia. We show in particular from RHD simulations
that a supergiant like Betelgeuse exhibits a photocentric noise
characterised by a standard deviation of the order of 0.1 AU. The
number of bright giant and supergiant stars whose Gaia parallaxes
will be altered by the photocentric noise ranges from a few tens to
several thousands, depending on the poorly known relation between the
size of the convective cells and the atmospheric pressure scale height
of supergiants, and to a lower extent, on the adopted prescription
for galactic extinction. In the worst situation, the degradation
of the astrometric fit caused by this photocentric noise will be
noticeable up to about 5 kpc for the brightest supergiants. Moreover,
parallaxes of Betelgeuse-like supergiants are affected by an error of
the order of a few percents. We also show that the photocentric noise,
as predicted by the 3D simulation, does account for a substantial part
of the supplementary "cosmic noise" that affects Hipparcos measurements
of Betelgeuse and Antares.
---------------------------------------------------------
Title: First stars. XIV. Sulfur abundances in extremely metal-poor
stars
Authors: Spite, M.; Caffau, E.; Andrievsky, S. M.; Korotin, S. A.;
Depagne, E.; Spite, F.; Bonifacio, P.; Ludwig, H. -G.; Cayrel, R.;
François, P.; Hill, V.; Plez, B.; Andersen, J.; Barbuy, B.; Beers,
T. C.; Molaro, P.; Nordström, B.; Primas, F.
2011A&A...528A...9S Altcode: 2010arXiv1012.4358S
Context. Precise S abundances are important in the study of the
early chemical evolution of the Galaxy. In particular the site of the
formation remains uncertain because, at low metallicity, the trend
of this α-element versus [Fe/H] remains unclear. Moreover, although
sulfur is not bound significantly in dust grains in the ISM, it seems
to behave differently in DLAs and old metal-poor stars. <BR /> Aims:
We attempt a precise measurement of the S abundance in a sample of
extremely metal-poor stars observed with the ESO VLT equipped with
UVES, taking into account NLTE and 3D effects. <BR /> Methods: The
NLTE profiles of the lines of multiplet 1 of S I were computed with a
version of the program MULTI, including opacity sources from ATLAS9
and based on a new model atom for S. These profiles were fitted to
the observed spectra. <BR /> Results: We find that sulfur in EMP stars
behaves like the other α-elements, with [S/Fe] remaining approximately
constant below [Fe/H] = -3. However, [S/Mg] seems to decrease slightly
with increasing [Mg/H]. The overall abundance patterns of O, Na, Mg,
Al, S, and K are most closely matched by the SN model yields by Heger
& Woosley. The [S/Zn] ratio in EMP stars is solar, as also found
in DLAs. We derive an upper limit to the sulfur abundance [S/Fe] <
+0.5 for the ultra metal-poor star CS 22949-037. This, along with a
previously reported measurement of zinc, argues against the conjecture
that the light-element abundance pattern of this star (and by analogy,
the hyper iron-poor stars HE 0107-5240 and HE 1327-2326) would be
due to dust depletion. <P />Based on observations obtained with the
ESO Very Large Telescope at Paranal (Large Programme "First Stars",
ID 165, N-0276, P.I.: Cayrel.
---------------------------------------------------------
Title: Extremely metal-poor stars in SDSS fields
Authors: Bonifacio, P.; Caffau, E.; François, P.; Sbordone, L.;
Ludwig, H. -G.; Spite, M.; Molaro, P.; Spite, F.; Cayrel, R.; Hammer,
F.; Hill, V.; Nonino, M.; Randich, S.; Stelzer, B.; Zaggia, S.
2011AN....332..251B Altcode: 2011arXiv1101.3139B
Some insight on the first generation of stars can be obtained from
the chemical composition of their direct descendants, extremely
metal-poor stars (EMP), with metallicity less than or equal to 1/1000
of the solar metallicity. Such stars are exceedingly rare, the most
successful surveys, for this purpose, have so far provided only about
100 stars with 1/1 000 the solar metallicity and 4 stars with about
1/10 000 of the solar metallicity. The Sloan Digital Sky Survey has
the potential to provide a large number of candidates of extremely
low metallicity. X-shooter has the unique capability of performing the
necessary follow-up spectroscopy providing accurate metallicities and
abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr, ...) for
EMP candidates. We here report on the results for the first two stars
observed in the course of our Franco-Italian X-shooter GTO. The two
stars were targeted to be of metallicity around -3.0, the analysis of
the X-shooter spectra showed them to be of metallicity around -2.0,
but with a low α to iron ratio, which explains the underestimate of
the metallicity from the SDSS spectra. The efficiency of X-shooter
allows an in situ study of the outer halo, for the two stars studied
here we estimate distances of 3.9 and 9.1 kpc, these are likely the
most distant dwarf stars studied in detail to date. <P />Based on
spectra obtained with X-shooter at the 8.2-m Kueyen ESO telescope,
GTO programmes 085.D-0194 and 086.D.0094.
---------------------------------------------------------
Title: The solar photospheric abundance of zirconium
Authors: Caffau, E.; Faraggiana, R.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.
2011AN....332..128C Altcode: 2010arXiv1012.1038C
Zirconium (Zr), together with strontium and yttrium, is an important
element in the understanding of the Galactic nucleosynthesis. In
fact, the triad Sr-Y-Zr constitutes the first peak of s-process
elements. Despite its general relevance not many studies of the solar
abundance of Zr were conducted. We derive the zirconium abundance in
the solar photosphere with the same CO<SUP>5</SUP>BOLD hydrodynamical
model of the solar atmosphere that we previously used to investigate
the abundances of C-N-O. We review the zirconium lines available in
the observed solar spectra and select a sample of lines to determine
the zirconium abundance, considering lines of neutral and singly
ionised zirconium. We apply different line profile fitting strategies
for a reliable analysis of Zr lines that are blended by lines of other
elements. The abundance obtained from lines of neutral zirconium is very
uncertain because these lines are commonly blended and weak in the solar
spectrum. However, we believe that some lines of ionised zirconium are
reliable abundance indicators. Restricting the set to Zr II lines,
from the CO<SUP>5</SUP>BOLD 3D model atmosphere we derive A(Zr)
{=2.62± 0.06}, where the quoted error is the RMS line-to-line scatter.
---------------------------------------------------------
Title: Gravitational redshifts in main-sequence and giant stars
Authors: Pasquini, L.; Melo, C.; Chavero, C.; Dravins, D.; Ludwig,
H. -G.; Bonifacio, P.; de La Reza, R.
2011A&A...526A.127P Altcode: 2010arXiv1011.4635P
Context. Precise analyses of stellar radial velocities is able to
reveal intrinsic causes of the wavelength shifts of spectral lines
(other than Doppler shifts due to radial motion), such as gravitational
redshifts and convective blueshifts. <BR /> Aims: Gravitational
redshifts in solar-type main-sequence stars are expected to be some
500 m s<SUP>-1</SUP> greater than those in giants. We search for this
difference in redshifts among groups of open-cluster stars that share
the same average space motion and thus have the same average Doppler
shift. <BR /> Methods: We observed 144 main-sequence stars and cool
giants in the M 67 open cluster using the ESO FEROS spectrograph and
obtained radial velocities by means of cross-correlation with a spectral
template. Binaries and doubtful members were not analyzed, and average
spectra were created for different classes of stars. <BR /> Results:
The M 67 dwarf and giant radial-velocity distributions are each well
represented by Gaussian functions, which share the same apparent average
radial velocity to within ≃100 m s<SUP>-1</SUP>. In addition, dwarfs
in M 67 appear to be dynamically hotter (σ = 0.90 km s<SUP>-1</SUP>)
than giants (σ = 0.68 km s<SUP>-1</SUP>). <BR /> Conclusions: We fail
to detect any difference in the gravitational redshifts of giants and
MS stars. This is probably because of the differential wavelength
shifts produced by the different hydrodynamics of dwarf and giant
atmospheres. Radial-velocity differences measured between unblended
lines in averaged spectra vary with line-strength: stronger lines
are more blueshifted in dwarfs than in giants, apparently removing
any effect of the gravitational redshift. Synthetic high-resolution
spectra are computed from three dimensional (3D) hydrodynamic model
atmospheres for both giants and dwarfs, and synthetic wavelength
shifts obtained. In agreement with observations, 3D models predict
substantially smaller wavelength-shift differences than expected from
gravitational redshifts only. The procedures developed could be used
to test 3D models for different classes of stars, but will ultimately
require high-fidelity spectra for measurements of wavelength shifts in
individual spectral lines. <P />Based on observations collected at ESO,
La Silla, Chile, during the agreement between the Observatorio Nacional
at Rio de Janeiro and ESO.Table 1 is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A> and also at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/526/A127">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/526/A127</A>
---------------------------------------------------------
Title: Solar Chemical Abundances Determined with a CO5BOLD 3D Model
Atmosphere
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.;
Bonifacio, P.
2011SoPh..268..255C Altcode: 2010SoPh..tmp...66C; 2010arXiv1003.1190C
In the last decade, the photospheric solar metallicity as determined
from spectroscopy experienced a remarkable downward revision. Part
of this effect can be attributed to an improvement of atomic data and
the inclusion of NLTE computations, but also the use of hydrodynamical
model atmospheres seemed to play a role. This "decrease" with time of
the metallicity of the solar photosphere increased the disagreement
with the results from helioseismology. With a CO<SUP>5</SUP>BOLD 3D
model of the solar atmosphere, the CIFIST team at the Paris Observatory
re-determined the photospheric solar abundances of several elements,
among them C, N, and O. The spectroscopic abundances are obtained by
fitting the equivalent width and/or the profile of observed spectral
lines with synthetic spectra computed from the 3D model atmosphere. We
conclude that the effects of granular fluctuations depend on the
characteristics of the individual lines, but are found to be relevant
only in a few particular cases. 3D effects are not responsible for
the systematic lowering of the solar abundances in recent years. The
solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Velocities of M67 main-sequence
and giant stars (Pasquini+, 2011)
Authors: Pasquini, L.; Melo, C.; Chavero, C.; Dravins, D.; Ludwig,
H. -G.; Bonifacio, P.; de La, Reza R.
2011yCat..35260127P Altcode: 2011yCat..35269127P
We observed 144 main-sequence stars and cool giants in the M67 open
cluster using the ESO FEROS spectrograph and obtained radial velocities
by means of cross-correlation with a spectral template. Binaries and
doubtful members were not analyzed, and average spectra were created
for different classes of stars. <P />(1 data file).
---------------------------------------------------------
Title: Asteroseismology of solar-type stars with Kepler I: Data
analysis
Authors: Karoff, C.; Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.;
Garcia, R. A.; Houdek, G.; Metcalfe, T. S.; Molenda-Żakowicz, J.;
Monteiro, M. J. P. F. G.; Thompson, M. J.; Christensen-Dalsgaard, J.;
Gilliland, R. L.; Kjeldsen, H.; Basu, S.; Bedding, T. R.; Campante,
T. L.; Eggenberger, P.; Fletcher, S. T.; Gaulme, P.; Handberg, R.;
Hekker, S.; Martic, M.; Mathur, S.; Mosser, B.; Regulo, C.; Roxburgh,
I. W.; Salabert, D.; Stello, D.; Verner, G. A.; Belkacem, K.; Biazzo,
K.; Cunha, M. S.; Gruberbauer, M.; Guzik, J. A.; Kupka, F.; Leroy,
B.; Ludwig, H. -G.; Mathis, S.; Noels, A.; Noyes, R. W.; Roca Cortes,
T.; Roth, M.; Sato, K. H.; Schmitt, J.; Suran, M. D.; Trampedach,
R.; Uytterhoeven, K.; Ventura, R.
2010AN....331..972K Altcode: 2010arXiv1005.0507K
We report on the first asteroseismic analysis of solar-type stars
observed by Kepler. Observations of three G-type stars, made at
one-minute cadence during the first 33.5 days of science operations,
reveal high signal-to-noise solar-like oscillation spectra in all three
stars: About 20 modes of oscillation can clearly be distinguished
in each star. We discuss the appearance of the oscillation spectra,
including the presence of a possible signature of faculae, and the
presence of mixed modes in one of the three stars.
---------------------------------------------------------
Title: Cu I resonance lines in turn-off stars of NGC 6752 and NGC
6397. Effects of granulation from CO5BOLD models
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.
2010A&A...524A..96B Altcode: 2010arXiv1009.1848B
Context. Copper is an element whose interesting evolution with
metallicity is not fully understood. Observations of copper abundances
rely on a very limited number of lines, the strongest are the Cu I
lines of Mult. 1 at 324.7 nm and 327.3 nm which can be measured even at
extremely low metallicities. <BR /> Aims: We investigate the quality of
these lines as abundance indicators. <BR /> Methods: We measure these
lines in two turn-off (TO) stars in the Globular Cluster NGC 6752 and
two TO stars in the Globular Cluster NGC 6397 and derive abundances
with 3D hydrodynamical model atmospheres computed with the CO5BOLD
code. These abundances are compared to the Cu abundances measured in
giant stars of the same clusters, using the lines of Mult. 2 at 510.5
nm and 578.2 nm. <BR /> Results: The abundances derived from the lines
of Mult. 1 in TO stars differ from the abundances of giants of the same
clusters. This is true both using CO5BOLD models and using traditional
1D model atmospheres. The LTE 3D corrections for TO stars are large,
while they are small for giant stars. <BR /> Conclusions: The Cu I
resonance lines of Mult. 1 are not reliable abundance indicators. It
is likely that departures from LTE should be taken into account to
properly describe these lines, although it is not clear if these alone
can account for the observations. An investigation of these departures
is indeed encouraged for both dwarfs and giants. Our recommendation to
those interested in the study of the evolution of copper abundances is
to rely on the measurements in giants, based on the lines of Mult. 2. We
caution, however, that NLTE studies may imply a revision in all the
Cu abundances, both in dwarfs and giants. <P />Based on observations
made with the ESO Very Large Telescope at Paranal Observatory, Chile
(Programmes 71.D-0155, 75.D-0807, 76.B-0133).
---------------------------------------------------------
Title: Photocentric variability of red supergiant stars and
consequences on Gaia measurements
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
Y.; Spang, A.; Chesneau, O.
2010sf2a.conf..339C Altcode:
Red supergiant stars are characterized by large convection-related
surface structures that cause surface inhomogeneities and shock
waves. We explore the impact of granulation on the photocentric motion
using 3D simulations of convection with CO5BOLD and post-processing
radiative transfer code Optim3D to compute spectra and intensity maps
in the Gaia G band [325 -- 1030~nm]. We found that the Gaia parallax
for Betelgeuse-like supergiants are characterized by a systematic
error of a few percents.
---------------------------------------------------------
Title: The metal-poor end of the Spite plateau. I. Stellar parameters,
metallicities, and lithium abundances
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N. T.; González Hernández, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010A&A...522A..26S Altcode: 2010arXiv1003.4510S
Context. The primordial nature of the Spite plateau is at odds with
the WMAP satellite measurements, implying a primordial Li production
at least three times higher than observed. It has also been suggested
that A(Li) might exhibit a positive correlation with metallicity below
[Fe/H] ~ -2.5. Previous samples studied comprised few stars below
[Fe/H] = -3. <BR /> Aims: We present VLT-UVES Li abundances of 28
halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have
[Fe/H] <-3. <BR /> Methods: We determined stellar parameters and
abundances using four different T<SUB>eff</SUB> scales. The direct
infrared flux method was applied to infrared photometry. Hα wings were
fitted with two synthetic grids computed by means of 1D LTE atmosphere
models, assuming two different self-broadening theories. A grid of Hα
profiles was finally computed by means of 3D hydrodynamical atmosphere
models. The Li i doublet at 670.8 nm has been used to measure A(Li)
by means of 3D hydrodynamical NLTE spectral syntheses. An analytical
fit of A(Li)<SUB>3D, NLTE</SUB> as a function of equivalent width,
T<SUB>eff</SUB>, log g, and [Fe/H] has been derived and is made
available. <BR /> Results: We confirm previous claims that A(Li)
does not exhibit a plateau below [Fe/H] = -3. We detect a strong
positive correlation with [Fe/H] that is insensitive to the choice of
T<SUB>eff</SUB> estimator. From a linear fit, we infer a steep slope
of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance
of 2-3σ. The slopes derived using the four T<SUB>eff</SUB> estimators
are consistent to within 1σ. A significant slope is also detected
in the A(Li)-T<SUB>eff</SUB> plane, driven mainly by the coolest
stars in the sample (T<SUB>eff</SUB> < 6250), which appear to be
Li-poor. However, when we remove these stars the slope detected in
the A(Li)-[Fe/H] plane is not altered significantly. When the full
sample is considered, the scatter in A(Li) increases by a factor
of 2 towards lower metallicities, while the plateau appears very
thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at
<A(Li)<SUB>3D, NLTE</SUB>> = 2.199±0.086. <BR /> Conclusions:
The meltdown of the Spite plateau below [Fe/H] ~ -3 is established,
but its cause is unclear. If the primordial A(Li) were that derived
from standard BBN, it appears difficult to envision a single depletion
phenomenon producing a thin, metallicity independent plateau above
[Fe/H] = -2.8, and a highly scattered, metallicity dependent
distribution below. That no star below [Fe/H] = -3 lies above the
plateau suggests that they formed at plateau level and experienced
subsequent depletion. <P />Based on observations made with the ESO Very
Large Telescope at Paranal Observatory, Chile (Programmes 076.A-0463
and 077.D-0299).Full Table 3 is available in electronic form at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26</A>IDL
code (appendix) is only available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Joint Discussion 10: 3D views on cool stellar atmospheres -
theory meets observation
Authors: Nagendra, K. N.; Bonifacio, P.; Ludwig, H. -G.
2010HiA....15..331N Altcode:
Much of what we know about the chemical composition of the Universe
actually stems from the chemical composition of stars, which is often
deciphered from the spectra emerging from their atmospheres. Cool,
low-mass and long-living stars allow to study the evolution of
the Universe's chemistry from a time shortly after the big bang
until today. The observation and interpretation of stellar spectra
is a classical field in astronomy but is still undergoing vivid
developments. The enormous increase in available computational
resources opened-up possibilities which led to a revolution in the
degree of realism to which modelers can mimic Nature. High-resolution,
high-stability, high-efficiency spectrographs are now routinely
providing stellar spectra whose full information content can only be
exploited if a very much refined description of a stellar atmosphere
is at hand.
---------------------------------------------------------
Title: CO5BOLD: COnservative COde for the COmputation of COmpressible
COnvection in a BOx of L Dimensions with l=2,3
Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven;
Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner;
Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga;
Steiner, Oskar
2010ascl.soft11014F Altcode:
CO5BOLD - nickname COBOLD - is the short form of "COnservative
COde for the COmputation of COmpressible COnvection in a BOx of L
Dimensions with l=2,3". <P />It is used to model solar and stellar
surface convection. For solar-type stars only a small fraction of the
stellar surface layers are included in the computational domain. In
the case of red supergiants the computational box contains the entire
star. Recently, the model range has been extended to sub-stellar objects
(brown dwarfs). <P />CO5BOLD solves the coupled non-linear equations
of compressible hydrodynamics in an external gravity field together
with non-local frequency-dependent radiation transport. Operator
splitting is applied to solve the equations of hydrodynamics (including
gravity), the radiative energy transfer (with a long-characteristics
or a short-characteristics ray scheme), and possibly additional 3D
(turbulent) diffusion in individual sub steps. The 3D hydrodynamics
step is further simplified with directional splitting (usually). The 1D
sub steps are performed with a Roe solver, accounting for an external
gravity field and an arbitrary equation of state from a table. <P
/>The radiation transport is computed with either one of three
modules: <P />MSrad module: It uses long characteristics. The lateral
boundaries have to be periodic. Top and bottom can be closed or open
("solar module"). <P />LHDrad module: It uses long characteristics
and is restricted to an equidistant grid and open boundaries at all
surfaces (old "supergiant module"). <P />SHORTrad module: It uses
short characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces (new "supergiant module"). <P />The
code was supplemented with an (optional) MHD version [Schaffenberger
et al. (2005)] that can treat magnetic fields. There are also modules
for the formation and advection of dust available. The current version
now contains the treatment of chemical reaction networks, mostly used
for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and
hydrogen ionization [Leenaarts & Wedemeyer-Böhm (2005)], too. <P
/>CO5BOLD is written in Fortran90. The parallelization is done with
OpenMP directives.
---------------------------------------------------------
Title: Galactic evolution of oxygen. OH lines in 3D hydrodynamical
model atmospheres
Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.; Behara, N. T.; Freytag, B.
2010A&A...519A..46G Altcode: 2010arXiv1005.3754G
Context. Oxygen is the third most common element in the Universe. The
measurement of oxygen lines in metal-poor unevolved stars, in
particular near-UV OH lines, can provide invaluable information
about the properties of the Early Galaxy. <BR /> Aims: Near-UV OH
lines constitute an important tool to derive oxygen abundances in
metal-poor dwarf stars. Therefore, it is important to correctly model
the line formation of OH lines, especially in metal-poor stars, where
3D hydrodynamical models commonly predict cooler temperatures than
plane-parallel hydrostatic models in the upper photosphere. <BR />
Methods: We have made use of a grid of 52 3D hydrodynamical model
atmospheres for dwarf stars computed with the code CO<SUP>5</SUP>BOLD,
extracted from the more extended CIFIST grid. The 52 models cover
the effective temperature range 5000-6500 K, the surface gravity
range 3.5-4.5 and the metallicity range -3 < [Fe/H] < 0. <BR />
Results: We determine 3D-LTE abundance corrections in all 52 3D models
for several OH lines and ion{Fe}{i} lines of different excitation
potentials. These 3D-LTE corrections are generally negative and reach
values of roughly -1 dex (for the OH 3167 with excitation potential
of approximately 1 eV) for the higher temperatures and surface
gravities. <BR /> Conclusions: We apply these 3D-LTE corrections
to the individual O abundances derived from OH lines of a sample
the metal-poor dwarf stars reported in Israelian et al. (1998, ApJ,
507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et
al. (1999, AJ, 117, 492) by interpolating the stellar parameters of the
dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio
still keeps a similar trend as the 1D-LTE, i.e., increasing towards
lower [Fe/H] values. We applied 1D-NLTE corrections to 3D ion{Fe}{i}
abundances and still see an increasing [O/Fe] ratio towards lower
metallicites. However, the Galactic [O/Fe] ratio must be revisited
once 3D-NLTE corrections become available for OH and Fe lines for a
grid of 3D hydrodynamical model atmospheres.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Fe Abundances in metal-poor stars
(Sbordone+ 2010)
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N. T.; Gonzalez Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; Francois, P.; Hill, V.
2010yCat..35220026S Altcode: 2010yCat..35229026S
Line-by-line abundances for FeI and FeII lines used to estimate
metallicity and gravity for the program stars. The first column lists
the star name, then the ion (FeI or FeII) The the wavelength in nm,
the loggf, the measured EW (pm) and the derived abundance assuming the
four stellar parameter sets used in the article, respectively 3D, BA,
ALI and IRFM. <P />(3 data files).
---------------------------------------------------------
Title: Sulphur abundances in halo stars from multiplet 3 at 1045 nm
Authors: Caffau, E.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P.;
Spite, M.
2010AN....331..725C Altcode: 2010arXiv1003.4914C
Sulphur is a volatile α-element which is not locked into dust grains
in the interstellar medium (ISM). Hence, its abundance does not need
to be corrected for dust depletion when comparing the ISM to the
stellar atmospheres. The abundance of sulphur in the photosphere of
metal-poor stars is a matter of debate: according to some authors,
[S/Fe] versus [Fe/H] forms a plateau at low metallicity, while,
according to other studies, there is a large scatter or perhaps a
bimodal distribution. In metal-poor stars sulphur is detectable by its
lines of multiplet 1 at 920 nm, but this range is heavily contaminated
by telluric absorptions, and one line of the multiplet is blended by the
hydrogen Paschen ζ line. We study the possibility of using multiplet 3
(at 1045 nm) for deriving the sulphur abundance because this range,
now observable at the VLT with the infra-red spectrograph CRIRES,
is little contaminated by telluric absorption and not affected by
blends at least in metal-poor stars. We compare the abundances derived
from multiplets 1 and 3, taking into account NLTE corrections and
3D effects. Here we present the results for a sample of four stars,
although the scatter is less pronounced than in previous analysis,
we cannot find a plateau in [S/Fe], and confirm the scatter of the
sulphur abundance at low metallicity. <P />Using data from CRIRES at
the ESO-VLT, Programme 079.D-0434.
---------------------------------------------------------
Title: The solar photospheric abundance of carbon. Analysis of atomic
carbon lines with the CO5BOLD solar model
Authors: Caffau, E.; Ludwig, H. -G.; Bonifacio, P.; Faraggiana, R.;
Steffen, M.; Freytag, B.; Kamp, I.; Ayres, T. R.
2010A&A...514A..92C Altcode: 2010arXiv1002.2628C
Context. The analysis of the solar spectra using hydrodynamical
simulations, with a specific selection of lines, atomic data, and method
for computing deviations from local thermodynamical equilibrium, has
led to a downward revision of the solar metallicity, Z. We are using
the latest simulations computed with the CO5BOLD code to reassess
the solar chemical composition. Our previous analyses of the key
elements, oxygen and nitrogen, have not confirmed any extreme downward
revision of Z, as derived in other works based on hydrodynamical
models. <BR /> Aims: We determine the solar photospheric carbon
abundance with a radiation-hydrodynamical CO5BOLD model and compute
the departures from local thermodynamical equilibrium by using the
Kiel code. <BR /> Methods: We measured equivalent widths of atomic C
I lines on high-resolution, high signal-to-noise ratio solar atlases
of disc-centre intensity and integrated disc flux. These equivalent
widths were analysed with our latest solar 3D hydrodynamical simulation
computed with CO5BOLD. Deviations from local thermodynamic equilibrium
we computed in 1D with the Kiel code, using the average temperature
structure of the hydrodynamical simulation as a background model. <BR />
Results: Our recommended value for the solar carbon abundance relies
on 98 independent measurements of observed lines and is A(C)=8.50
± 0.06. The quoted error is the sum of statistical and systematic
errors. Combined with our recent results for the solar oxygen and
nitrogen abundances, this implies a solar metallicity of Z = 0.0154
and Z/X = 0.0211. <BR /> Conclusions: Our analysis implies a solar
carbon abundance that is about 0.1 dex higher than what was found in
previous analyses based on different 3D hydrodynamical computations. The
difference is partly driven by our equivalent width measurements
(we measure, on average, larger equivalent widths than the other work
based on a 3D model), in part because of the different properties of
the hydrodynamical simulations and the spectrum synthesis code. The
solar metallicity we obtain from the CO5BOLD analyses is in slightly
better agreement with the constraints of helioseismology than the
previous 3D abundance results.
---------------------------------------------------------
Title: Commission 36: Theory of Stellar Atmospheres
Authors: Asplund, Martin; Puls, Joachim; Landstreet, John; Allende
Prieto, Carlos; Ayres, Thomas; Berdyugina, Svetlana; Gustafsson,
Bengt; Hubeny, Ivan; Ludwig, Hans Günter; Mashonkina, Lyudmila;
Randich, Sofia
2010IAUTB..27..197A Altcode:
The members of the Commission 36 Organizing Committee attending the IAU
General Assembly in Rio de Janeiro met for a business session on August
7. Both members from the previous (2006-2009) and the new (2009-2012)
Organizing Committee partook in the discussions. Past president John
Landstreet described the work he had done over the past three years
in terms of supporting proposed conferences on the topic. He has
also spent significant amount of time establishing an updated mailing
list of all >350 members of the commission, which is unfortunately
not provided automatically by the IAU. Such a list is critical for a
rapid dissemination of information to the commission members and for
a correct and smooth running of elections of IAU officials. Everyone
present thanked John effusively for all of his hard work over the past
three years to stimulate a high level of activity within the discipline.
---------------------------------------------------------
Title: A 3D-NLTE study of the 670 nm solar lithium feature
Authors: Caffau, Elisabetta; Ludwig, Hans-Günter; Steffen, Matthias;
Bonifacio, Piercarlo
2010IAUS..268..329C Altcode:
We derive the 3D-NLTE lithium abundance in the solar photosphere from
the Lii line at 670 nm as measured in several solar atlases. The Li
abundance is obtained from line profile fitting with 1D/3D-LTE/3D-NLTE
synthetic spectra, considering several possibilities for the
atomic parameters of the lines blending the Li feature. The 670 nm
spectral region shows considerable differences in the two available
disc-centre solar atlases, while the two integrated disc spectra are
very similar. We obtain A(Li)<SUB>3D-NLTE</SUB> = 1.03. The 1D-LTE
abundance is 0.07 dex smaller. The line-lists giving the best fit
for the Sun may fail for other stars, while some line-lists fail to
reproduce the solar profile satisfactorily. We need a better knowledge
of the atomic parameters of the lines blending the Li feature in order
to be able to reproduce both the solar spectrum and the spectra of
other stars. An improved line-list is also required to derive reliable
estimates of the isotopic Li ratio in solar-metallicity stars.
---------------------------------------------------------
Title: The role of convection, overshoot, and gravity waves for the
transport of dust in M dwarf and brown dwarf atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2010A&A...513A..19F Altcode: 2010arXiv1002.3437F
Context. Observationally, spectra of brown dwarfs indicate the presence
of dust in their atmospheres while theoretically it is not clear what
prevents the dust from settling and disappearing from the regions of
spectrum formation. Consequently, standard models have to rely on ad
hoc assumptions about the mechanism that keeps dust grains aloft in
the atmosphere. <BR /> Aims: We apply hydrodynamical simulations to
develop an improved physical understanding of the mixing properties of
macroscopic flows in M dwarf and brown dwarf atmospheres, in particular
of the influence of the underlying convection zone. <BR /> Methods: We
performed two-dimensional radiation hydrodynamics simulations including
a description of dust grain formation and transport with the CO5BOLD
code. The simulations cover the very top of the convection zone and
the photosphere including the dust layers for a sequence of effective
temperatures between 900 K and 2800 K, all with log g = 5 assuming solar
chemical composition. <BR /> Results: Convective overshoot occurs in the
form of exponentially declining velocities with small scale heights, so
that it affects only the region immediately above the almost adiabatic
convective layers. From there on, mixing is provided by gravity waves
that are strong enough to maintain thin dust clouds in the hotter
models. With decreasing effective temperature, the amplitudes of the
waves become smaller but the clouds become thicker and develop internal
convective flows that are more efficient in transporting and mixing
material than gravity waves. The presence of clouds often leads to a
highly structured appearance of the stellar surface on short temporal
and small spatial scales (presently inaccessible to observations). <BR
/> Conclusions: We identify convectively excited gravity waves as an
essential mixing process in M dwarf and brown dwarf atmospheres. Under
conditions of strong cloud formation, dust convection is the dominant
self-sustaining mixing component.
---------------------------------------------------------
Title: Three carbon-enhanced metal-poor dwarf stars from the
SDSS. Chemical abundances from CO<SUP>5</SUP>BOLD 3D hydrodynamical
model atmospheres
Authors: Behara, N. T.; Bonifacio, P.; Ludwig, H. -G.; Sbordone, L.;
González Hernández, J. I.; Caffau, E.
2010A&A...513A..72B Altcode: 2010arXiv1002.1670B
Context. The origin of carbon-enhanced metal-poor stars enriched
with both s and r elements is highly debated. Detailed abundances of
these types of stars are crucial to understand the nature of their
progenitors. <BR /> Aims: The aim of this investigation is to study
in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS
J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital
Sky Survey. <BR /> Methods: Using high resolution VLT/UVES spectra
(R ~ 30 000) we determine abundances for Li, C, N, O, Na, Mg, Al,
Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We
made use of CO<SUP>5</SUP>BOLD 3D hydrodynamical model atmospheres
in the analysis of the carbon, nitrogen and oxygen abundances. NLTE
corrections for Ci and Oi lines were computed using the Kiel code. <BR
/> Results: We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s
stars. SDSS J1036+1212 belongs to the class CEMP-no/s, with enhanced
Ba, but deficient Sr, of which it is the third member discovered to
date. Radial-velocity variations have been observed in SDSS J1349-0229,
providing evidence that it is a member of a binary system. <BR />
Conclusions: The chemical composition of the three stars is generally
compatible with mass transfer from an AGB companion. However, many
details remain difficult to explain. Most notably of those are the
abundance of Li at the level of the Spite plateau in SDSS J1036+1212
and the large over-abundance of the pure r-process element Eu in all
three stars. <P />Based on observations obtained with the ESO Very
Large Telescope at Paranal Observatory, Chile (programmes 078.D-0217
and 383.D-0927).
---------------------------------------------------------
Title: Convection and <SUP>6</SUP>Li in the atmospheres of metal-poor
halo stars
Authors: Steffen, Matthias; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2010IAUS..268..215S Altcode: 2010arXiv1001.3274S
Based on 3D hydrodynamical model atmospheres computed with the
CO<SUP>5</SUP>BOLD code and 3D non-LTE (NLTE) line formation
calculations, we study the effect of the convection-induced line
asymmetry on the derived <SUP>6</SUP>Li abundance for a range in
effective temperature, gravity, and metallicity covering the stars
of the Asplund et al. (2006) sample. When the asymmetry effect
is taken into account for this sample of stars, the resulting
<SUP>6</SUP>Li/<SUP>7</SUP>Li ratios are reduced by about 1.5% on
average with respect to the isotopic ratios determined by Asplund et
al. (2006). This purely theoretical correction diminishes the number
of significant <SUP>6</SUP>Li detections from 9 to 4 (2σ criterion),
or from 5 to 2 (3σ criterion). In view of this result the existence
of a <SUP>6</SUP>Li plateau appears questionable. A careful reanalysis
of individual objects by fitting the observed lithium 6707 Å doublet
both with 3D NLTE and 1D LTE synthetic line profiles confirms that the
inferred <SUP>6</SUP>Li abundance is systematically lower when using
3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with
unquestionable <SUP>6</SUP>Li detection do exist even if analyzed in
3D-NLTE, the most prominent example being HD 84937.
---------------------------------------------------------
Title: The Asteroseismic Potential of Kepler: First Results for
Solar-Type Stars
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García,
R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz,
J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.;
Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki,
W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.;
Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt,
H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan,
G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.;
Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe,
R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic,
M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.;
Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.;
Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.;
Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi,
D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.;
Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.;
Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss,
L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev,
A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.;
Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes,
R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.;
Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre,
V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.;
Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A.
2010ApJ...713L.169C Altcode: 2010arXiv1001.0506C
We present preliminary asteroseismic results from Kepler on three G-type
stars. The observations, made at one-minute cadence during the first
33.5 days of science operations, reveal high signal-to-noise solar-like
oscillation spectra in all three stars: about 20 modes of oscillation
may be clearly distinguished in each star. We discuss the appearance of
the oscillation spectra, use the frequencies and frequency separations
to provide first results on the radii, masses, and ages of the stars,
and comment in the light of these results on prospects for inference
on other solar-type stars that Kepler will observe.
---------------------------------------------------------
Title: The metal-poor end of the Spite plateau: gravity sensitivity
of the Hα wings fitting.
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010IAUS..268..355S Altcode:
We recently presented (Sbordone et al., 2009a) the largest sample to
date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and
Turn-Off (TO) stars. One of the most crucial aspects in estimating Li
abundances is the T<SUB>eff</SUB> determination, since the Li I 670.8
nm doublet is highly temperature sensitive. In this short contribution
we concentrate on the T<SUB>eff</SUB> determination based on Hα wings
fitting, and on its sensitivity to the chosen stellar gravity.
---------------------------------------------------------
Title: Main-sequence and sub-giant stars in the globular cluster
NGC 6397: The complex evolution of the lithium abundance
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.;
Zaggia, S.
2010IAUS..268..257G Altcode: 2009arXiv0912.4105G
Thanks to the high multiplex and efficiency of Giraffe at the VLT
we have been able for the first time to observe the Li I doublet in
the Main Sequence stars of a globular cluster. At the same time we
observed Li in a sample of Sub-Giant stars of the same B-V colour. <P
/>Our final sample is composed of 84 SG stars and 79 MS stars. In
spite of the fact that SG and MS span the same temperature range we
find that the equivalent widths of the Li I doublet in SG stars are
systematically larger than those in MS stars, suggesting a higher Li
content among SG stars. This is confirmed by our quantitative analysis
carried out making use of 1D hydrostatic plane-parallel models and
3D hydrodynamical simulations of the stellar atmospheres. <P />We
derived the effective temperatures of stars in our the sample from Hα
fitting. Theoretical profiles were computed using 3D hydrodynamical
simulations and 1D ATLAS models. Therefore, we are able to determined
1D and 3D-based effective temperatures. We then infer Li abundances
taking into account non-local thermodynamical equilibrium effects when
using both 1D and 3D models. <P />We find that SG stars have a mean
Li abundance higher by 0.1 dex than MS stars. This result is obtained
using both 1D and 3D models. We also detect a positive slope of Li
abundance with effective temperature, the higher the temperature the
higher the Li abundance, both for SG and MS stars, although the slope
is slightly steeper for MS stars. These results provide an unambiguous
evidence that the Li abundance changes with evolutionary status. <P
/>The physical mechanisms responsible for this behaviour are not yet
clear, and none of the existing models seems to describe accurately
these observations. Based on these conclusions, we believe that the
cosmological lithium problem still remains an open question.
---------------------------------------------------------
Title: <SUP>6</SUP>Li in metal-poor halo stars: real or spurious?
Authors: Steffen, M.; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2010IAUS..265...23S Altcode: 2009arXiv0910.5917S
The presence of convective motions in the atmospheres of metal-poor
halo stars leads to systematic asymmetries of the emergent spectral
line profiles. Since such line asymmetries are very small, they can be
safely ignored for standard spectroscopic abundance analysis. However,
when it comes to the determination of the <SUP>6</SUP>Li/<SUP>7</SUP>Li
isotopic ratio, q(Li)=n(<SUP>6</SUP>Li)/n(<SUP>7</SUP>Li), the
intrinsic asymmetry of the <SUP>7</SUP>Li line must be taken into
account, because its signature is essentially indistinguishable from
the presence of a weak <SUP>6</SUP>Li blend in the red wing of the
<SUP>7</SUP>Li line. In this contribution we quantity the error of the
inferred <SUP>6</SUP>Li/<SUP>7</SUP>Li isotopic ratio that arises if
the convective line asymmetry is ignored in the fitting of the λ6707
Å lithium blend. Our conclusion is that <SUP>6</SUP>Li/<SUP>7</SUP>Li
ratios derived by Asplund et al. (2006), using symmetric line profiles,
must be reduced by typically Δq(Li) ≈ 0.015. This diminishes the
number of certain <SUP>6</SUP>Li detections from 9 to 4 stars or less,
casting some doubt on the existence of a <SUP>6</SUP>Li plateau.
---------------------------------------------------------
Title: Can we trust elemental abundances derived in late-type giants
with the classical 1D stellar atmosphere models?
Authors: Kučinskas, A.; Dobrovolskas, V.; Ivanauskas, A.; Ludwig,
H. -G.; Caffau, E.; Blaževičius, K.; Klevas, J.; Prakapavičius, D.
2010IAUS..265..209K Altcode: 2009arXiv0910.3397K
We compare the abundances of various chemical species as derived
with 3D hydrodynamical and classical 1D stellar atmosphere codes in
a late-type giant characterized by T<SUB>eff</SUB> =3640 K, log g =
1.0, [M/H]= 0.0. For this particular set of atmospheric parameters the
3D-1D abundance differences are generally small for neutral atoms and
molecules but they may reach up to 0.3-0.4 dex in case of ions. The
3D-1D differences generally become increasingly more negative at
higher excitation potentials and are typically largest in the optical
wavelength range. Their sign can be both positive and negative, and
depends on the excitation potential and wavelength of a given spectral
line. While our results obtained with this particular late-type giant
model suggest that 1D stellar atmosphere models may be safe to use
with neutral atoms and molecules, care should be taken if they are
exploited with ions.
---------------------------------------------------------
Title: Detailed analyses of three neutron-capture-rich carbon-enhanced
metal-poor stars
Authors: Behara, N. T.; Bonifacio, P.; Ludwig, H. -G.; Sbordone, L.;
González Hernández, J. I.; Caffau, E.
2010IAUS..265..122B Altcode: 2009arXiv0909.0180B
Approximately 20% of very metal-poor stars ([Fe/H] < -2.0)
are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are
referred to as carbon-enhanced metal-poor (CEMP) stars. We present a
chemical abundance analysis based on high resolution spectra acquired
with UVES at the VLT of three dwarf CEMP stars: SDSS J1349-0229, SDSS
J0912+0216 and SDSS J1036+1212. These very metal-poor stars, with
[Fe/H] < -2.5, were selected from our ongoing survey of extremely
metal-poor dwarf candidates from the SDSS. <P />Among these CEMPs,
SDSS J1349-0229 has been identified as a carbon star ([C/O] >
+1.0). First and second peak s-process elements, as well as second
peak r-process elements have been detected in all stars. In addition,
elements from the third r-process peak were detected in one of the
stars, SDSS J1036+1212. We present the abundance results of these
stars in the context of neutron-capture nucleosynthesis theories.
---------------------------------------------------------
Title: Solar abundances and 3D model atmospheres
Authors: Ludwig, Hans-Günter; Caffau, Elisabetta; Steffen, Matthias;
Bonifacio, Piercarlo; Freytag, Bernd; Cayrel, Roger
2010IAUS..265..201L Altcode: 2009arXiv0911.4248L
We present solar photospheric abundances for 12 elements from optical
and near-infrared spectroscopy. The abundance analysis was conducted
employing 3D hydrodynamical (CO<SUP>5</SUP>BOLD) as well as standard
1D hydrostatic model atmospheres. We compare our results to others
with emphasis on discrepancies and still lingering problems, in
particular exemplified by the pivotal abundance of oxygen. We argue
that the thermal structure of the lower solar photosphere is very
well represented by our 3D model. We obtain an excellent match of
the observed center-to-limb variation of the line-blanketed continuum
intensity, also at wavelengths shortward of the Balmer jump.
---------------------------------------------------------
Title: The metal-poor end of the Spite plateau
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
2010IAUS..265...75S Altcode:
We present the largest sample available to date of lithium abundances in
extremely metal poor (EMP) Halo dwarfs. Four T<SUB>eff</SUB> estimators
are used, including IRFM and Hα wings fitting against 3D hydrodynamical
synthetic profiles. Lithium abundances are computed by means of 1D and
3D-hydrodynamical NLTE computations. Below [Fe/H]~-3, a strong positive
correlation of A(Li) with [Fe/H] appears, not influenced by the choice
of the Teff estimator. A linear fit finds a slope of about 0.30 dex in
A(Li) per dex in [Fe/H], significant to 2-3 σ, and consistent within
1 σ among all the T<SUB>eff</SUB> estimators. The scatter in A(Li)
increases significantly below [Fe/H]~-3. Above, the plateau lies at
<A(Li)<SUB>3D, NLTE</SUB>> = 2.199 ± 0.086. If the primordial
A(Li) is the one derived from standard Big Bang Nucleosynthesis
(BBN), it appears difficult to envision a single depletion phenomenon
producing a thin, metallicity independent plateau above [Fe/H] = -2.8,
and a highly scattered, metallicity dependent distribution below.
---------------------------------------------------------
Title: Accuracy of spectroscopy-based radioactive dating of stars
Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Bonifacio, P.;
Sbordone, L.
2010A&A...509A..84L Altcode: 2009arXiv0911.4251L
Context. Combined spectroscopic abundance analyses of stable and
radioactive elements can be applied for deriving stellar ages. The
achievable precision depends on factors related to spectroscopy,
nucleosynthesis, and chemical evolution. <BR /> Aims: We quantify the
uncertainties arising from the spectroscopic analysis, and compare these
to the other error sources. <BR /> Methods: We derive formulae for the
age uncertainties arising from the spectroscopic abundance analysis,
and apply them to spectroscopic and nucleosynthetic data compiled
from the literature for the Sun and metal-poor stars. <BR /> Results:
We obtained ready-to-use analytic formulae of the age uncertainty for
the cases of stable+unstable and unstable+unstable chronometer pairs,
and discuss the optimal relation between to-be-measured age and mean
lifetime of a radioactive species. Application to the literature
data indicates that, for a single star, the achievable spectroscopic
accuracy is limited to about ±20% for the foreseeable future. At
present, theoretical uncertainties in nucleosynthesis and chemical
evolution models form the precision bottleneck. For stellar clusters,
isochrone fitting provides a higher accuracy than radioactive dating,
but radioactive dating becomes competitive when applied to many cluster
members simultaneously, reducing the statistical errors by a factor
√{N}. <BR /> Conclusions: Spectroscopy-based radioactive stellar
dating would benefit from improvements in the theoretical understanding
of nucleosynthesis and chemical evolution. Its application to clusters
can provide strong constraints for nucleosynthetic models.
---------------------------------------------------------
Title: The CoRoT target HD 49933 . II. Comparison of theoretical
mode amplitudes with observations
Authors: Samadi, R.; Ludwig, H. -G.; Belkacem, K.; Goupil, M. J.;
Benomar, O.; Mosser, B.; Dupret, M. -A.; Baudin, F.; Appourchaux,
T.; Michel, E.
2010A&A...509A..16S Altcode: 2009arXiv0910.4037S
Context. The seismic data obtained by CoRoT for the star HD 49933 enable
us for the first time to measure directly the amplitudes and linewidths
of solar-like oscillations for a star other than the Sun. From those
measurements it is possible, as was done for the Sun, to constrain
models of the excitation of acoustic modes by turbulent convection. <BR
/> Aims: We compare a stochastic excitation model described in Paper
I with the asteroseismology data for HD 49933, a star that is rather
metal poor and significantly hotter than the Sun. <BR /> Methods:
Using the seismic determinations of the mode linewidths detected by
CoRoT for HD 49933 and the theoretical mode excitation rates computed
in Paper I for the specific case of HD 49933, we derive the expected
surface velocity amplitudes of the acoustic modes detected in HD
49933. Using a calibrated quasi-adiabatic approximation relating
the mode amplitudes in intensity to those in velocity, we derive the
expected values of the mode amplitude in intensity. <BR /> Results:
Except at rather high frequency, our amplitude calculations are within
1-σ error bars of the mode surface velocity spectrum derived with
the HARPS spectrograph. The same is found with respect to the mode
amplitudes in intensity derived for HD 49933 from the CoRoT data. On
the other hand, at high frequency (ν ⪆ 1.9 mHz), our calculations
depart significantly from the CoRoT and HARPS measurements. We show
that assuming a solar metal abundance rather than the actual metal
abundance of the star would result in a larger discrepancy with the
seismic data. Furthermore, we present calculations which assume the
“new” solar chemical mixture to be in better agreement with the
seismic data than those that assumed the “old” solar chemical
mixture. <BR /> Conclusions: These results validate in the case of a
star significantly hotter than the Sun and α Cen A the main assumptions
in the model of stochastic excitation. However, the discrepancies seen
at high frequency highlight some deficiencies of the modelling, whose
origin remains to be understood. We also show that it is important
to take the surface metal abundance of the solar-like pulsators into
account. <P />The CoRoT space mission, launched on December 27 2006,
has been developped and is operated by CNES, with the contribution of
Austria, Belgium, Brasil, ESA, Germany and Spain.
---------------------------------------------------------
Title: The CoRoT target HD 49933 . I. Effect of the metal abundance
on the mode excitation rates
Authors: Samadi, R.; Ludwig, H. -G.; Belkacem, K.; Goupil, M. J.;
Dupret, M. -A.
2010A&A...509A..15S Altcode: 2009arXiv0910.4027S
Context. Solar-like oscillations are stochastically excited by turbulent
convection at the surface layers of the stars. <BR /> Aims: We study the
role of the surface metal abundance on the efficiency of the stochastic
driving in the case of the CoRoT target HD 49933. <BR /> Methods:
We compute two 3D hydrodynamical simulations representative - in
effective temperature and gravity - of the surface layers of the CoRoT
target HD 49933, a star that is rather metal poor and significantly
hotter than the Sun. One 3D simulation has a solar metal abundance, and
the other has a surface iron-to-hydrogen, [Fe/H], abundance ten times
smaller. For each 3D simulation we match an associated global 1D model,
and we compute the associated acoustic modes using a theoretical model
of stochastic excitation validated in the case of the Sun and α Cen
A. <BR /> Results: The rate at which energy is supplied per unit time
into the acoustic modes associated with the 3D simulation with [Fe/H] =
-1 is found to be about three times smaller than those associated with
the 3D simulation with [Fe/H] = 0. As shown here, these differences
are related to the fact that low metallicity implies surface layers
with a higher mean density. In turn, a higher mean density favors
smaller convective velocities and hence less efficient driving of the
acoustic modes. <BR /> Conclusions: Our result shows the importance of
taking the surface metal abundance into account in the modeling of the
mode driving by turbulent convection. A comparison with observational
data is presented in a companion paper using seismic data obtained for
the CoRoT target HD 49933. <P />The CoRoT space mission, launched on
December 27, 2006, has been developped and is operated by CNES, with
the contribution of Austria, Belgium, Brasil, ESA, Germany and Spain.
---------------------------------------------------------
Title: Chemical abundances in metal-poor giants: limitations imposed
by the use of classical 1D stellar atmosphere models
Authors: Dobrovolskas, V.; Kucinskas, A.; Ludwig, H. G.; Caffau, E.;
Klevas, J.; Prakapavicius, D.
2010nuco.confE.288D Altcode: 2010arXiv1010.2507D; 2010PoS...100E.288D
In this work we have used 3D hydrodynamical (CO5BOLD) and 1D hydrostatic
(LHD) stellar atmosphere models to study the importance of convection
and horizontal temperature inhomogeneities in stellar abundance work
related to late-type giants. We have found that for a number of key
elements, such as Na, Mg, Si, Ca, Ti, Fe, Ni, Zn, Ba, Eu, differences
in abundances predicted by 3D and 1D models are typically minor (<
0.1 dex) at solar metallicity. However, at [M/H] = -3 they become
larger and reach to -0.5...-0.8 dex. In case of neutral atoms and fixed
metallicity, the largest abundance differences were obtained for the
spectral lines with lowest excitation potential, while for ionized
species the largest 3D-1D abundance differences were found for lines
of highest excitation potential. The large abundance differences at
low metallicity are caused by large horizontal temperature fluctuations
and lower mean temperature in the outer layers of the 3D hydrodynamical
model compared with its 1D counterpart.
---------------------------------------------------------
Title: Lithium abundances of main-sequence and subgiant stars in
the globular cluster NGC 6397
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.;
Zaggia, S.
2010IAUS..266..407G Altcode: 2009arXiv0910.2305G
We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large
Telescope (VLT). Using these observations, we have been able (for the
first time) to observe the Lii doublet in the main-sequence (MS) stars
of a globular cluster. We also observed Li in a sample of subgiant (SG)
stars of the same B - V colour. Our final sample is composed of 84 SG
and 79 MS stars. In spite of the fact that SG and MS stars span the same
temperature range, we find that the equivalent widths of the Lii doublet
in SG stars are systematically greater than in MS stars, suggesting a
higher Li content among SG stars. This is confirmed by our quantitative
analysis, which makes use of both 1D and 3D model atmospheres. We find
that SG stars show, on average, a higher Li abundance, by 0.1 dex, than
MS stars. We also detect a positive slope of Li abundance with effective
temperature: the higher the temperature the higher the Li abundance,
both for SG and MS stars, although the slope is slightly steeper for MS
stars. These results provide unambiguous evidence that the Li abundance
changes with evolutionary state. The physical mechanisms that contribute
to this are not yet clear, since none of the proposed models seem to
describe accurately the observations. Whether such a mechanism can
explain the cosmological lithium problem is still an open question.
---------------------------------------------------------
Title: Perspectives for Determining Stellar Surface Parameters
Authors: Ludwig, H. -G.
2010EAS....45..251L Altcode: 2011EAS....45..251L
I present ideas of Gaia's impact on the determination of the properties
of stars primarily connected to the study of their atmospheres. This
mainly relates to effective temperatures, gravities and high-fidelity
chemical abundances obtained by combining envisioned Gaia measurements
with ground-based spectroscopy ranging from single objects to
well-selected stellar populations. I further discuss the impact of
Gaia on the study of the kinematics of atmospheric flows.
---------------------------------------------------------
Title: Sulfur in the globular clusters 47 Tuc and NGC 6752
Authors: Sbordone, L.; Chieffi, A.; Limongi, M.; Caffau, E.; Ludwig,
H. -G.; Bonifacio, P.
2010IAUS..266..537S Altcode:
The light elements Li, O, Na, Al, and Mg are known to show star-to-star
variations in the globular clusters 47 Tuc and NGC 6752. We have
investigated the behavior of the α element sulfur, for which no
previous measurements exist in any Galactic globular cluster. We
used high-resolution UVES spectra of Si multiplet 1 around 923 nm,
and determined S abundances by means of ATLAS static plano-parallel
models. NLTE corrections were applied and 3D corrections were also
computed from co5bold 3D hydrodynamical models. Sulfur has been measured
in four subgiant stars in NGC 6752, leading to an average value of
[S/Fe] = +0.49 ± 0.15 dex, consistent with what is observed in field
stars of similar metallicity. In 47 Tuc, we measured S in four turnoff
(TO) and five subgiant (SG) stars, for an average value of [S/Fe] =
0.18 ± 0.14 dex. While the measurement errors are consistent with a
constant value among all cluster stars analyzed, we detected a highly
significant correlation with sodium abundance, as well as a tentative
one with silicon. The sulfur-sodium correlation is difficult to explain
in terms of nucleosynthesis. Given its high statistical significance,
it is also difficult to dismiss it as fortuitous. Until better data for
more stars are available, the question as to its origin remains open.
---------------------------------------------------------
Title: Local stars formed at z>10: a sample extracted from the SDSS
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. G.
2010nuco.confE.294S Altcode: 2010arXiv1009.5210S; 2010PoS...100E.294S
As the Universe emerged from its initial hot and dense phase, its
chemical composition was extremely simple, being limited to stable H
and He isotopes, and traces of Li. The first stars that formed had such
initial composition. However, they quickly began to produce a whole
array of heavier nuclei, polluting the interstellar medium. While none
among these first stars has been detected to date, an increasing sample
exists of their direct descendant, stars with heavy elements content of
the order of 1/1000 of the solar value, or less. In most cases, such
stars should have formed at redshift of about 10 or beyond, and their
chemical composition can provide crucial constraints to the nature of
the very first stars. Extremely metal poor (EMP) stars are exceedingly
rare. We used the low resolution spectra obtained by the Sloan Digital
Sky Survey (SDSS) to search for EMP candidates: results of VLT-UVES
high resolution follow-up for 16 of them is presented here. A newly
developed automatic abundance analysis and parameter determination
code, MyGIsFOS, has been employed to analyze the detailed chemical
abundances of such stars.
---------------------------------------------------------
Title: Gaia spectroscopy: processing, performances and scientific
returns
Authors: Katz, D.; Cropper, M.; Meynadier, F.; Jean-Antoine, A.;
Allende Prieto, C.; Baker, S.; Benson, K.; Berthier, J.; Bigot, L.;
Blomme, R.; Boudreault, S.; Chemin, L.; Crifo, F.; Damerdji, Y.; David,
M.; David, P.; Delle Luche, C.; Dolding, C.; Frémat, Y.; Gerbier,
N.; Gerssen, J.; Gómez, A.; Gosset, E.; Guerrier, A.; Guy, L.;
Hall, D.; Hestroffer, D.; Huckle, H.; Jasniewicz, G.; Ludwig, H. -G.;
Martayan, C.; Morel, T.; Nguyen, A. -T.; Ocvirk, P.; Parr, C.; Royer,
F.; Sartoretti, P.; Seabroke, G.; Simon, E.; Smith, M.; Soubiran, C.;
Steinmetz, M.; Thévenin, F.; Turon, C.; Udry, S.; Veltz, L.; Viala, Y.
2010EAS....45..189K Altcode: 2011EAS....45..189K
During the five years of the mission, the Gaia spectrograph, the Radial
Velocity Spectrometer (RVS) will repeatedly survey the celestial sphere
down to magnitude V ~ 17-18. This talk presents: (i) the system which
is currently developed within the Gaia Data Processing and Analysis
Consortium (DPAC) to reduce and calibrate the spectra and to derive the
radial and rotational velocities, (ii) the RVS expected performances
and (iii) scientific returns.
---------------------------------------------------------
Title: Radial Velocity Standard Stars for the Gaia RVS
Authors: Jasniewicz, G.; Crifo, F.; Soubiran, C.; Hestroffer, D.;
Siebert, A.; Veltz, L.; Bigot, L.; Chemin, L.; David, P.; Guerrier,
A.; Katz, D.; Ludwig, H. -G.; Richard, P.; Royer, F.; Sartoretti,
P.; Udry, S.
2010EAS....45..195J Altcode: 2011EAS....45..195J
The calibration of the Radial Velocity Zero-Point (RVZP) of the Radial
Velocity Spectrometer (RVS) will be performed with the help of Radial
Velocity (RV) standard stars and asteroids. A full-sky list of RV
standard stars candidates has been built for this purpose within the
Development Unit (DU) 640 of the Gaia DPAC Consortium. A ground-based
campaign of RV observations has been initiated to eliminate unsuitable
candidates. Simulations have also been performed in order to estimate
in advance the number of RV standard stars and asteroids crossing the
RVS field of view. These standard stars and asteroids will also be
very helpful for controlling any harmful effect (especially the CCD
radiation damage) on calibration and RVZP during the lifetime of the
Gaia project. Kinematic RV are expected to be published at the end of
the project for the brightest stars, taking into account gravitational
redshift and convective shifts for the lines present in the RVS spectral
wavelength range.
---------------------------------------------------------
Title: 3D hydrodynamical CO5BOLD model atmospheres of late-type
giants: stellar abundances from molecular lines
Authors: Ivanauskas, A.; Kucinskas, A.; Ludwig, H. G.; Caffau, E.
2010nuco.confE.290I Altcode: 2010PoS...100E.290I; 2010arXiv1010.1722I
We investigate the influence of convection on the formation of
molecular spectral lines in the atmospheres of late-type giants. For
this purpose we use the 3D hydrodynamical CO5BOLD and classical 1D
LHD stellar atmosphere codes and synthesize a number of fictitious
lines belonging to a number of astrophysically relevant molecules, C2,
CH, CN, CO, NH, OH. We find that differences between the abundances
obtained from molecular lines using the 3D and 1D model atmospheres are
generally small at [M/H]=0.0, but they quickly increase at sub-solar
metallicities where for certain molecules they may reach -2.0
dex. The 3D-1D abundance differences show a significant dependence
on the spectral line parameters, such as wavelength and excitation
potential. Our comparison, therefore, reveals a complex interplay
between the spectral line formation and convection that can not be
properly accounted for with the classical 1D model atmospheres.
---------------------------------------------------------
Title: 3D simulations of M star atmosphere velocities and their
influence on molecular FeH lines
Authors: Wende, S.; Reiners, A.; Ludwig, H. -G.
2009A&A...508.1429W Altcode: 2009arXiv0910.3493W
Context: The measurement of line broadening in cool stars is in general
a difficult task. In order to detect slow rotation or weak magnetic
fields, an accuracy of 1 km s<SUP>-1</SUP> is needed. In this regime
the broadening from convective motion becomes important. We present
an investigation of the velocity fields in early to late M-type
star hydrodynamic models, and we simulate their influence on FeH
molecular line shapes. The M star model parameters range between
log{g} of 3.0-5.0 and effective temperatures from 2500 K to 4000
K.<BR /> Aims: Our aim is to characterize the T<SUB>eff</SUB>- and
log{g}-dependence of the velocity fields and express them in terms of
micro- and macro-turbulent velocities in the one dimensional sense. We
present a direct comparison between 3D hydrodynamical velocity fields
and 1D turbulent velocities. The velocity fields strongly affect the
line shapes of FeH, and it is our goal to give a rough estimate of
the log{g} and T<SUB>eff</SUB> parameter range in which 3D spectral
synthesis is necessary and where 1D synthesis suffices. We want to
distinguish between the velocity-broadening from convective motion and
the rotational- or Zeeman-broadening in M-type stars we are planning
to measure. For the latter, FeH lines are an important indicator.<BR
/> Methods: In order to calculate M-star structure models, we employ
the 3D radiative-hydrodynamics (RHD) code CO^5BOLD. The spectral
synthesis in these models is performed with the line synthesis code
LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian
standard deviations and project them onto the line of sight to include
geometrical and limb-darkening effects. The micro- and macro-turbulent
velocities are determined with the “curve of growth” method
and convolution with a Gaussian velocity profile, respectively. To
characterize the log{g} and T<SUB>eff</SUB> dependence of FeH lines,
the equivalent width, line width, and line depth are examined.<BR />
Results: The velocity fields in M-stars strongly depend on log{g}
and T<SUB>eff</SUB>. They become stronger with decreasing log{g} and
increasing T<SUB>eff</SUB>. The projected velocities from the 3D models
agree within 100 m s<SUP>-1</SUP> with the 1D micro- and macro-turbulent
velocities. The FeH line quantities systematically depend on log{g}
and T<SUB>eff</SUB>.<BR /> Conclusions: The influence of hydrodynamic
velocity fields on line shapes of M-type stars can well be reproduced
with 1D broadening methods. FeH lines turn out to provide a means to
measure log{g} and T<SUB>eff</SUB> in M-type stars. Since different
FeH lines all behave in a similar manner, they provide an ideal measure
for rotational and magnetic broadening.
---------------------------------------------------------
Title: GAIA RVS data reduction : the 6^{th} dimension
Authors: Meynadier, F.; Crifo, F.; Katz, D.; Thévenin, F.; Berthier,
J.; Bigot, L.; Delle Luche, C.; Doressoundiram, A.; Gomez, A.;
Guerrier, A.; Hestroffer, D.; Hubert, A. -M. .; Jasniewicz, G.;
Jean-Antoine, A.; Ludwig, H.; Martayan, C.; Nguyen, A. -T.; Ocvirk,
P.; Pichon, B.; Royer, F.; Sartoretti, P.; Siebert, A.; Soubiran,
C.; Turon, C.; Veltz, L.; Viala, Y.
2009sf2a.conf...63M Altcode:
This poster describes the current organisation of RVS data processing
among the Gaia-DPAC (Data Processing & Analysis Consortium),
with a particular focus on the French community's contribution.
---------------------------------------------------------
Title: Hydrodynamical simulations of convection-related stellar
micro-variability. II. The enigmatic granulation background of the
CoRoT target HD 49933
Authors: Ludwig, H. -G.; Samadi, R.; Steffen, M.; Appourchaux, T.;
Baudin, F.; Belkacem, K.; Boumier, P.; Goupil, M. -J.; Michel, E.
2009A&A...506..167L Altcode: 2009arXiv0905.2695L
Context: Local-box hydrodynamical model atmospheres provide
statistical information about a star's emergent radiation field
which allows one to predict the level of its granulation-related
micro-variability. Space-based photometry is now sufficiently
accurate to test model predictions. <BR />Aims: We aim to model the
photometric granulation background of HD 49933 as well as the Sun,
and compare the predictions to the measurements obtained by the
<sf>CoRoT</sf> and <sf>SOHO</sf> satellite
missions. <BR />Methods: We construct hydrodynamical model atmospheres
representing HD 49933 and the Sun, and use a previously developed
scaling technique to obtain the observable disk-integrated brightness
fluctuations. We further performed exploratory magneto-hydrodynamical
simulations to gauge the impact of small scale magnetic fields
on the synthetic light-curves. <BR />Results: We find that the
granulation-related brightness fluctuations depend on metallicity. We
obtain a satisfactory correspondence between prediction and observation
for the Sun, validating our approach. For HD 49933, we arrive at
a significant over-estimation by a factor of two to three in total
power. Locally generated magnetic fields are unlikely to be responsible,
otherwise existing fields would need to be rather strong to sufficiently
suppress the granulation signal. Presently suggested updates on the
fundamental stellar parameters do not improve the correspondence;
however, an ad-hoc increase of the HD 49933 surface gravity by about
0.2 dex would eliminate most of the discrepancy. <BR />Conclusions:
We diagnose a puzzling discrepancy between the predicted and observed
granulation background in HD 49933, with only rather ad-hoc ideas for
remedies at hand. <P />The <sf>CoRoT</sf> space mission,
launched on December 27th 2006, has been developed and is operated by
CNES, with the contribution of Austria, Belgium, Brazil, ESA, Germany
and Spain. CIFIST Marie Curie Excellence Team.
---------------------------------------------------------
Title: Theoretical amplitudes and lifetimes of non-radial solar-like
oscillations in red giants
Authors: Dupret, M. -A.; Belkacem, K.; Samadi, R.; Montalban, J.;
Moreira, O.; Miglio, A.; Godart, M.; Ventura, P.; Ludwig, H. -G.;
Grigahcène, A.; Goupil, M. -J.; Noels, A.; Caffau, E.
2009A&A...506...57D Altcode: 2009arXiv0906.3951D
Context: Solar-like oscillations have been observed in numerous red
giants from ground and from space. An important question arises:
could we expect to detect non-radial modes probing the internal
structure of these stars? <BR />Aims: We investigate under what physical
circumstances non-radial modes could be observable in red giants; what
would be their amplitudes, lifetimes and heights in the power spectrum
(PS)? <BR />Methods: Using a non-radial non-adiabatic pulsation
code including a non-local time-dependent treatment of convection,
we compute the theoretical lifetimes of radial and non-radial modes
in several red giant models. Next, using a stochastic excitation
model, we compute the amplitudes of these modes and their heights in
the PS. <BR />Results: Distinct cases appear. Case A corresponds to
subgiants and stars at the bottom of the ascending giant branch. Our
results show that the lifetimes of the modes are mainly proportional to
the inertia I, which is modulated by the mode trapping. The predicted
amplitudes are lower for non-radial modes. But the height of the peaks
in the PS are of the same order for radial and non-radial modes as
long as they can be resolved. The resulting frequency spectrum is
complex. Case B corresponds to intermediate models in the red giant
branch. In these models, the radiative damping becomes high enough to
destroy the non-radial modes trapped in the core. Hence, only modes
trapped in the envelope have significant heights in the PS and could
be observed. The resulting frequency spectrum of detectable modes is
regular for ℓ=0 and 2, but a little more complex for ℓ=1 modes
because of less efficient trapping. Case C corresponds to models
of even higher luminosity. In these models the radiative damping of
non-radial modes is even larger than in the previous case and only
radial and non-radial modes completely trapped in the envelope could be
observed. The frequency pattern is very regular for these stars. The
comparison between the predictions for radial and non-radial modes
is very different if we consider the heights in the PS instead of the
amplitudes. This is important as the heights (not the amplitudes) are
used as detection criterion. <P />CIFIST Marie Curie Excellence Team.
---------------------------------------------------------
Title: Lithium in the globular cluster NGC 6397. Evidence for
dependence on evolutionary status
Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.;
Steffen, M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel,
R.; Zaggia, S.
2009A&A...505L..13G Altcode: 2009arXiv0909.0983G
Context: Most globular clusters are believed to host a single
stellar population. They can thus be considered a good place to
study the Spite plateau and to search for possible evolutionary
modifications of the Li content. <BR />Aims: We want to determine the
Li content of subgiant (SG) and main sequence (MS) stars of the old,
metal-poor globular cluster NGC 6397. This work was aimed not only
at studying possible Li abundance variations but also to investigate
the cosmological Li discrepancy. <BR />Methods: Here, we present
FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in
NGC 6397 selected in a narrow range of B-V colour and, therefore,
effective temperatures. We determine both effective temperatures and
Li abundances using three-dimensional hydrodynamical model atmospheres
for all the MS and SG stars of the sample. <BR />Results: We find
a significant difference in the Li abundance between SG stars and
MS stars, the SG stars having an abundance higher by almost 0.1
dex on average. We also find a decrease in the lithium abundance
with decreasing effective temperature, both in MS and SG stars,
albeit with a significantly different slope for the two classes of
stars. This suggests that the lithium abundance in these stars is,
indeed, altered by some process, which is temperature-dependent. <BR
/>Conclusions: The lithium abundance pattern observed in NGC 6397 is
different from what is found among field stars, casting some doubt on
the use of globular cluster stars as representative of Population II
with respect to the lithium abundance. None of the available theories
of Li depletion appears to satisfactorily describe our observations. <P
/>Based on observations obtained with FLAMES/GIRAFFE at VLT Kueyen
8.2 m telescope in programme 079.D-0399(A). Table and Figs. 3-10 are
only available in electronic form at http://www.aanda.org Table 2
is available in electronic form at http://www.aanda.org and at the
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/L13
---------------------------------------------------------
Title: The Solar Photospheric Nitrogen Abundance: Determination with
3D and 1D Model Atmospheres
Authors: Maiorca, E.; Caffau, E.; Bonifacio, P.; Busso, M.; Faraggiana,
R.; Steffen, M.; Ludwig, H. -G.; Kamp, I.
2009PASA...26..345M Altcode: 2009arXiv0912.0375M
We present a new determination of the solar nitrogen abundance
making use of 3D hydrodynamical modelling of the solar photosphere,
which is more physically motivated than traditional static 1D
models. We selected suitable atomic spectral lines, relying on
equivalent width measurements already existing in the literature. For
atmospheric modelling we used the co <SUP>5</SUP> bold 3D radiation
hydrodynamics code. We investigated the influence of both deviations
from local thermodynamic equilibrium (non-LTE effects) and photospheric
inhomogeneities (granulation effects) on the resulting abundance. We
also compared several atlases of solar flux and centre-disc intensity
presently available. As a result of our analysis, the photospheric
solar nitrogen abundance is A(N) = 7.86 +/- 0.12.
---------------------------------------------------------
Title: Teff and log g dependence of velocity fields in M-stars
Authors: Wende, S.; Reiners, A.; Ludwig, H. -G.
2009AIPC.1171..323W Altcode: 2009arXiv0908.0820W
We present an investigation of velocity fields in early to late M-type
hydrodynamic stellar atmosphere models. These velocities will be
expressed in classical terms of micro- and macro-turbulent velocities
for usage in 1D spectral synthesis. The M-star model parameters range
between log g of 3.0-5.0 and Teff of 2500 K-4000 K. We characterize
the Teff- and log g-dependence of the hydrodynamical velocity fields
in these models with a binning method, and for the determination of
micro-turbulent velocities, the Curve of Growth method is used. The
macro-turbulent velocities are obtained by convolutions with Gaussian
profiles. Velocity fields in M-stars strongly depend on log g and
Teff. Their velocity amplitudes increase with decreasing log g and
increasing Teff. The 3D hydrodynamical and 1D macro-turbulent velocities
range from ~100 m/s for cool high gravity models to ~800 m/s-1000 m/s
for hot models or models with low log g. The micro-turbulent velocities
range in the order of ~100 m/s for cool models, to ~600 m/s for hot or
low log g models. Our M-star structure models are calculated with the
3D radiative-hydrodynamics (RHD) code CO<SUP>5</SUP>BOLD. The spectral
synthesis on these models is performed with the line synthesis code
LINFOR3D.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Lithium in NGC 6397 (Gonzalez
Hernandez+, 2009)
Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Steffen,
M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel, R.; Zaggia, S.
2009yCat..35059013G Altcode:
Photometric data of the dwarf and subgiant stars of the globular
cluster NGC 6397. We also provide the signal-to-noise of the spectra,
the 3D and 1D Halpha-based effective temperatures, 3D Li abundances,
and the equivalent widths and errors: dEWa: Error of the equivalent
width measurements estimated from a fitting routine that uses as
free parameters the velocity shift, the continuum location, and the
equivalent width of the Li line. dEWb: Error of the equivalent width
associated to the signal-to-noise ratio and the wavelength dispersion
of the spectra, derived using Cayrel's formula (Cayrel, 1988, IAU
Symp. 132: The Impact of Very High S/N Spectroscopy on Stellar Physics,
132, 345). <P />(1 data file).
---------------------------------------------------------
Title: Impact of granulation effects on the use of Balmer lines as
temperature indicators
Authors: Ludwig, H. -G.; Behara, N. T.; Steffen, M.; Bonifacio, P.
2009A&A...502L...1L Altcode: 2009arXiv0906.4697L
Context: Balmer lines serve as important indicators of stellar
effective temperatures in late-type stellar spectra. One of their
modelling uncertainties is the influence of convective flows on
their shape. <BR />Aims: We aim to characterize the influence of
convection on the wings of Balmer lines. <BR />Methods: We perform
a differential comparison of synthetic Balmer line profiles obtained
from 3D hydrodynamical model atmospheres and 1D hydrostatic standard
ones. The model parameters are appropriate for F, G, K dwarf and
subgiant stars of metallicity ranging from solar to 10<SUP>-3</SUP>
solar. <BR />Results: The shape of the Balmer lines predicted by 3D
models can never be exactly reproduced by a 1D model, irrespective of
its effective temperature. We introduce the concept of a 3D temperature
correction, as the effective temperature difference between a 3D model
and a 1D model which provides the closest match to the 3D profile. The
temperature correction is different for the different members of
the Balmer series and depends on the adopted mixing-length parameter
α<SUB>MLT</SUB> in the 1D model. Among the investigated models, the
3D correction ranges from -300 K to +300 K. Horizontal temperature
fluctuations tend to reduce the 3D correction. <BR />Conclusions:
Accurate effective temperatures cannot be derived from the wings of
Balmer lines, unless the effects of convection are properly accounted
for. The 3D models offer a physically well justified way of doing
so. The use of 1D models treating convection with the mixing-length
theory do not appear to be suitable for this purpose. In particular,
there are indications that it is not possible to determine a single
value of α<SUB>MLT</SUB> which will optimally reproduce the Balmer
lines for any choice of atmospheric parameters. The investigation
of a more extended grid and direct comparison with observed Balmer
profiles will be carried out in the near future. <P />Appendices is
only available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Sulfur in the globular clusters <ASTROBJ>47 Tucanae</ASTROBJ>
and <ASTROBJ>NGC 6752</ASTROBJ>
Authors: Sbordone, L.; Limongi, M.; Chieffi, A.; Caffau, E.; Ludwig,
H. -G.; Bonifacio, P.
2009A&A...503..121S Altcode: 2009arXiv0904.1417S
Context: The light elements Li, O, Na, Al, and Mg are known to
show star-to-star variations in the globular clusters <ASTROBJ>47
Tuc</ASTROBJ> and <ASTROBJ>NGC 6752</ASTROBJ>. Such variations are
interpreted as coming from processing in a previous generation of
stars. <BR />Aims: In this paper we investigate the abundances of
the α-element sulfur, for which no previous measurements exist. In
fact this element has not been investigated in any Galactic globular
cluster so far. The only globular cluster for which such measurements
are available is <ASTROBJ>Terzan 7</ASTROBJ>, which belongs to the
<ASTROBJ>Sgr dSph</ASTROBJ>. <BR />Methods: We use high-resolution
spectra of the S i Mult. 1, acquired with the UVES spectrograph at the
8.2 m VLT-Kueyen telescope, for turn-off and giant stars in the two
globular clusters. The spectra were analysed making use of ATLAS static
plane parallel model atmospheres and SYNTHE spectrum synthesis. We
also compute 3D corrections from CO^5BOLD hydrodynamic models and
apply corrections due to NLTE effects taken from the literature. <BR
/>Results: In the cluster NGC 6752 sulfur has been measured only in
four subgiant stars. We find no significant star-to-star scatter and a
mean <[S/Fe]> = +0.49 ± 0.15, consistent with what is observed in
field stars of the same metallicity. In the cluster 47 Tuc we measured
S in 4 turn-off and 5 subgiant stars with a mean <[S/Fe]> =
+0.18 ± 0.14. While this result is compatible with no star-to-star
scatter we notice a statistically significant correlation of the sulfur
abundance with the sodium abundance and a tentative correlation with
the silicon abundance. <BR />Conclusions: The sulfur-sodium correlation
is not easily explained in terms of nucleosynthesis. An origin due to
atomic diffusion can be easily dismissed. The correlation cannot be
easily dismissed either, in view of its statistical significance, until
better data for more stars is available. <P />Based on observations
made with the ESO VLT-Kueyen telescope at the Paranal Observatory,
Chile, in the course of the ESO-Large Programme 165.L-0263.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Extremely metal-poor turnoff
stars abundances (Bonifacio+, 2009)
Authors: Bonifacio, P.; Spite, M.; Cayrel, R.; Hill, V.; Spite,
F.; Francois, P.; Plez, B.; Ludwig, H. -G.; Caffau, E.; Molaro, P.;
Depagne, E.; Andersen, J.; Barbuy, B.; Beers, T. C.; Nordstroem, B.;
Primas, F.
2009yCat..35010519B Altcode:
The detailed chemical abundances of extremely metal-poor (EMP) stars
are key guides to understanding the early chemical evolution of the
Galaxy. Most existing data, however, treat giant stars that may have
experienced internal mixing later. We aim to compare the results for
giants with new, accurate abundances for all observable elements in
18 EMP turnoff stars. VLT/UVES spectra at ~45000 and S/N ~130 per
pixel (330-1000nm) are analysed with OSMARCS model atmospheres and
the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc,
Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. For Ca, Ni, Sr, and Ba, we find
excellent consistency with our earlier sample of EMP giants, at all
metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co
are ~0.2dex larger than in giants of similar metallicity. Mg and Si
abundances are ~0.2dex lower (the giant [Mg/Fe] values are slightly
revised), while Zn is again ~0.4dex higher than in giants of similar
[Fe/H] (6 stars only). For C, the dwarf/giant discrepancy could
possibly have an astrophysical cause, but for the other elements it
must arise from shortcomings in the analysis. Approximate computations
of granulation (3D) effects yield smaller corrections for giants than
for dwarfs, but suggest that this is an unlikely explanation, except
perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide
consistent abundances between dwarfs and giants, unlike the LTE results,
and would be highly desirable for the other discrepant elements as
well. Meanwhile, we recommend using the giant abundances as reference
data for Galactic chemical evolution models. <P />(3 data files).
---------------------------------------------------------
Title: First stars XII. Abundances in extremely metal-poor turnoff
stars, and comparison with the giants
Authors: Bonifacio, P.; Spite, M.; Cayrel, R.; Hill, V.; Spite, F.;
François, P.; Plez, B.; Ludwig, H. -G.; Caffau, E.; Molaro, P.;
Depagne, E.; Andersen, J.; Barbuy, B.; Beers, T. C.; Nordström, B.;
Primas, F.
2009A&A...501..519B Altcode: 2009arXiv0903.4174B
Context: The detailed chemical abundances of extremely metal-poor (EMP)
stars are key guides to understanding the early chemical evolution
of the Galaxy. Most existing data, however, treat giant stars that
may have experienced internal mixing later. <BR />Aims: We aim to
compare the results for giants with new, accurate abundances for all
observable elements in 18 EMP turnoff stars. <BR />Methods: VLT/UVES
spectra at R ~ 45 000 and S/N ~ 130 per pixel (λλ 330-1000 nm)
are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM
code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co,
Ni, Zn, Sr, and Ba. <BR />Results: For Ca, Ni, Sr, and Ba, we find
excellent consistency with our earlier sample of EMP giants, at all
metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and
Co are ~0.2 dex larger than in giants of similar metallicity. Mg
and Si abundances are ~0.2 dex lower (the giant [Mg/Fe] values are
slightly revised), while Zn is again ~0.4 dex higher than in giants
of similar [Fe/H] (6 stars only). <BR />Conclusions: For C, the
dwarf/giant discrepancy could possibly have an astrophysical cause,
but for the other elements it must arise from shortcomings in the
analysis. Approximate computations of granulation (3D) effects yield
smaller corrections for giants than for dwarfs, but suggest that this
is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE
computations for Na and Al provide consistent abundances between dwarfs
and giants, unlike the LTE results, and would be highly desirable for
the other discrepant elements as well. Meanwhile, we recommend using
the giant abundances as reference data for Galactic chemical evolution
models. <P />Based on observations obtained with the ESO Very Large
Telescope at Paranal Observatory, Chile (Large Programme “First
Stars”, ID 165.N-0276; P.I.: R. Cayrel, and Programme 078.B-0238;
P.I.: M. Spite). Appendices A-C are only available in electronic form
at http://www.aanda.org Table 7 is only available in electronic form
at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)
or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/519
---------------------------------------------------------
Title: The solar photospheric nitrogen abundance. Analysis of atomic
transitions with 3D and 1D model atmospheres
Authors: Caffau, E.; Maiorca, E.; Bonifacio, P.; Faraggiana, R.;
Steffen, M.; Ludwig, H. -G.; Kamp, I.; Busso, M.
2009A&A...498..877C Altcode: 2009arXiv0903.3406C
Context: In recent years, the solar chemical abundances have been
studied in considerable detail because of discrepant values of
solar metallicity inferred from different indicators, i.e., on the
one hand, the “sub-solar” photospheric abundances resulting
from spectroscopic chemical composition analyses with the aid of
3D hydrodynamical models of the solar atmosphere, and, on the other
hand, the high metallicity inferred by helioseismology. <BR />Aims:
After investigating the solar oxygen abundance using a CO^5BOLD 3D
hydrodynamical solar model in previous work, we undertake a similar
approach studying the solar abundance of nitrogen, since this element
accounts for a significant fraction of the overall solar metallicity,
Z. <BR />Methods: We used a selection of atomic spectral lines to
determine the solar nitrogen abundance, relying mainly on equivalent
width measurements in the literature. We investigate the influence on
the abundance analysis, of both deviations from local thermodynamic
equilibrium (“NLTE effects”) and photospheric inhomogeneities
(“granulation effects”). <BR />Results: We recommend use of a solar
nitrogen abundance of A(N) = 7.86 ± 0.12, whose error bar reflects
the line-to-line scatter. <BR />Conclusions: The solar metallicity
implied by the CO^5BOLD-based nitrogen and oxygen abundances is in the
range 0.0145≤ Z ≤ 0.0167. This result is a step towards reconciling
photospheric abundances with helioseismic constraints on Z. Our most
suitable estimates are Z=0.0156 and Z/X=0.0213.
---------------------------------------------------------
Title: Near-surface stellar magneto-convection: simulations for the
Sun and a metal-poor solar analog
Authors: Steffen, Matthias; Ludwig, H. -G.; Steiner, O.
2009IAUS..259..233S Altcode: 2009arXiv0902.2753S
We present 2D local box simulations of near-surface radiative
magneto-convection with prescribed magnetic flux, carried out with the
MHD version of the CO<SUP>5</SUP>BOLD code for the Sun and a solar-like
star with a metal-poor chemical composition (metal abundances reduced by
a factor 100, [M/H] = -2). The resulting magneto-hydrodynamical models
can be used to study the influence of the metallicity on the properties
of magnetized stellar atmospheres. A preliminary analysis indicates
that the horizontal magnetic field component tends to be significantly
stronger in the optically thin layers of metal-poor stellar atmospheres.
---------------------------------------------------------
Title: Observable properties of late-type giants predicted by 3D
hydrodynamical and 1D stellar atmosphere models
Authors: Kucinskas, A.; Ludwig, H. -G.; Ivanauskas, A.; Caffau, E.
2009IAUS..254P..37K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Halo chemistry and first stars. The chemical composition of
the matter in the early Galaxy, from C to Mg†
Authors: Spite, M.; Bonifacio, P.; Cayrel, R.; Spite, F.; Francois,
P.; Ludwig, H. G.; Caffau, E.; Andrievsky, S.; Barbuy, B.; Plez, B.;
Molaro, P.; Andersen, J.; Beers, T.; Depagne, E.; Nordström, B.;
Primas, F.
2009IAUS..254..349S Altcode:
From NLTE computations of the magnesium abundance in a sample
of extremely metal-poor giants we derive [Mg/Fe]=+0.7, leading to
[Al/Mg]=-0.80 and [Na/Mg]=-0.85 in the early Galaxy. The ratio [O/Mg]
should be near to the solar value. Measurements of nitrogen abundances
derived from the analysis of the NH band in eight more stars confirm
the large scatter of the ratios [N/Fe] and [N/O] in the early Galaxy.
---------------------------------------------------------
Title: Convective mixing and dust clouds in the atmospheres of
brown dwarfs
Authors: Freytag, Bernd; Allard, France; Ludwig, Hans-Günter; Homeier,
Derek; Steffen, Matthias; Sharp, Christopher
2009AIPC.1094..489F Altcode: 2009csss...15..489F
Observed spectra of brown dwarfs demonstrate that their atmospheres
are influenced by dust. To investigate the mechanism that controls
the formation and gravitational settling of dust grains as well as the
mixing of fresh condensable material into the atmosphere, we performed
2D radiation-hydrodynamics simulations with CO5BOLD. The models comprise
the upper part of the convection zone and the atmosphere with the
dust cloud layers. We find that direct convective overshoot does not
play a major role. Instead, the mixing in the clouds is controlled by
gravity waves.
---------------------------------------------------------
Title: Effective temperatures of cool metal-poor stars derived from
the analysis of 3D Balmer lines
Authors: Behara, N. T.; Ludwig, H. -G.; Steffen, M.; Bonifacio, P.
2009AIPC.1094..784B Altcode: 2009csss...15..784B
Balmer lines are recognized as accurate indicators of the effective
temperature of late-type stars. The influence of convection on
the shape of Balmer line profiles has been investigated using LTE
3D hydrodynamical model atmospheres and `classical' LTE 1D stellar
atmospheres, where convection is modeled within the simplistic picture
of mixing-length theory. Models and line profiles computed with the
CO<SUP>5</SUP>BOLD and Linfor3D codes have been used to determine
the effective temperatures of the Sun and three well known metal-poor
stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit
thus far for the solar Hα temperature using the Barklem theory. The
resulting (3D-1D) Teff biases related to the different treatment of
convection in the 1D and 3D models are presented.
---------------------------------------------------------
Title: Teff and log g dependence of FeH in M-dwarfs
Authors: Wende, S.; Reiners, A.; Ludwig, H. -G.
2009AIPC.1094..816W Altcode: 2008arXiv0812.2146W; 2009csss...15..816W
We present synthetic FeH band spectra in the z-filter range for several
M-dwarf models with log g = 3.0-5.0 [cgs] and Teff = 2800 K-3450
K. Our aim is to characterize convective velocities in M-dwarfs and to
give a rough estimate of the range in which 3D-atmosphere treatment is
necessary and where 1D-atmosphere models suffice for the interpretation
of molecular spectral features. This is also important in order
to distinguish between the velocity-broadening and the rotational-
or Zeeman-broadening. The synthetic spectra were calculated using 3D
CO5BOLD radiative-hydrodynamic (RHD) models and the line synthesis code
LINFOR3D. We used complete 3D-models and high resolution 3D spectral
synthesis for the detailed study of some well isolated FeH lines. The
FeH line strength shows a dependence on surface gravity and effective
temperature and could be employed to measure both quantities in M-type
objects. The line width is related to the velocity-field in the model
stars, which depends strongly on surface gravity. Furthermore, we
investigate the velocity-field in the 3D M-dwarf models together with
the related micro- and macro-turbulent velocities in the 1D case. We
also search for effects on the lineshapes.
---------------------------------------------------------
Title: Commission 36: Theory of Stellar Atmospheres
Authors: Landstreet, John D.; Asplund, Martin; Spite, Monique;
Balachandran, Suchitra B.; Berdyugina, Svetlana V.; Hauschildt, Peter
H.; Ludwig, Hans G.; Mashonkina, Lyudmila I.; Nagendra, K. N.; Puls,
Joachim; Randich, M. Sofia; Tautvaisiene, Grazina
2009IAUTA..27..222L Altcode:
Commission 36 covers the whole field of the physics of stellar
atmospheres. The scientific activity in this large subject has been very
intense during the last triennium and led to the publication of a large
number of papers, which makes a complete report quite impractical. We
have therefore decided to keep the format of the preceding report:
first a list of areas of current research, then Web links for obtaining
further information.
---------------------------------------------------------
Title: Micro- and macroturbulence derived from 3D hydrodynamical
stellar atmospheres .
Authors: Steffen, M.; Ludwig, H. -G.; Caffau, E.
2009MmSAI..80..731S Altcode: 2009arXiv0909.2831S
The theoretical prediction of micro- and macroturbulence (xi_mic
and xi_mac ) as a function of stellar parameters can be useful for
spectroscopic work based on 1D model atmospheres in cases where an
empirical determination of xi_mic is impossible due to a lack of
suitable lines and/or macroturbulence and rotational line broadening
are difficult to separate. In an effort to exploit the CIFIST 3D model
atmosphere grid for deriving the theoretical dependence of xi_mic and
xi_mac on effective temperature, gravity, and metallicity, we discuss
different methods to derive xi_mic from the numerical simulations,
and report first results for the Sun and Procyon. In both cases the
preliminary analysis indicates that the microturbulence found in the
simulations is significantly lower than in the real stellar atmospheres.
---------------------------------------------------------
Title: The ESO Large Programme “First Stars”
Authors: Bonifacio, P.; Andersen, J.; Andrievsky, S. M.; Barbuy, B.;
Beers, T. C.; Caffau, E.; Cayrel, R.; Depagne, E.; François, P.;
González Hernández, J. I.; Hansen, C. J.; Herwig, F.; Hill, V.;
Korotin, S. A.; Ludwig, H. -G.; Molaro, P.; Nordström, B.; Plez,
B.; Primas, F.; Sivarani, T.; Spite, F.; Spite, M.
2009ASSP....9...31B Altcode: 2008arXiv0801.1293B; 2009svlt.conf...31B
In ESO period 65 (April-September 2000) the large programme 165.N-0276,
led by Roger Cayrel, began making use of UVES at the Kueyen VLT
telescope. Known within the Team and outside as "First Stars", it was
aimed at obtaining high resolution, high signal-to-noise ratio spectra
in the range 320 nm-1000 nm for a large sample of extremely metal-poor
(EMP) stars identified from the HK objective prism survey [T.C. Beers,
G.W. Preston, S.A. Shectman in Astron. J. 90, 2089 (1985); T.C. Beers,
G.W. Preston, S.A. Shectman in Astron. J. 103, 1987 (1992)]. The goal
was to use these spectra to determine accurate atmospheric parameters
and chemical composition of these stars which are among the oldest
objects amenable to our detailed study. Although these stars are not
the first generation of stars they must be very close descendants of
the first generation. One may hope to gain insight on the nature of
the progenitors from detailed information on the descendants.
---------------------------------------------------------
Title: The CIFIST 3D model atmosphere grid.
Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Freytag, B.;
Bonifacio, P.; Kučinskas, A.
2009MmSAI..80..711L Altcode: 2009arXiv0908.4496L
Grids of stellar atmosphere models and associated synthetic spectra
are numerical products which have a large impact in astronomy due to
their ubiquitous application in the interpretation of radiation from
individual stars and stellar populations. 3D model atmospheres are
now on the verge of becoming generally available for a wide range
of stellar atmospheric parameters. We report on efforts to develop
a grid of 3D model atmospheres for late-type stars within the CIFIST
Team at Paris Observatory. The substantial demands in computational
and human labor for the model production and post-processing render
this apparently mundane task a challenging logistic exercise. At
the moment the CIFIST grid comprises 77 3D model atmospheres with
emphasis on dwarfs of solar and sub-solar metallicities. While the
model production is still ongoing, first applications are already
worked upon by the CIFIST Team and collaborators.
---------------------------------------------------------
Title: Accounting for convective blue-shifts in the determination
of absolute stellar radial velocities.
Authors: Allende Prieto, C.; Koesterke, L.; Ramírez, I.; Ludwig,
H. -G.; Asplund, M.
2009MmSAI..80..622A Altcode: 2009arXiv0909.0470A
For late-type non-active stars, gravitational redshifts and convective
blueshifts are the main source of biases in the determination of
radial velocities. If ignored, these effects can introduce systematic
errors of the order of ∼ 0.5 km s<SUP>-1</SUP>. We demonstrate that
three-dimensional hydrodynamical simulations of solar surface convection
can be used to predict the convective blue-shifts of weak spectral
lines in solar-like stars to ∼ 0.070 km s<SUP>-1</SUP>. Using accurate
trigonometric parallaxes and stellar evolution models, the gravitational
redshifts can be constrained with a similar uncertainty, leading to
absolute radial velocities accurate to ∼ 0.1 km s<SUP>-1</SUP>.
---------------------------------------------------------
Title: Effects of granulation on neutral copper resonance lines in
metal-poor stars
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.
2009MmSAI..80..739B Altcode: 2009arXiv0910.4730B
We make use of three dimensional hydrodynamical simulations to
investigate the effects of granulation on the Cu I lines of Mult. 1
in the near UV, at 324.7 nm and 327.3 nm. These lines remain strong
even at very low metallicity and provide the opportunity to study the
chemical evolution of Cu in the metal-poor populations. We find very
strong granulation effects on these lines. In terms of abundances the
neglect of such effects can lead to an overestimate of the A(Cu) by
as much as 0.8 dex in dwarf stars. Comparison of our computations with
stars in the metal-poor Globular Clusters NGC 6752 and NGC 6397, show
that there is a systematic discrepancy between the copper abundances
derived from Mult. 2 in TO stars and those derived in giant stars of the
same cluster from the lines of Mult. 2 at at 510.5 nm and 587.2 nm. We
conclude that the Cu I resonance lines are not reliable indicators of
Cu abundance and we believe that an investigations of departures from
LTE is mandatory to make use of these lines.
---------------------------------------------------------
Title: Solar abundances and granulation effects
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.
2009MmSAI..80..643C Altcode: 2009arXiv0910.4733C
The solar abundances have undergone a major downward revision
in the last decade, reputedly as a result of employing 3D
hydrodynamical simulations to model the inhomogeneous structure of
the solar photosphere. The very low oxygen abundance advocated by
\citet{asplund04}, A(O)=8.66, together with the downward revision of
the carbon and nitrogen abundances, has created serious problems for
solar models to explain the helioseismic measurements. <P />In an
effort to contribute to the dispute we have re-derived photospheric
abundances of several elements independently of previous analysis. We
applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation
of the solar granulation as well as different 1D model atmospheres
for the line by line spectroscopic abundance determinations. The
analysis is based on both standard disc-centre and disc-integrated
spectral atlases; for oxygen we acquired in addition spectra at
different heliocentric angles. The derived abundances are the result
of equivalent width and/or line profile fitting of the available atomic
lines. We discuss the different granulation effects on solar abundances
and compare our results with previous investigations. According to
our investigations hydrodynamical models are important in the solar
abundance determination, but are not responsible for the recent downward
revision in the literature of the solar metallicity.
---------------------------------------------------------
Title: NLTE Abundances of Sodium, Magnesium and Barium in the Globular
Clusters M10 and M71
Authors: Mishenina, T. V.; Kučinskas, A.; Andrievsky, S. M.; Korotin,
S. A.; Dobrovolskas, V.; Ivanauskas, A.; Caffau, E.; Ludwig, H. -G.;
Steffen, M.; Sperauskas, J.; Klochkova, V. G.; Panchuk, V. E.
2009BaltA..18..193M Altcode: 2009OAst...18..193M
We derive NLTE abundances of Na, Mg and Ba in four late-type giants
belonging to globular clusters M10 and M71. The obtained relative
[Na/Fe] ratios, which were measured only in M10, are positive, with
the average value [Na/Fe] = +0.3. The ratios [Mg/Fe] in both clusters
are supersolar, +0.15 to +0.28, while [Ba/Fe] scatter between --0.14
and +0.09. Differences between the NLTE abundances derived in this
work and those obtained in LTE by Mishenina et al. (2003) are small,
typically within ±0.1 dex. We also perform numerical simulations with
the CO<SUP>5</SUP>BOLD 3D hydrodynamical stellar atmosphere code to
investigate the influence of convection on the formation of spectral
lines used in our NLTE study. For this purpose we use a model of
late-type giant with T<SUB></SUB> eff = 4020 K, log g = 1.0, [M/H] =
--1.0 and find that for Na, Mg and Ba the 3D--1D abundance corrections
are below ∼ 0.02 dex. However, their size strongly depends on the
value of microturbulent velocity used with the 1D model.
---------------------------------------------------------
Title: Simulations of dust clouds in the atmospheres of substellar
objects. Theory toddles after observations
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2009MmSAI..80..670F Altcode:
The atmospheres of brown dwarfs allow the formation of dust grains
and their rain-out into deeper, invisible layers. However, observed
spectra of L dwarfs can only be reproduced when static 1D models
account for dust formation and its resulting greenhouse effect in the
visible layers. Time-dependent hydrodynamical processes can mix up the
material giving rise to complex unsteady weather phenomena on these
objects. We performed radiation hydrodynamics simulations in two and
three dimensions of the atmospheres of brown dwarfs with CO5BOLD,
including a treatment of dust particles. We find that exponential
overshoot (close to the gas convection zone), gravity waves (weak
omni-present mixing), and convection within dust layers (in the thick
clouds in cooler models) contribute to the atmospheric mixing, which
is far from being a stationary process. The presence of dust in the
atmospheres is accompanied by large temporal and spatial intensity
fluctuations.
---------------------------------------------------------
Title: 3D hydrodynamical simulations of stellar photospheres with
the CO<SUP>5</SUP>BOLD code. Photometric colors of a late-type giant
Authors: Kučinskas, A.; Ludwig, H. -G.; Caffau, E.; Steffen, M.
2009MmSAI..80..723K Altcode: 2009arXiv0910.3412K
We present synthetic broad-band photometric colors of a late-type
giant located close to the RGB tip (T_eff≈3640 K, log g=1.0 and
[M/H]=0.0). Johnson-Cousins-Glass BVRIJHK colors were obtained from
the spectral energy distributions calculated using 3D hydrodynamical
and 1D classical stellar atmosphere models. The differences between
photometric magnitudes and colors predicted by the two types of models
are significant, especially at optical wavelengths where they may
reach, e.g., Delta V≈0.16, Delta R≈0.13 and Delta (V-I)≈0.14,
Delta (V-K)≈0.20. Differences in the near-infrared are smaller but
still non-negligible (e.g., Delta K≈ 0.04). Such discrepancies may
lead to noticeably different photometric parameters when these are
inferred from photometry (e.g., effective temperature will change by
Delta T_eff≈60 K due to difference of Delta (V-K)≈0.20).
---------------------------------------------------------
Title: 3D views on cool stellar atmospheres: theory meets observation
Authors: Nagendra, K. N.; Bonifacio, P.; Ludwig, H. -G.
2009MmSAI..80..601N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: 3D molecular line formation in dwarf carbon-enhanced metal-poor
stars.
Authors: Behara, N. T.; Ludwig, H. -G.; Bonifacio, P.; Sbordone, L.;
González Hernández, J. I.; Caffau, E.
2009MmSAI..80..735B Altcode: 2009arXiv0909.1010B
We present a detailed analysis of the carbon and nitrogen abundances
of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229
and SDSS J0912+0216. We also report the oxygen abundance of SDSS
J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5,
and were selected from our ongoing survey of extremely metal-poor
dwarf candidates from the Sloan Digital Sky Survey (SDSS). The carbon,
nitrogen and oxygen abundances rely on molecular lines which form in the
outer layers of the stellar atmosphere. It is known that convection in
metal-poor stars induces very low temperatures which are not predicted
by `classical' 1D stellar atmospheres. To obtain the correct temperature
structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D
hydrodynamical model atmospheres and the Linfor3D line formation code,
molecular lines of CH, NH, OH and C_2 were computed, and 3D carbon,
nitrogen and oxygen abundances were determined. The resulting carbon
abundances were compared to abundances derived using atomic C I lines
in 1D LTE and NLTE. For one star, SDSS J1349-0229, we were able to
compare the 3D oxygen abundance from OH lines to O I lines in 1D LTE
and NLTE. There is not a good agreement between the carbon abundances
determined from C_2 bands and from the CH band, and molecular lines
do not agree with the atomic C I lines. Although this may be partly
due to uncertainties in the transition probabilities of the molecular
bands it certainly has to do with the temperature structure of the
outer layers of the adopted model atmosphere. In fact the discrepancy
between C_2 and CH is in opposite directions when using 3D and 1D
models. Confronted with this inconsistency, we explore the influence
of the 3D model properties on the molecular abundance determination. In
particular, the choice of the number of opacity bins used in the model
calculations and its subsequent effects on the temperature structure
and molecular line formation is discussed.
---------------------------------------------------------
Title: Models of surface convection and dust clouds in brown dwarfs
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
Steffen, M.
2008PhST..133a4005F Altcode:
The influence of dust grains on the atmospheres of brown dwarfs is
visible in observed spectra. To investigate what prevents the dust
grains from falling down, or how fresh condensable material is mixed
up in the atmosphere to allow new grains to form, we performed 2D
radiation-hydrodynamics simulations with CO5BOLD of the upper part
of the convection zone and the atmosphere containing the dust cloud
layers. We find that unlike in models of Cepheids, the convective
overshoot does not play a major role. Instead, the mixing in the dust
clouds is controlled by gravity waves.
---------------------------------------------------------
Title: Extremely metal-poor stars from the SDSS
Authors: Ludwig, H. -G.; Bonifacio, P.; Caffau, E.; Behara, N. T.;
González Hernández, J. I.; Sbordone, L.
2008PhST..133a4037L Altcode: 2008arXiv0809.2948L
We give a progress report on the activities within the CIFIST Team
related to the search for extremely metal-poor (EMP) stars in the
Sloan Digital Sky Survey's (SDSS) spectroscopic catalogue. So far,
the search has provided 25 candidates with metallicities around or
smaller than -3. For 15 candidates, high-resolution spectroscopy with
UVES at the VLT has confirmed their EMP status. Work is under way to
extend the search to the SDSS's photometric catalogue by augmenting
the SDSS photometry and by gauging the capabilities of X-shooter when
going to significantly fainter targets.
---------------------------------------------------------
Title: Modeling stochastic excitation of acoustic modes in stars:
present status and perspectives
Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Ludwig, H. -G.;
Dupret, M. -A.
2008CoAst.157..130S Altcode:
Solar-like oscillations have now been detected for more than ten years
and their frequencies measured for a still growing number of stars with
various characteristics (e.g. mass, chemical composition, evolutionary
stage ...). Excitation of such oscillations is attributed to turbu-
lent convection and takes place in the uppermost part of the convective
envelope. Since the pioneering work of Goldreich & Keely (1977),
more sophisticated theoretical models of stochastic excitation were
developed, which differ from each other both by the way turbulent
convection is modeled and by the assumed sources of excitation. We
briefly review here the different underlying approximations and
assumptions of those models. A second part shows that computed
mode excitation rates crucially depend on the way time-correlations
between eddies are described but also on the surface metal abundance
of the star.
---------------------------------------------------------
Title: he models comprise the upper part of the convection zone
and the atmosphere with %the dust cloud layers. We find that direct
convective overshoot does not play a major role. Instead, the mixing
in the clouds is controlled by gravity waves.
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen,
M.; Sharp, C.
2008sf2a.conf..469F Altcode:
To investigate the mechanism that controls the formation and
gravitational settling of dust grains as well as the mixing of fresh
condensable material into the atmosphere of brown dwarfs, we performed
2D radiation-hydrodynamics simulations with CO5BOLD.
---------------------------------------------------------
Title: Radiation-hydrodynamics simulations of surface convection in
low-mass stars: connections to stellar structure and asteroseismology
Authors: Ludwig, Hans-G.; Caffau, Elisabetta; Kučinskas, A.
2008IAUS..252...75L Altcode: 2008arXiv0809.2939L
Radiation-hydrodynamical simulations of surface convection in low-mass
stars can be exploited to derive estimates of i) the efficiency of
the convective energy transport in the stellar surface layers; ii)
the convection-related photometric micro-variability. We comment
on the universality of the mixing-length parameter, and point out
potential pitfalls in the process of its calibration which may be in
part responsible for the contradictory findings about its variability
across the Hertzsprung-Russell digramme. We further comment on the
modelling of the photometric micro-variability in HD 49933 one of the
first main COROT targets.
---------------------------------------------------------
Title: 3D model atmospheres and the solar photospheric oxygen
abundance
Authors: Caffau, E.; Ludwig, H. -G.
2008IAUS..252...35C Altcode:
In recent years the photospheric solar oxygen abundance experienced a
significant downward revision. However, a low photospheric abundance
is incompatible with the value in the solar interior inferred
from helioseismology. For contributing to the dispute whether the
solar oxygen abundance is “high” or “low”, we re-derived its
photospheric abundance independently of previous analyses. We applied
3D (CO5BOLD) as well as 1D model atmospheres. We considered standard
disc-centre and disc-integrated spectral atlases, as well as newly
acquired solar intensity spectra at different heliocentric angles. We
determined the oxygen abundances from equivalent width and/or line
profile fitting of a number of atomic lines. As preliminary result,
we find an oxygen abundance in the range 8.73 8.79, encompassing the
value obtained by Holweger (2001), and somewhat higher than the value
obtained by Asplund et al. (2005).
---------------------------------------------------------
Title: The Solar Photospheric Oxygen Abundance and the Role of 3D
Model Atmospheres
Authors: Caffau, E.; Steffen, M.; Ludwig, H. -G.
2008ESPM...12..3.7C Altcode:
The solar oxygen abundance has undergone a major downward revision in
the last decade, reputedly as a result of employing 3D hydrodynamical
simulations to model the inhomogeneous structure of the solar
photosphere. <P />The very low oxygen abundance advocated by Asplund
et al. 2004, A(O)=8.66, together with the downward revision of the
abundances of other key elements, has created serious problems for solar
models to explain the helioseismic measurements. <P />In an effort to
contribute to the dispute of whether the Sun has "solar" or "sub-solar"
abundances, we have re-derived its photospheric abundance of oxygen,
nitrogen, and other elements, independently of previous analyses. <P
/>We applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation
of the solar granulation as well as different 1D model atmospheres for
the line by line spectroscopic abundance determinations. The analysis
is based on both standard disk-center and full-disk spectral atlases;
for oxygen we acquired in addition spectra at different heliocentric
angles. The derived abundances are the result of equivalent width
and/or line profile fitting of the available atomic lines. Our
recommended oxygen abundance is A(O)=8.76+- 0.07, 0.1 dex higher
than the value of Asplund et al. (2004). Our current estimate of the
overall solar metallicity is 0.014< Z<0.016. <P />Questions we
discuss include: (i) Is the general downward revision of the solar
abundances a 3D effect? (ii) How large are the abundance corrections
due to horizontal inhomogeneities? (iii) What is the main reason for
the differences between the abundances obtained in our study and those
derived by Apslund and coworkers? (iv) How large are the uncertainties
in the observed solar spectra? (v) What is the reason why the two
forbidden oxygen lines, [OI] lambda 630 nm and [OI] lambda 636.3 nm,
give significantly different answers for the solar oxygen abundance?
---------------------------------------------------------
Title: The photospheric solar oxygen project. I. Abundance analysis
of atomic lines and influence of atmospheric models
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Ayres, T. R.;
Bonifacio, P.; Cayrel, R.; Freytag, B.; Plez, B.
2008A&A...488.1031C Altcode: 2008arXiv0805.4398C
Context: The solar oxygen abundance has undergone a major downward
revision in the past decade, the most noticeable one being the
update including 3D hydrodynamical simulations to model the solar
photosphere. Up to now, such an analysis has only been carried out
by one group using one radiation-hydrodynamics code. <BR />Aims:
We investigate the photospheric oxygen abundance considering lines
from atomic transitions. We also consider the relationship between
the solar model used and the resulting solar oxygen abundance, to
understand whether the downward abundance revision is specifically
related to 3D hydrodynamical effects. <BR />Methods: We performed
a new determination of the solar photospheric oxygen abundance by
analysing different high-resolution high signal-to-noise ratio atlases
of the solar flux and disc-centre intensity, making use of the latest
generation of CO5BOLD 3D solar model atmospheres. <BR />Results: We
find 8.73 ≤ log (N_O/N_H) +12 ≤ 8.79. The lower and upper values
represent extreme assumptions on the role of collisional excitation
and ionisation by neutral hydrogen for the NLTE level populations
of neutral oxygen. The error of our analysis is ± (0.04± 0.03)
dex, the last being related to NLTE corrections, the first error
to any other effect. The 3D “granulation effects” do not play a
decisive role in lowering the oxygen abundance. <BR />Conclusions:
Our recommended value is log (N_O/N_H) = 8.76 ± 0.07, considering our
present ignorance of the role of collisions with hydrogen atoms on the
NLTE level populations of oxygen. The reasons for lower O abundances in
the past are identified as (1) the lower equivalent widths adopted and
(2) the choice of neglecting collisions with hydrogen atoms in the
statistical equilibrium calculations for oxygen. <P />This paper is
dedicated to the memory of Hartmut Holweger.
---------------------------------------------------------
Title: Radiation hydrodynamics simulations of stellar surface
convection
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Guenter;
Wedemeyer-Boehm, Sven
2008asd..soft...36F Altcode:
The code is used to perform radiation hydrodynamics simulations of
the convective surface layers and the photosphere of cool stars.
---------------------------------------------------------
Title: The solar photospheric abundance of europium. Results from
CO5BOLD 3D hydrodynamical model atmospheres
Authors: Mucciarelli, A.; Caffau, E.; Freytag, B.; Ludwig, H. -G.;
Bonifacio, P.
2008A&A...484..841M Altcode: 2008arXiv0803.0863M
Context: Europium is an almost pure r-process element, which may be
useful as a reference in nucleocosmochronology. <BR />Aims: Determine
the photospheric solar abundance using CO5BOLD 3D hydrodynamical
model atmospheres. <BR />Methods: Disc-centre and integrated-flux
observed solar spectra are used. The europium abundance is derived
using equivalent-width measurements. As a reference, one-dimensional
model atmospheres are in addition used. <BR />Results: The europium
photospheric solar abundance (0.52 ± 0.02) agrees with previous
determinations. We determine the photospheric isotopic fraction of
<SUP>151</SUP>Eu to be 49% ± 2.3% using the intensity spectra, and 50%
± 2.3% using the flux spectra. This compares well to the meteoritic
isotopic fraction 47.8%. We explore 3D corrections for dwarfs and
sub-giants in the temperature range ~5000 K to ~6500 K and solar and
1/10-solar metallicities and find them to be negligible for all models
investigated. <BR />Conclusions: Our photospheric Eu abundance agrees
well with previous determinations based on 1D models. This is in line
with our conclusion that 3D effects for this element are negligible
in the case of the Sun.
---------------------------------------------------------
Title: The solar photospheric abundance of hafnium and
thorium. Results from CO<SUP>5</SUP>BOLD 3D hydrodynamic model
atmospheres
Authors: Caffau, E.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P.;
Steffen, M.; Behara, N. T.
2008A&A...483..591C Altcode: 2008arXiv0803.3585C
Context: The stable element hafnium (Hf) and the radioactive element
thorium (Th) were recently suggested as a suitable pair for radioactive
dating of stars. The applicability of this elemental pair needs to
be established for stellar spectroscopy. <BR />Aims: We aim at a
spectroscopic determination of the abundance of Hf and Th in the
solar photosphere based on a CO<SUP>5</SUP>BOLD 3D hydrodynamical
model atmosphere. We put this into a wider context by investigating 3D
abundance corrections for a set of G- and F-type dwarfs. <BR />Methods:
High-resolution, high signal-to-noise solar spectra were compared to
line synthesis calculations performed on a solar CO<SUP>5</SUP>BOLD
model. For the other atmospheres, we compared synthetic spectra
of CO<SUP>5</SUP>BOLD 3D and associated 1D models. <BR />Results:
For Hf we find a photospheric abundance A(Hf) = 0.87 ± 0.04, in good
agreement with a previous analysis, based on 1D model atmospheres. The
weak Th II 401.9 nm line constitutes the only Th abundance indicator
available in the solar spectrum. It lies in the red wing of a Ni-Fe
blend exhibiting a non-negligible convective asymmetry. Accounting for
the asymmetry-related additional absorption, we obtain A(Th) = 0.08 ±
0.03, consistent with the meteoritic abundance, and about 0.1 dex lower
than obtained in previous photospheric abundance determinations. <BR
/>Conclusions: Only for the second time, to our knowledge, has a
non-negligible effect of convective line asymmetries on an abundance
derivation been highlighted. Three-dimensional hydrodynamical
simulations should be employed to measure Th abundances in dwarfs
if similar blending is present, as in the solar case. In contrast,
3D effects on Hf abundances are small in G- to mid F-type dwarfs and
sub-giants, and 1D model atmospheres can be conveniently used.
---------------------------------------------------------
Title: Radiation-hydrodynamical model atmospheres across the
Hertzsprung-Russell diagram
Authors: Ludwig, H. -G.
2008iac..talk..177L Altcode: 2008iac..talk....1L
No abstract at ADS
---------------------------------------------------------
Title: Hydrodynamical Model Atmospheres of Metal-Poor Stars
Authors: Ludwig, Hans-Günter; González Hernández, Jonay I.; Behara,
Natalie; Caffau, Elisabetta; Steffen, Matthias
2008AIPC..990..268L Altcode:
Standard one-dimensional (1D) model atmospheres rely on the assumption
of radiative equilibrium in the non-convective part of the stellar
photosphere. However, gas-dynamical effects can lead to dramatic
deviations from radiative equilibrium conditions, especially in
metal-poor stellar atmospheres. These can be taken into account in
3D stellar atmosphere models representing the detailed interplay of
hydrodynamics and radiation. During the last two years efforts have been
invested to compute such 3D models for metal-poor atmospheres with the
CO<SUP>5</SUP> BOLD code within the CIFIST (Cosmological Impact of the
FIrst STars) Team, an European Union funded research group dedicated
to the study of metal-poor stars. Based on the available models we will
give an account of the radiation-hydrodynamical processes at work, and
discuss consequences for the temperature scale and abundance analysis
of metal-poor stars.
---------------------------------------------------------
Title: First stars XI. Chemical composition of the extremely
metal-poor dwarfs in the binary CS 22876-032
Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.; Spite, M.; Spite, F.; Cayrel, R.; Molaro, P.; Hill, V.;
François, P.; Plez, B.; Beers, T. C.; Sivarani, T.; Andersen, J.;
Barbuy, B.; Depagne, E.; Nordström, B.; Primas, F.
2008A&A...480..233G Altcode: 2007arXiv0712.2949G
Context: Unevolved metal-poor stars constitute a fossil record of the
early Galaxy, and can provide invaluable information on the properties
of the first generations of stars. Binary systems also provide direct
information on the stellar masses of their member stars. <BR />Aims:
The purpose of this investigation is a detailed abundance study of the
double-lined spectroscopic binary CS 22876-032, which comprises the two
most metal-poor dwarfs known. <BR />Methods: We used high-resolution,
high-S/N ratio spectra from the UVES spectrograph at the ESO VLT
telescope. Long-term radial-velocity measurements and broad-band
photometry allowed us to determine improved orbital elements and
stellar parameters for both components. We used OSMARCS 1D models and
the turbospectrum spectral synthesis code to determine the abundances
of Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also
used the CO^5BOLD model atmosphere code to compute the 3D abundance
corrections, notably for Li and O. <BR />Results: We find a metallicity
of [Fe/H] ~ -3.6 for both stars, using 1D models with 3D corrections of
~-0.1 dex from averaged 3D models. We determine the oxygen abundance
from the near-UV OH bands; the 3D corrections are large, -1 and -1.5
dex for the secondary and primary respectively, and yield [O/Fe] ~
0.8, close to the high-quality results obtained from the [OI] 630
nm line in metal-poor giants. Other [ α/Fe] ratios are consistent
with those measured in other dwarfs and giants with similar [Fe/H],
although Ca and Si are somewhat low ([X/Fe] ⪉ 0). Other element
ratios follow those of other halo stars. The Li abundance of the
primary star is consistent with the Spite plateau, but the secondary
shows a lower abundance; 3D corrections are small. <BR />Conclusions:
The Li abundance in the primary star supports the extension of the Spite
Plateau value at the lowest metallicities, without any decrease. The
low abundance in the secondary star could be explained by endogenic
Li depletion, due to its cooler temperature. If this is not the case,
another, yet unknown mechanism may be causing increased scatter in A(Li)
at the lowest metallicities.
---------------------------------------------------------
Title: CS 22876-032: The Most Metal-Poor Dwarfs. Abundances and
3D Effects
Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.; Spite, M.; Spite, F.; Cayrel, R.; Molaro, P.; Hill, V.;
François, P.; Plez, B.; Beers, T. C.; Sivarani, T.; Andersen, J.;
Barbuy, B.; Depagne, E.; Nordström, B.; Primas, F.
2008AIPC..990..175G Altcode: 2008AIPC..990..175H
Unevolved extremely metal-poor stars offer us a unique tool to infer
knowledge of the first generation of stars. We have analysed UVES
high-resolution spectra of the double-lined spectroscopic binary CS
22876-032 which comprises the two most metal-poor dwarfs currently
known. In particular, we determine the oxygen (from OH lines in the
near-UV) and lithium abundances taking into account 3D effects. <P
/>The long-time baseline radial velocity measurements and photometric
data available allowed us to determine the orbital elements as well as
stellar parameters of both components. We use OSMARCS 1D models and the
TURBOSPECTRUM spectral synthesis code to determine the abundances of Li,
O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also use the
CO<SUP>5</SUP> BOLD 3D model atmosphere code to predict the 3D abundance
corrections, mainly for Li, O and Fe. <P />We find a metallicity
of [Fe/H]~-3.6 for both stars using 1D models with 3D corrections
of ~-0.1 dex from horizontal and temporal averaged 3D models. The
[α/Fe] ratios are consistent with those found for metal-poor giants
with similar [Fe/H], although Ca and Si are rather low, [X/Fe]~=0. The
1D O abundance, [O/Fe]~2 for both stars, is very large, but 3D models
predict abundance corrections of roughly -1.0 dex and -1.5 dex for the
secondary and primary stars, respectively. These 3D corrections bring
the O abundances derived from near-UV OH bands in these two dwarfs
closer to other high-quality measurements from the forbidden [OI]
630 nm line in metal-poor giants. The Li abundance is consistent with
the Spite plateau, although the secondary star shows a lower abundance.
---------------------------------------------------------
Title: The Metal-Poor End of the Lithium Plateau
Authors: Sbordone, L.; Bonifacio, P.; González Hernández, J. I.;
Cayrel, R.; Behara, N.; Molaro, P.; Plez, B.; Francois, P.; Christlieb,
N.; Ludwig, H. -G.; Sivarani, T.; Beers, T. C.; van't Veer, C.
2008AIPC..990..339S Altcode:
We present our current sample of Lithium abundances in 28 low
metallicity dwarf and Turn Off (TO) stars ([Fe/H] between -2.5
and -3.5), based on high resolution, high signal to noise echelle
spectra. Nine new stars have been added to the Bonifacio et al. [1]
sample, and the full sample has been reanalyzed in order to take into
account the effect of two different possible temperature scales. In
fact, the Li abundance measurement based on the 670.8 nm line is
highly sensitive to temperature, and Teff scales are still poorly
calibrated at low metallicities. First, the effective temperature has
been derived from Hα profile fitting, and second, directly from the
star's infrared flux. The two methods offer similar precision but are
affected by different uncertainties and systematics. The infrared flux
method (IRFM) leads to a larger Teff dispersion than the Hα profile
fitting, while also producing an offset of about 150 K towards hotter
temperatures. This leads to a contraction of the metallicity scale
of the sample, which encompasses [Fe/H] = -3.7 to -2.5 when using
Hα calibrated temperatures, and [Fe/H] = -3.4 to -2.5 when using
IRFM. The higher average IRFM temperature increases somewhat the mean
Li abundance, changing from A(Li)Hα = 2.10 to A(Li)IRFM = 2.18
---------------------------------------------------------
Title: Hydrodynamical Model Atmospheres and 3D Spectral Synthesis
Authors: Ludwig, Hans-Günter; Steffen, Matthias
2008psa..conf..133L Altcode: 2007arXiv0704.1176L
In this paper we discuss three issues in the context of
three-dimensional (3D) hydrodynamical model atmospheres for late-type
stars, related to spectral line shifts, radiative transfer in metal-poor
3D models, and the solar oxygen abundance. To establish the context we
start by giving a brief overview about the model construction, taking
the radiation-hydrodynamics code CO<SUP>5</SUP>BOLD (Conservative COde
for the Computation of COmpressible COnvection in a BOx of L Dimensions
with L=2,3; [3]) and the related spectral synthesis package Linfor3D
as examples.
---------------------------------------------------------
Title: Spectral analyses of three carbon-enhanced metal-poor stars
Authors: Behara, N.; Bonifacio, P.; Ludwig, H. G.; Sbordone, L.;
Gonzales Hernandez, J. I.; Caffau, E.
2008nuco.confE..68B Altcode: 2008arXiv0809.4204B; 2008PoS....53E..68B
We are conducting a high-resolution follow-up of candidate EMP stars
extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000)
using UVES at the VLT. Three of the programme stars, SDSS J0912+0216,
SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as
CEMP stars since a strong $G$ band was evident from the SDSS spectra
and the weakness of the Ca {\sc ii} K line testified their very
low metallicity. The UVES high resolution follow-up confirmed the
original findings ([Fe/H] $<-2.50$) and allowed a more detailed
investigation of their chemical composition. We determined the carbon
abundance from molecular lines which form in the outer layers of the
stellar atmosphere. It is known that convection in metal-poor stars
induces very low temperatures which are not predicted by classical
1D stellar atmospheres. To obtain the correct temperature structure,
one needs full 3D hydrodynamical models. 3D carbon abundances were
determined for all three stars, using CO$^5$BOLD 3D hydrodynamical
model atmospheres. 3D effects on the carbon abundance are found to be
quite significant for these stars, with 3D corrections of up to --0.7
dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit
an overabundance of neutron capture elements which classifies them as
CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of
CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is
the third member discovered to date.
---------------------------------------------------------
Title: Overview of the Li problem in metal-poor stars and new results
on 6Li
Authors: Cayrel, R.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.
2008nuco.confE...2C Altcode: 2008arXiv0810.4290C; 2008PoS....53E...2C
Two problems are discussed here. The first one is the 0.4 dex
discrepancy between the 7Li abundance derived from the spectra
of metal-poor halo stars on the one hand, and from Big Bang
nucleosynthesis, based on the cosmological parameters constrained
by the WMAP measurements, on the other hand. Lithium, indeed,
can be depleted in the convection zone of unevolved stars. The
understanding of the hydrodynamics of the crucial zone near the
bottom of the convective envelope in dwarfs or turn-off stars of solar
metallicity has recently made enormous progress with the inclusion of
internal gravity waves. However, similar work for metal-poor stars is
still lacking. Therefore it is not yet clear whether the depletion
occurring in the metal-poor stars themselves is adequate to produce
a 7Li plateau. The second problem pertains to the large amount of
6Li recently found in metal-poor halo stars. The convection-related
asymmetry of the 7Li line could mimic the signal attributed so far
to the weak blend of 6Li in the red wing of the 7Li line. Theoretical
computations show that the signal generated by the asymmetry of 7Li is
2.0, 2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively
(Teff =6250 K and log g=4.0 [cgs]). In addition we re-investigate
the statistical properties of the 6Li plateau and show that previous
analyses were biased. Our conclusion is that the 6Li plateau can be
reinterpreted in terms of intrinsic line asymmetry, without the need
to invoke a contribution of 6Li. (abridged)
---------------------------------------------------------
Title: Limb Darkening: Getting Warmer
Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P.; Mérand,
A.; Ridgway, S. T.; Coudé du Foresto, V.; sten Brummelaar, T. A.;
Berger, D. H.; Sturmann, J.; Turner, N. H.
2008poii.conf...71A Altcode: 2008poio.conf...71A
We present interferometric observations and model atmosphere analyses
of three stars: the F-type subgiant Procyon, the A-type supergiant
Deneb, and the B-type supergiant Rigel. We use VLTI/VINCI and Mark III
observations of Procyon to test recent multiwavelength limb-darkening
predictions from 3-D hydrodynamic atmosphere simulations with no
free parameters for convection. We also investigate the effects of
different 1-D atmospheric convection treatments on limb-darkening
predictions. We show that the 3-D model predictions are confirmed
and we find that 1-D models fail to reproduce Procyon's UV spectral
energy distribution, a result consistent with models of granulation for
Procyon's surface. We use observations employing the longest baselines
of the CHARA Array together with the FLUOR beam combiner to determine
precise angular diameters for the two early-type supergiants and test
limb-darkening predictions from expanding atmosphere models of these
tars' stellar winds. For Deneb, we derive angular diameters consistent
with previous measurements, but which vary with position angle at
the ≃3% level. Observations of the 2nd lobe of Deneb's visibility
curve are more consistent with expanding atmosphere predictions than
hydrostatic atmosphere predictions. For Rigel, we derive from the
CHARA/FLUOR observations a limb-darkened angular diameter consistent
with a recent VLTI/IONIC measurement and 8% larger than reported from
the Intensity Interferometer.
---------------------------------------------------------
Title: Towards the Interferometric Imaging of Red Supergiants
Authors: Ludwig, Hans-Günter; Beckers, Jacques
2008poii.conf..485L Altcode: 2008poio.conf..485L
No abstract at ADS
---------------------------------------------------------
Title: The solar photospheric abundance of phosphorus: results from
CO^5BOLD 3D model atmospheres
Authors: Caffau, E.; Steffen, M.; Sbordone, L.; Ludwig, H. -G.;
Bonifacio, P.
2007A&A...473L...9C Altcode: 2007arXiv0708.1607C
Aims:We determine the solar abundance of phosphorus using CO^5BOLD
3D hydrodynamic model atmospheres. <BR />Methods: High-resolution,
high signal-to-noise solar spectra of the P i lines of Multiplet 1 at
1051-1068 nm are compared to line-formation computations performed
on a CO^5BOLD solar model atmosphere. <BR />Results: We find A(P)
= 5.46 ± 0.04, in good agreement with previous analyses based on
1D model atmospheres, due to the P i lines of Mult. 1 not being
affected much by 3D effects. We cannot confirm an earlier claim by
other authors of a downward revision of the solar P abundance by 0.1
dex when employing a 3D model atmosphere. Concerning other stars,
we find modest (<0.1 dex) 3D abundance corrections for P among
four F-dwarf model atmospheres of different metallicities, and these
corrections are largest at lowest metallicity. <BR />Conclusions:
We conclude that 3D abundance corrections are generally rather small
for the P i lines studied in this work. They are marginally relevant
for metal-poor stars, but may be neglected in the Sun. <P />Tables
2-4 are only available in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Line shift, line asymmetry, and the ^6Li/^7Li isotopic ratio
determination
Authors: Cayrel, R.; Steffen, M.; Chand, H.; Bonifacio, P.; Spite,
M.; Spite, F.; Petitjean, P.; Ludwig, H. -G.; Caffau, E.
2007A&A...473L..37C Altcode: 2007arXiv0708.3819C
Context: Line asymmetries are generated by convective Doppler shifts in
stellar atmospheres, especially in metal-poor stars, where convective
motions penetrate to higher atmospheric levels. Such asymmetries are
usually neglected in abundance analyses. The determination of the
^6Li/^7Li isotopic ratio is prone to suffering from such asymmetries,
as the contribution of ^6Li is a slight blending reinforcement of the
red wing of each component of the corresponding ^7Li line, with respect
to its blue wing. <BR />Aims: The present paper studies the halo star
HD 74000 and estimates the impact of convection-related asymmetries
on the Li isotopic ratio determination. <BR />Methods: Two methods
are used to meet this aim. The first, which is purely empirical,
consists in deriving a template profile from another element that can
be assumed to originate in the same stellar atmospheric layers as Li
I, producing absorption lines of approximately the same equivalent
width as individual components of the ^7Li I resonance line. The
second method consists in conducting the abundance analysis based on
NLTE line formation in a 3D hydrodynamical model atmosphere, taking
into account the effects of photospheric convection. <BR />Results:
The results of the first method show that the convective asymmetry
generates an excess absorption in the red wing of the ^7Li absorption
feature that mimics the presence of ^6Li at a level comparable to
the hitherto published values. This opens the possibility that only
an upper limit on ^6Li/^7Li has thus far been derived. The second
method confirms these findings. <BR />Conclusions: From this work,
it appears that a systematic reappraisal of former determinations of
^6Li abundances in halo stars is warranted. <P />Based on observations
carried out at the European Southern Observatory (ESO), under prog. ID
75.D-0600. Tables 1-3, and additional references are only available
in electronic form at http://www.aanda.org
---------------------------------------------------------
Title: 3D spectral synthesis and rotational line broadening
Authors: Ludwig, H. -G.
2007A&A...471..925L Altcode: 2007arXiv0707.3347L
Context: Spectral synthesis calculations based on three-dimensional
stellar atmosphere models are limited by the affordable angular
resolution of the radiation field. This hampers an accurate treatment
of rotational line broadening. <BR />Aims: We aim to find a treatment
of rotational broadening of a spherical star when the radiation field
is only available at a modest number of limb-angles. <BR />Methods: We
apply a combination of analytical considerations of the line-broadening
process and numerical tests. <BR />Results: We obtain a method which
is closely related to classical flux convolution and which performs
noticeably better than a previously suggested procedure. It can be
applied to rigid as well as differential rotation.
---------------------------------------------------------
Title: Inter-network regions of the Sun at millimetre wavelengths
Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts,
J.; Freytag, B.
2007A&A...471..977W Altcode: 2007arXiv0705.2304W
Aims:The continuum intensity at wavelengths around 1 mm provides an
excellent way to probe the solar chromosphere and thus valuable input
for the ongoing controversy on the thermal structure and the dynamics of
this layer. The synthetic continuum intensity maps for near-millimetre
wavelengths presented here demonstrate the potential of future
observations of the small-scale structure and dynamics of internetwork
regions on the Sun. <BR />Methods: The synthetic intensity/brightness
temperature maps are calculated on basis of three-dimensional
radiation (magneto-)hydrodynamic (MHD) simulations. The assumption
of local thermodynamic equilibrium (LTE) is valid for the source
function. The electron densities are also treated in LTE for most maps
but also in non-LTE for a representative model snapshot. Quantities
like intensity contrast, intensity contribution functions, spatial
and temporal scales are analysed in dependence on wavelength and
heliocentric angle. <BR />Results: While the millimetre continuum
at 0.3 mm originates mainly from the upper photosphere, the longer
wavelengths considered here map the low and middle chromosphere. The
effective formation height increases generally with wavelength and
also from disk-centre towards the solar limb. The average intensity
contribution functions are usually rather broad and in some cases they
are even double-peaked as there are contributions from hot shock waves
and cool post-shock regions in the model chromosphere. The resulting
shock-induced thermal structure translates to filamentary brightenings
and fainter regions in between. Taking into account the deviations from
ionisation equilibrium for hydrogen gives a less strong variation of
the electron density and with it of the optical depth. The result is
a narrower formation height range although the intensity maps still
are characterised by a highly complex pattern. The average brightness
temperature increases with wavelength and towards the limb although
the wavelength-dependence is reversed for the MHD model and the NLTE
brightness temperature maps. The relative contrast depends on wavelength
in the same way as the average intensity but decreases towards the
limb. The dependence of the brightness temperature distribution on
wavelength and disk-position can be explained with the differences in
formation height and the variation of temperature fluctuations with
height in the model atmospheres. The related spatial and temporal
scales of the chromospheric pattern should be accessible by future
instruments. <BR />Conclusions: Future high-resolution millimetre
arrays, such as the Atacama Large Millimeter Array (ALMA), will
be capable of directly mapping the thermal structure of the solar
chromosphere. Simultaneous observations at different wavelengths
could be exploited for a tomography of the chromosphere, mapping its
three-dimensional structure, and also for tracking shock waves. The
new generation of millimetre arrays will be thus of great value for
understanding the dynamics and structure of the solar atmosphere.
---------------------------------------------------------
Title: Sulphur abundances from the S i near-infrared triplet at
1045 nm
Authors: Caffau, E.; Faraggiana, R.; Bonifacio, P.; Ludwig, H. -G.;
Steffen, M.
2007A&A...470..699C Altcode: 2007arXiv0704.2335C
Context: Unlike silicon and calcium, sulphur is an α-element
that does not form dust. Some of the available observations of
the evolution of sulphur with metallicity indicate an increased
scatter of sulphur-to-iron ratios at low metallicities or even
a bimodal distribution, with some stars showing constant S/Fe at
all metallicities and others showing an increasing S/Fe ratio with
decreasing metallicity. In metal-poor stars S i lines of Multiplet
1 at 920 nm are not yet too weak to permit the measurement of the
sulphur abundance A(S); however, in ground-based observations they
are severely affected by telluric lines. <BR />Aims: We investigate
the possibility of measuring sulphur abundances from S iMult. 3 at
1045 nm lines. These lie in the near infrared and are slightly weaker
than those of Mult. 1, but lie in a range not affected by telluric
lines. <BR />Methods: We investigated the lines of Mult. 3 in the Sun
(G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V), and ɛ Eri (HD
22049, K2V). For the Sun and Procyon the analysis was performed with
CO^5BOLD 3D hydrodynamical model atmospheres, while the three other
stars, for which hydrodynamical simulations are not available, were
analysed using 1D model atmospheres. <BR />Results: For our sample of
stars we find a global agreement between A(S) from lines of different
multiplets. <BR />Conclusions: Our results suggest that the infrared
lines of Mult. 3 are a viable indicator of the sulphur abundance
that, because of the intrinsic strength of this multiplet, should be
suitable for studying the trend of [S/Fe] at low metallicities. <P
/>Based on data from the UVES Paranal Observatory Project (ESO DDT
Program ID 266.D-5655). Appendix is only available in electronic form
at http://www.aanda.org
---------------------------------------------------------
Title: Formation of convective structures in stellar atmospheres
Authors: Freytag, Bernd; Ludwig, Hans-Günter
2007sf2a.conf..481F Altcode:
Convection is a ubiquitous phenomenon in cool stars. Its interplay
with radiation leads to the formation of coherent flow structures --
granular cells -- on the visible surfaces of these stars. We model the
processes with the 3D radiation-hydrodynamics code CO5BOLD in stars
of different atmospheric parameters. We find that the granular cell
size scales with the atmospheric pressure scale height for stars with
a surface gravity log g>1.0 . However, the scaling breaks down for
red supergiants having lower surface gravities. This qualitatively
different behaviour is likely linked to sphericity effects and mainly
to a larger contribution of radiation to the energy transport in the
stellar envelope.
---------------------------------------------------------
Title: The forbidden 1082 nm line of sulphur:. the photospheric
abundance of sulphur in the Sun and 3D effects
Authors: Caffau, E.; Ludwig, H. -G.
2007A&A...467L..11C Altcode: 2007astro.ph..3423C
Context: Sulphur is an element which is formed in the α-process and
is easily measured in the gaseous phase in external galaxies. Since
it does not form dust, it is the preferred indicator for α-elements,
rather than Si or Mg, for which dust corrections are necessary. The
measurement of the sulphur abundance in stars is not an easy
task, relying mainly on high excitation lines with non-negligible
deviations from LTE. The 1082 nm sulphur forbidden transition is
less sensitive to departures from LTE and is less dependent on
temperature uncertainties than other sulphur lines usually employed
as abundance indicators. Therefore it should provide a more robust
abundance diagnostics. <BR />Aims: To derive the solar photospheric
abundance of sulphur from the 1082 nm [SI] line and to investigate
3D effects present in G- and F-type atmospheres at solar and lower
metallicity. <BR />Methods: High-resolution, high signal-to-noise
solar intensity and flux spectra were used to measure the sulphur
abundance from the [SI] 1082 nm line. CO^5BOLD hydrodynamical model
atmospheres were applied to predict 3D abundance corrections for the
[SI] line. <BR />Results: The solar sulphur abundance is derived to be
7.15± (0.01)_stat ± (0.05)_sys, where the statistical uncertainty
represents the scatter in the determination using four different
solar spectra and the systematic uncertainty is due to the modelling
of the blending lines. Sulphur abundances obtained from this line are
insensitive to the micro-turbulence. 3D abundance corrections, found
from strictly differential comparisons between 1D and 3D models, are
negligible in the Sun, but become sizable for more metal-poor dwarfs.
---------------------------------------------------------
Title: Prospects of using simulations to study the photospheres of
brown dwarfs
Authors: Ludwig, Hans-Günter
2007IAUS..239..205L Altcode: 2007arXiv0704.1296L
We discuss prospects of using multi-dimensional time-dependent
simulations to study the atmospheres of brown dwarfs and extrasolar
giant planets, including the processes of convection, radiation,
dust formation, and rotation. We argue that reasonably realistic
simulations are feasible, however, separated into two classes of local
and global models. Numerical challenges are related to potentially
large dynamic ranges, and the treatment of scattering of radiation in
multi-D geometries.
---------------------------------------------------------
Title: Energy transport, overshoot, and mixing in the atmospheres
of M-type main- and pre-main-sequence objects
Authors: Ludwig, H. -G.; Allard, F.; Hauschildt, P. H.
2006A&A...459..599L Altcode: 2006astro.ph..8264L
We constructed hydrodynamical model atmospheres for mid M-type main-,
as well as pre-main-sequence (PMS) objects. Despite the complex
chemistry encountered in these cool atmospheres a reasonably accurate
representation of the radiative transfer is possible, even in the
context of time-dependent and three-dimensional models. The models
provide detailed information about the morphology of M-type granulation
and statistical properties of the convective surface flows. In
particular, we determined the efficiency of the convective energy
transport, and the efficiency of mixing by convective overshoot. The
convective transport efficiency was expressed in terms of an equivalent
mixing-length parameter α_MLT in the formulation of mixing-length
theory (MLT) given by Mihalas (1978). α_MLT amounts to values around
≈2 for matching the entropy of the deep, adiabatically stratified
regions of the convective envelope, and lies between 2.5 and 3.0 for
matching the thermal structure of the deep photosphere. For current
spectral analysis of PMS objects this implies that MLT models based
on α_MLT=2.0 overestimate the effective temperature by 100 K and
surface gravities by 0.25 dex. The average thermal structure of the
formally convectively stable layers is little affected by convective
overshoot and wave heating, i.e., stays close to radiative equilibrium
conditions. Our models suggest that the rate of mixing by convective
overshoot declines exponentially with geometrical distance to the
Schwarzschild stability boundary. It increases at given effective
temperature with decreasing gravitational acceleration.
---------------------------------------------------------
Title: Comments on the granulation background in the Sun, Procyon,
and η Bootis
Authors: Ludwig, H. -G.
2006IAUJD..17E..24L Altcode:
We performed radiation-hydrodynamical simulations to derive estimates
for the background signal due to granulation in the Sun, Procyon,
and η Boo. We present an intercomparison of the theoretical results,
and discuss them in the light of recent observational findings, in
particular of the MOST photometry mission.
---------------------------------------------------------
Title: Dust in the atmospheres of brown dwarfs and young planets:
the effects of gravitational settling and convective overshoot
Authors: Homeier, D.; Ludwig, H. -G.; Allard, F.; Hauschildt, P.;
Dehn, M.
2006IAUS..232..328H Altcode:
Dwarfs of the spectral types late-M, L and T span mass regimes from
very-low-mass stars through brown dwarfs down to young planetary
objects. They all show massive molecular line-blanketing and the
condensation of refractory species with decreasing T_{eff}, leading
to changes in chemical equilibrium composition and absorption due to
dust grains. The spectral evolution from late-M through L to mid- and
late-T classes is now understood as chiefly due to increasing amounts
of condensates in the visible photosphere up to mid-L types, and the
settling of dust clouds into deeper regions at the transition from L
to T, resulting in a depletion of condensable elements in the upper
atmosphere. The ensuing photospheric cooling also drives a change in
carbon chemistry leading to the hallmark methane absorption features
of T dwarfs.Recent observations of brown dwarfs in the L-T spectral
sequence and model atmosphere calculations have shown that these changes
in spectral features reveal differences in the efficiency of cloud
removal that seem to be triggered by an additional parameter besides
effective temperature. We present models describing the settling
of particle clouds as an equilibrium process between condensation,
gravitational sedimentation and convective and turbulent mixing, based
on 3D-hydrodynamical simulations for the description of the velocity
field. These calculations predict a strong dependence of the settling
on gravity, and can therefore explain observed differences between the
spectral energy distributions of brown dwarfs of equal luminosity as an
effect of different mass. Based on these calculations we would expect
even stronger deviations in the colours of young brown dwarfs of very
low mass from those commonly observed in the field. Such predictions
seem to be in aggreement with the first observations of candidate
planetary-mass objects, and would imply that extrasolar planets at very
young ages have spectral energy distributions significantly different
from previous models. Our models al so predict characteristic time
scales for the cloud formation processes that may be compared to
observed variability in brown dwarfs.
---------------------------------------------------------
Title: Photometric colors of late-type giants: theory versus
observations
Authors: Kucinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott,
I.; Vansevicius, V.; Lindegren, L.; Tanabé, T.; Allard, F.
2006IAUS..232..276K Altcode: 2005astro.ph.12354K
Late-type giants (i.e., stars on the red and asymptotic giant branches,
RGB/AGB, respectively) are dominant contributors to the overall spectral
appearance of intermediate age and old stellar populations, especially
in the red/near-infrared part of the spectrum. Being intrinsically
bright, they are well suited for probing distant/obscured populations,
especially those that can not be studied with their fainter members,
like main sequence turn-off stars or subgiants. Late-type giants and
supergiants will be the only stellar types accessible in intermediate
age and old populations beyond the distances of several Mpc with
the future 30-50 m class extremely large telescopes (Olsen et
al. 2003). Indeed, proper understanding of their observable properties
by means of theoretical models is of key importance for studying the
evolution of stellar populations and their host galaxies.
---------------------------------------------------------
Title: Hydrodynamical simulations of convection-related stellar
micro-variability. I. Statistical relations for photometric and
photocentric variability
Authors: Ludwig, H. -G.
2006A&A...445..661L Altcode: 2005astro.ph..9441L
Local-box hydrodynamical model atmospheres provide statistical
information about the spatial dependence, as well as temporal evolution,
of a star's emergent radiation field. Here, we consider late-type
stellar atmospheres for which temporal changes of the radiative output
are primarily related to convective (granular) surface flows. We derived
relations for evaluating the granulation-induced, disk-integrated
thus observable fluctuations of the stellar brightness and location
of the photocenter from radiation intensities available from a local
model. Apart from their application in the context of hydrodynamical
stellar atmospheres, these formulae provide some broader insight
into the nature of the fluctuations under consideration. Brightness
fluctuations scale inversely proportional to the square root of the
number of convective cells (the statistically independently radiating
surface elements) present on the stellar surface and increase
with more pronounced limb-darkening. Fluctuations of the stellar
photocentric position do not depend on the number of cells and are
largely insensitive to the degree of limb-darkening. They amount to
a small fraction of the typical cell size, and can become a limiting
factor for high-precision astrometry in the case of extreme giants. The
temporal brightness and positional fluctuations are statistically
uncorrelated but closely related in magnitude.
---------------------------------------------------------
Title: Convection and observable properties of late-type giants
Authors: Kucinskas, A.; Ludwig, H. -G.; Hauschildt, P. H.
2006IAUS..232..498K Altcode: 2005astro.ph.12353K
We show that contrary to what is expected from 1D stationary model
atmospheres, 3D hydrodynamical modeling predicts a considerable
influence of convection on the spectral properties of late-type
giants. This is due to the fact that convection overshoots into the
formally stable outer atmospheric layers producing a notable granulation
pattern in the 3D hydrodynamical models, which has a direct influence
on the observable spectra and colors. Within the framework of standard
1D model atmospheres the average thermal stratification of the 3D
hydro model can not be reproduced with any reasonable choice of the
mixing length parameter and formulation of the turbulent pressure. The
differences in individual photometric colors - in terms of 3D versus
1D - reach up to ∼0.2 mag, or Δ T_{eff}∼70 K. We discuss the
impact of full 3D hydrodynamical models on the interpretation of
observable properties of late-type giants, briefly mentioning problems
and challenges which need to be solved for bringing these models to
a routine use within the astronomical community in 5-10 years from now.
---------------------------------------------------------
Title: Status of the physics of substellar objects
Authors: Jones, H. R. A.; Viti, S.; Tennyson, J.; Barber, B.; Harris,
G.; Pickering, J. C.; Blackwell-Whitehead, R.; Champion, J. -P.;
Allard, F.; Hauschildt, P. H.; Jorgensen, U. G.; Ehrenfreund, P.;
Stachowska, E.; Ludwig, H. -G.; Martin, E.; Pavlenko, Ya.; Lyubchik,
Yu.; Kurucz, R. L.
2005AN....326..920J Altcode:
A full understanding of the properties of substellar objects is one
of the major challenges facing astrophysics. Since their discovery
in 1995, we have discovered hundreds of brown dwarfs and extrasolar
planets. While these discoveries have enabled important comparisons
with theory, observational progress has been much more rapid than
the theoretical understanding of cool atmospheres. The reliable
determination of mass, abundances, gravities and temperatures is not
yet possible. The key problem is that substellar objects emit their
observable radiation in the infrared region of the spectrum where our
knowledge of atomic, molecular and line broadening data is poor. Here
we report on the status of PoSSO (Physics of SubStellar Objects). In
order to understand brown dwarfs and extrasolar planets increasing
more like those in our solar system, we are studying a wide range of
processes. Here we give an update on the project and sketch an outline
of atoms, molecules and processes requiring study.
---------------------------------------------------------
Title: On the Limb Darkening, Spectral Energy Distribution, and
Temperature Structure of Procyon
Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P.
2005ApJ...633..424A Altcode: 2005astro.ph..7336A
We have fit synthetic visibilities from three-dimensional
(CO<SUP>5</SUP>BOLD+PHOENIX) and one-dimensional (PHOENIX, ATLAS
12) model stellar atmospheres of Procyon (F5 IV) to high-precision
interferometric data from the VLT Interferometer (K band) and from the
Mark III interferometer (500 and 800 nm). These data sets provide a test
of theoretical wavelength-dependent limb-darkening predictions. The work
of Allende Prieto et al. has shown that the temperature structure from a
spatially and temporally averaged three-dimensional hydrodynamic model
produces significantly less limb darkening at 500 nm relative to the
temperature structure of a one-dimensional MARCS model atmosphere with
a standard mixing-length approximation for convection. Our direct fits
to the interferometric data confirm this prediction. A one-dimensional
ATLAS 12 model with “approximate overshooting” provides the required
temperature gradient. We show, however, that one-dimensional models
cannot reproduce the ultraviolet spectrophotometry below 160 nm
with effective temperatures in the range constrained by the measured
bolometric flux and angular diameter. We find that a good match to the
full spectral energy distribution can be obtained with a composite model
consisting of a weighted average of 12 one-dimensional model atmospheres
based on the surface intensity distribution of a three-dimensional
granulation simulation. We emphasize that one-dimensional models with
overshooting may realistically represent the mean temperature structure
of F-type stars such as Procyon, but the same models will predict redder
colors than observed because they lack the multicomponent temperature
distribution expected for the surfaces of these stars.
---------------------------------------------------------
Title: Broad-band photometric colors and effective temperature
calibrations for late-type giants. I. Z = 0.02
Authors: Kučinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott,
I.; Vansevičius, V.; Lindegren, L.; Tanabé, T.; Allard, F.
2005A&A...442..281K Altcode: 2005astro.ph.10434K
We present new synthetic broad-band photometric colors for
late-type giants based on synthetic spectra calculated with the
PHOENIX model atmosphere code. The grid covers effective temperatures
T_eff=3000dots 5000 K, gravities log g=-0.5dots{+3.5}, and metallicities
[M/H]=+0.5dots{-4.0}. We show that individual broad-band photometric
colors are strongly affected by model parameters such as molecular
opacities, gravity, microturbulent velocity, and stellar mass. Our
exploratory 3D modeling of a prototypical late-type giant shows that
convection has a noticeable effect on the photometric colors too,
as it alters significantly both the vertical and horizontal thermal
structures in the outer atmosphere. The differences between colors
calculated with full 3D hydrodynamical and 1D model atmospheres are
significant (e.g., Δ(V-K)∼0.2 mag), translating into offsets in
effective temperature of up to 70 K. For a sample of 74 late-type
giants in the Solar neighborhood, with interferometric effective
temperatures and broad-band photometry available in the literature,
we compare observed colors with a new PHOENIX grid of synthetic
photometric colors, as well as with photometric colors calculated with
the MARCS and ATLAS model atmosphere codes. We find good agreement of
the new synthetic colors with observations and published T_eff-color
and color-color relations, especially in the T_eff-(V-K), T_eff-(J-K)
and (J-K)-(V-K) planes. Deviations from the observed trends in the
T_eff-color planes are generally within ±100 K for T_eff=3500 to 4800
K. Synthetic colors calculated with different stellar atmosphere models
agree to ±100 K, within a large range of effective temperatures and
gravities. The comparison of the observed and synthetic spectra of
late-type giants shows that discrepancies result from the differences
both in the strengths of various spectral lines/bands (especially
those of molecular bands, such as TiO, H2O, CO) and the continuum
level. Finally, we derive several new T_eff-log g-color relations for
late-type giants at solar-metallicity (valid for T_eff=3500 to 4800
K), based both on the observed effective temperatures and colors of
the nearby giants, and synthetic colors produced with PHOENIX, MARCS
and ATLAS model atmospheres.
---------------------------------------------------------
Title: Model atmospheres of substellar atmospheres at a young age:
influence of gravity and dust.
Authors: Homeier, D.; Allard, F.; Ludwig, H. -G.; Hauschildt, P.;
Dehn, M.
2005AN....326Q.628H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: VizieR Online Data Catalog: Late-type giants BVRIJHKL and
Teff calibration (Kucinskas+, 2005)
Authors: Kucinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott,
I.; Vansevicius, V.; Lindegren, L.; Tanabe, T.; Allard, F.
2005yCat..34420281K Altcode:
Table 2 contains synthetic broad-band photometric colors of late-type
giants in the Johnson-Cousins-Glass photometric system. Colors are
based on the synthetic spectra calculated with the PHOENIX stellar
model atmosphere code. Photometric filter definitions used are those
from Bessell (1990PASP..102.1181B) for the Johnson-Cousins BVRI bands,
and from Bessell & Brett (1988PASP..100.1134B) for the Johnson-Glass
JHKL bands. <P />(1 data file).
---------------------------------------------------------
Title: Dynamo action in M-dwarfs
Authors: Dorch, S. B. F.; Gudiksen, B. V.; Ludwig, H. -G.
2005ESASP.560..515D Altcode: 2005csss...13..515D; 2004astro.ph..9219D
Magnetic activity in M-dwarfs present enigmatic questions: On the
one hand they have higher field strengths and larger filling factors
than the magnetic field on the Sun, on the other hand, they are fully
convective and their atmospheres are more neutral, hence they do not
have an undershoot layer for magnetic flux storage and as we show here,
cannot have small-scale dynamo action in their photospheres either. We
present a discussion of these facts and propose a new numerical model
to investigate M-dwarf magnetism.
---------------------------------------------------------
Title: Hydrodynamical simulations of convection-related stellar
micro-variability
Authors: Svensson, F.; Ludwig, H. -G.
2005ESASP.560..979S Altcode: 2004astro.ph..9714S; 2005csss...13..979S
We used a series of COBOLD hydrodynamical model atmospheres covering
stellar objects from white dwarfs to red giants to derive theoretical
estimates of the photometric and photocentric stellar variability in
wavelength-integrated light across the Hertzsprung-Russell diagram. We
validated our models against solar measurements from the SOHO/VIRGO
instrument. Within our set of models we find a systematic increase
of the photometric as well as photocentric variability - which turn
out to be closely connected - with decreasing surface gravity. The
estimated absolute levels of the photocentric variability do not affect
astrometric observations on a precision level expected to be achieved
by the GAIA mission - with the exception of close-by giants. The case
of supergiants remains to be investigated. In view of the ongoing
debate about the photometric non-detection of p-modes in Procyon
by the Canadian MOST satellite we remark that we obtain a factor of
ca. 3 in amplitude between the granular background noise in the Sun
and Procyon. This statement refers to a particular representation of
temporal power spectra as discussed in Sect. 5.
---------------------------------------------------------
Title: The shock-patterned solar chromosphere in the light of ALMA
Authors: Wedemeyer-Böhm, S.; Ludwig, H. -G.; Steffen, M.; Freytag,
B.; Holweger, H.
2005ESASP.560.1035W Altcode: 2005csss...13.1035W; 2005astro.ph..9747W
Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer
et al. (2004) suggest that the solar chromosphere is highly structured
in space and time on scales of only 1000 km and 20-25 sec, resp.. The
resulting pattern consists of a network of hot gas and enclosed cool
regions which are due to the propagation and interaction of shock
fronts. In contrast to many other diagnostics, the radio continuum at
millimeter wavelengths is formed in LTE, and provides a rather direct
measure of the thermal structure. It thus facilitates the comparison
between numerical model and observation. While the involved time
and length scales are not accessible with todays equipment for that
wavelength range, the next generation of instruments, such as the
Atacama Large Millimeter Array (ALMA), will provide a big step towards
the required resolution. Here we present results of radiative transfer
calculations at mm and sub-mm wavelengths with emphasis on spatial
and temporal resolution which are crucial for the ongoing discussion
about the chromospheric temperature structure.
---------------------------------------------------------
Title: 3D simulation of convection and spectral line formation in
A-type stars
Authors: Steffen, M.; Freytag, B.; Ludwig, H. -G.
2005ESASP.560..985S Altcode: 2005astro.ph..9464S; 2005csss...13..985S
We present first realistic numerical simulations of 3D radiative
convection in the surface layers of main sequence A-type stars with Teff
= 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the
CO5BOLD radiation hydrodynamics code. The resulting models are used to
investigate the structure of the H+HeI and the HeII convection zones
in comparison with the predictions of local and non-local convection
theories, and to determine the amount of "overshoot" into the stable
layers below the HeII convection zone. The simulations also predict
how the topology of the photospheric granulation pattern changes from
solar to A-type star convection. The influence of the photospheric
temperature fluctuations and velocity fields on the shape of spectral
lines is demonstrated by computing synthetic line profiles and line
bisectors for some representative examples, allowing us to confront
the 3D model results with observations.
---------------------------------------------------------
Title: Wavelength shifts in solar-type spectra
Authors: Dravins, D.; Lindegren, L.; Ludwig, H. -G.; Madsen, S.
2005ESASP.560..113D Altcode: 2004astro.ph..9212D; 2005csss...13..113D
Spectral-line displacements away from the wavelengths naively expected
from the Doppler shift caused by stellar radial motion may originate as
convective shifts (correlated velocity and brightness patterns in the
photosphere), as gravitational redshifts, or perhaps be induced by wave
motions. Absolute lineshifts, in the past studied only for the Sun, are
now accessible also for other stars thanks to astrometric determination
of stellar radial motion, and spectrometers with accurate wavelength
calibration. Comparisons between spectroscopic apparent radial
velocities and astrometrically determined radial motions reveal greater
spectral blueshifts in F-type stars than in the Sun (as theoretically
expected from their more vigorous convection), further increasing in
A-type stars (possibly due to atmospheric shockwaves). An important
near-future development to enable a further analysis of stellar surface
structure will be the study of wavelength variations across spatially
resolved stellar disks, e.g., the center-to-limb wavelength changes
along a stellar diameter, and their spatially resolved time variability.
---------------------------------------------------------
Title: Status and future of hydrodynamical model atmospheres
Authors: Ludwig, H. -G.; Kučinskas, A.
2005ESASP.560..319L Altcode: 2005csss...13..319L; 2004astro.ph..9712L
Since about 25 years ago work has been dedicated to the development
of hydrodynamical model atmospheres for cool stars (of A to T spectral
type). Despite their obviously sounder physical foundation in comparison
with standard hydrostatic models, their general application has been
rather limited. In order to understand why this is, and how to progress,
we review the present status of hydrodynamical modelling of cool star
atmospheres. The development efforts were and are motivated by the
theoretical interest of understanding the dynamical processes operating
in stellar atmospheres. To show the observational impact, we discuss
examples in the fields of spectroscopy and stellar structure where
hydrodynamical modelling provided results on a level qualitatively
beyond standard models. We stress present modelling challenges,
and highlight presently possible and future observations that would
be particularly valuable in the interplay between model validation
and interpretation of observables, to eventually widen the usage of
hydrodynamical model atmospheres within the astronomical community.
---------------------------------------------------------
Title: Excitation of P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
2005HiA....13..411S Altcode:
We describe the stochastic excitation of p-mode oscillations by solar
convection. We discuss the role of Reynolds stresses and entropy
fluctuations what controls the excitation spectrum the depth of the
driving and the location of the driving. We then present results for
a range of other stars and discuss the similarities and differences
with the Sun.
---------------------------------------------------------
Title: Theoretical Modelling of Late-Type Giant Atmospheres: Preparing
for Gaia
Authors: Kucinskas, A.; Brott, I.; Hauschildt, P. H.; Ludwig, H. -G.;
Lindegren, L.; Tanabé, T.; Vansevicius, V.
2005ESASP.576..591K Altcode: 2005tdug.conf..591K; 2005astro.ph..3208K
Late-type giants (RGB/AGB stars) will be important tracers of the
Galactic morphology and evolution in the framework of Gaia, as they are
intrinsically bright and thus can probe distant stellar populations or
those obscured by interstellar extinction. A realistic representation
of their atmospheres and spectra with stellar atmosphere models is thus
of crucial importance, both for the design and optimization of Gaia
instruments, as well as the interpretation of provided astrophysical
data. Our analysis of synthetic photometric colours of latetype giants
based on PHOENIX, MARCS and ATLAS model atmospheres indicates a general
agreement between the current theoretical predictions and observations
in the framework of stationary 1-D model atmospheres. Presently
available models allow temperature determinations of RGB/AGB stars to
an accuracy of ∼ ±100 K. In an exploratory study we try to quantify
possible residual systematic effects due to the approximations made
in 1-D models using full 3-D hydrodynamical models. We find that
differences in broad-band photometric colours calculated with 1-D and
3-D models are significant, translating to the offsets in effective
temperature of up to μTeff ∼70 K. Clearly, full 3-D hydrodynamical
models will help to alleviate such ambiguities in current theoretical
modelling. Additionally, they will allow to study new phenomena, to
open qualitatively new windows for stellar astrophysics in the Gaia-era.
---------------------------------------------------------
Title: PoSSO Physics of SubStellar Objects
Authors: Jones, Hugh; Viti, Serena; Tennyson, Jonathan; Barber, Bob;
Pickering, Juliet; Blackwell-Whitehead, Richard; Champion, Jean-Paul;
Allard, France; Hauschildt, Peter; Jørgensen, Uffe; Ehrenfreund,
Pascale; Stachowska, Ewa; Ludwig, Hans-Günter; Pavlenko, Yakiv;
Lyubchik, Yuri; Kurucz, Robert
2005hris.conf..477J Altcode:
A full understanding of the properties of substellar objects is one
of the major challenges facing astrophysics. Since their discovery
in 1995, we have discovered hundreds of brown dwarfs and extrasolar
planets. While these discoveries have enabled important comparisons
with theory, observational progress has been much more rapid than the
theoretical understanding of cool atmospheres. The determination of
mass, abundances, gravities and temperatures is not yet possible. The
key problem is that substellar objects emit their observable radiation
in the infrared region of the spectrum where our knowledge of atomic,
molecular and line broadening data is poor. In order to understand
these objects, and extra-solar planets increasing more like those our
Solar System, we urge the wider physical chemistry community to engage
in this exciting new field. Here we sketch an outline of the atoms,
molecules and processes requiring study.
---------------------------------------------------------
Title: Hypervelocity impacts on HST solar arrays and the debris and
meteoroids population
Authors: Moussi, A.; Drolshagen, G.; McDonnell, J. A. M.; Mandeville,
J. -C.; Kearsley, A. T.; Ludwig, H.
2005AdSpR..35.1243M Altcode:
Accurate debris and meteoroid flux models are crucial for the design
of manned and unmanned space missions. For the most abundant particle
sizes smaller than a few millimetres, knowledge of the populations can
only be gained from in situ detectors or the analysis of retrieved space
hardware. The measurement of impact flux from exposed surfaces improves
with increased surface area and exposure time. A post-flight impact
investigation was initiated by the European Space Agency to record and
analyse the impact fluxes and any potential resulting damage on the two
flexible solar arrays of the Hubble Space Telescope. The arrays were
deployed during the first Hubble Space Telescope servicing mission in
December 1993 and retrieved in March 2002. They have a total exposed
surface area of roughly 120 m <SUP>2</SUP>, including 42 m <SUP>2</SUP>
covered with solar cells. This new Hubble post-flight impact study
follows a similar activity undertaken after the retrieval of one of
the first solar arrays, in 1993. The earlier study provided the first
opportunity for a numerical survey of damage to exposed surfaces from
more than 600 km altitude, and of impacts from particles larger than 1
mm. The results have proven very valuable in validation of important
flux model regimes. The second set of Hubble solar arrays has again
provided an unrivalled opportunity to measure the meteoroid and debris
environment, now sampled during a long interval in low Earth orbit,
and to identify changes in the space debris environment since the
previous survey. The retrieved solar array wings exhibit thousands
of craters, many of which are visible to the naked eye. A few hundred
impacts have completely penetrated the 0.7 mm thick array. The largest
impact features are about 7-8 mm in diameter. The cover glass of the
solar cells is particularly well suited to the recognition of small
impact features by optical and electron microscopy. In this paper, we
present the first results of the impact survey. Data upon the abundance
of craters of specific measured size ranges are plotted as cumulative
flux curves, and compared to the results of model predictions. The
most significant change to the particle flux since 1993 is a decrease
in the small debris population.
---------------------------------------------------------
Title: Procyon: Constraining Its Temperature Structure with
High-Precision Interferometry and 3-D Model Atmospheres
Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P.
2004AAS...205.1203A Altcode: 2004BAAS...36.1357A
We have fit synthetic visibilities from 3-D (CO<SUP>5</SUP>BOLD +
PHOENIX) and 1-D (PHOENIX, ATLAS12) model stellar atmospheres for
Procyon (F5 IV) to high-precision interferometric data from the
VINCI instrument at the VLT Interferometer (K-band) and from the
Mark III interferometer (500 nm, 800 nm). These data provide a test
of theoretical wavelength-dependent limb-darkening predictions, and
therefore Procyon's atmospheric temperature structure. Earlier work
(Allende Prieto et al. 2002 ApJ 567, 544) has shown that the temperature
structure from a spatially and temporally averaged 3-D hydrodynamical
model produces significantly less limb darkening at 500 nm relative to
the temperature structure from a 1-D MARCS model atmosphere which uses
a mixing-length approximation for convective flux transport. Our direct
fits to the interferometric data confirm this prediction, however we
find that not all 1-D models fail to reproduce the observations. The key
to matching the interferometric data is a shallower temperature gradient
than provided by the standard 1-D mixing-length approximation. We find
that in addition to our best fitting 3-D hydrodynamical model, a 1-D
ATLAS12 model, with an additional free parameter for “approximate
overshooting”, provides the required temperature gradient. We
estimate that an interferometric precision better than 0.1% will
be required to distinguish between the 3-D model and the ATLAS12
model. This overshooting approximation has been shown to match Solar
limb-darkening observations reasonably well (Castelli et al 1997 A&A
324, 432), however published work since using Strömgren photometry of
solar-type stars has cast doubt on the importance of overshooting. We
have also compared synthetic spectral energy distributions for Procyon
to ultraviolet, optical and near-infrared spectrophotometry and find
differences from comparisons to Strömgren photometry alone. <P />This
work was performed in part contract with the Jet Propulsion Laboratory
(JPL) funded by NASA through the Michelson Fellowship Program. JPL is
managed for NASA by the California Institute of Technology.
---------------------------------------------------------
Title: Intrinsic Wavelength Shifts in Stellar Spectra
Authors: Dravins, D.; Lindegren, L.; Ludwig, H. -G.; Madsen, S.
2004AAS...20517004D Altcode: 2004BAAS...36.1624D
Wavelengths of stellar spectral lines do not have the precise values
`naively' expected from laboratory wavelengths merely Doppler-shifted
by stellar radial motion. Slight displacements may originate as
convective shifts (correlated velocity and brightness patterns in the
photosphere), as gravitational redshifts, or perhaps be induced by wave
motions. Intrinsic lineshifts thus reveal stellar surface structure,
while possible periodic changes (during a stellar activity cycle,
say) need to be segregated from variability induced by orbiting
exoplanets. <P />Absolute lineshifts can now be studied also in some
stars other than the Sun, thanks to astrometric determinations of
stellar radial motion. Comparisons between spectroscopic apparent radial
velocities and astrometrically determined radial motions reveal greater
spectral blueshifts in F-type stars than in the Sun (as theoretically
expected from their more vigorous convection), further increasing
in A-type stars (possibly due to atmospheric shockwaves). <P />Solar
spectral atlases, and high-resolution spectra (from UVES on ESO VLT) of
a dozen solar-type stars are being surveyed for `unblended' photospheric
lines of most atomic species with accurate laboratory wavelengths
available. One aim is to understand the ultimate information content
of stellar spectra, and in what detail it will be feasible to verify
models of stellar atmospheric hydrodynamics. These may predict line
asymmetries (bisectors) and shifts for widely different classes of
lines, but there will not result any comparison with observations if
such lines do not exist in real spectra. <P />An expected near-future
development in stellar physics is spatially resolved spectroscopy across
stellar disks, enabled by optical interferometry and adaptive optics
on very large telescopes. Stellar surface structure manifests itself
in the center-to-limb wavelength changes along a stellar diameter,
and their spatially resolved time variability, diagnostics which
already now can be theoretically modeled.
---------------------------------------------------------
Title: Excitation of Radial P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
2004SoPh..220..229S Altcode:
P-mode oscillations in the Sun and stars are excited stochastically
by Reynolds stress and entropy fluctuations produced by convection in
their outer envelopes. The excitation rate of radial oscillations of
stars near the main sequence from K to F and a subgiant K IV star have
been calculated from numerical simulations of their surface convection
zones. P-mode excitation increases with increasing effective temperature
(until envelope convection ceases in the F stars) and also increases
with decreasing gravity. The frequency of the maximum excitation
decreases with decreasing surface gravity.
---------------------------------------------------------
Title: Numerical simulation of the three-dimensional structure and
dynamics of the non-magnetic solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2004A&A...414.1121W Altcode: 2003astro.ph.11273W
Three-dimensional numerical simulations with CO<SUP>5</SUP>, a
new radiation hydrodynamics code, result in a dynamic, thermally
bifurcated model of the non-magnetic chromosphere of the quiet Sun. The
3D model includes the middle and low chromosphere, the photosphere,
and the top of the convection zone, where acoustic waves are excited
by convective motions. While the waves propagate upwards, they steepen
into shocks, dissipate, and deposit their mechanienergy as heat in
the chromosphere. Our numerical simulations show for the first time
a complex 3D structure of the chromospheric layers, formed by the
interaction of shock waves. Horizontal temperature cross-sections of
the model chromosphere exhibit a network of hot filaments and enclosed
cool regions. The horizontal pattern evolves on short time-scales of
the order of typically 20-25 s, and has spatial scales comparable to
those of the underlying granulation. The resulting thermal bifurcation,
i.e., the co-existence of cold and hot regions, provides temperatures
high enough to produce the observed chromospheric UV emission and -
at the same time - temperatures cold enough to allow the formation
of molecules (e.g., carbon monoxide). Our 3D model corroborates the
finding by \citet{carlsson94} that the chromospheric temperature rise
of semi-empirical models does not necessarily imply an increase in
the average gas temperature but can be explained by the presence of
substantial spatial and temporal temperature inhomogeneities.
---------------------------------------------------------
Title: Hypervelocity impacts on HST solar arrays and the debris
population
Authors: Drolshagen, G.; McDonnell, J. A. M.; Mandeville, J. -C.;
Moussi, A.; Ludwig, H.
2004cosp...35..969D Altcode: 2004cosp.meet..969D
Accurate debris and meteoroid flux models are crucial for the design
of manned and unmanned space missions. For the most abundant particle
sizes smaller than a few millimetres, knowledge on the populations can
only be gained by in-situ detectors or the analysis of retrieved space
hardware. The impact flux information, which can be obtained from
exposed surfaces, increases with surface area and exposure time. A
Post-Flight Impact Investigation was initiated by ESA to record and
analyze the impact fluxes and any potential resulting damage on the
two flexible solar arrays of the Hubble Space Telescope. They were
deployed during the first HST servicing mission in December 1993
and retrieved in March 2002. They have a total exposed surface area
of roughly 120 m<SUP>2</SUP>, including 41 m<SUP>2</SUP> covered
with solar cells. The HST post-flight impact study follows a similar
activity undertaken after the retrieval of one of the first HST solar
arrays. That analysis has been very successful and already resulted in
a validation of certain flux model regimes. For the first time exposed
surfaces from more then 600 km altitude could be analysed and impacts
from particles larger than 1 mm could be observed. The second set of
HST solar arrays provide 4 times the area x time product of the first
array and extend the measurements to the largest particle sizes ever
recorded. The retrieved HST solar array wings exhibit thousands of
craters, which are visible to the naked eye. A few hundred impacts
have completely penetrated the 0.7 mm thick array. The largest impact
features are about 7-8 mm in diameter. Measured fluxes of craters
larger than 10 microns and 1 mm are in the order of 3 x 10<SUP>-5</SUP>
m<SUP>-2</SUP> s<SUP>-1</SUP> and 1.3 x 10<SUP>-7</SUP> m<SUP>-2</SUP>
s<SUP>-1</SUP>, respectively. First results of the impact survey are
presented here and compared to model predictions. Flux predictions are
based on the latest meteoroid and debris (e.g. MASTER 2001) models
and on crater size equations derived specifically for the HST solar
arrays. A second paper presented during this conference is dealing
more specifically on the identification of projectile remnants and
discrimination between natural particles and man-made orbital debris.
---------------------------------------------------------
Title: Revising the abundance of Pr in the solar photosphere
Authors: Ivarsson, S.; Wahlgren, G. M.; Ludwig, H. -G.
2003AAS...20313404I Altcode: 2003BAAS...35.1421I
New atomic data (Ivarsson et al. 2001, Physica Scripta 64, 455)
have opened the possibility of re-evaluating the solar photospheric
abundance of the rare-earth element praseodymium. The experimental
spectra were recorded in the region 2000 to 7000 Å with the Lund UV
Fourier transform spectrometer and a hollow-cathode light source. These
data allowed us to measure accurate wavelengths, re-analyse energy
levels and determine the hyperfine structure for transitions of Pr II
and Pr III. In addition, energy level lifetimes were measured at the
Lund Laser Centre, which combined with the new experimental branching
fractions lead to revisions of the gf values for many lines. The
abundance of Pr in the solar photosphere was determined from synthetic
spectrum fitting of three Pr II lines found in high-resolution solar
observations from Kitt-Peak (NOAO) and Junfraujoch observatories. The
1D modelling was accomplished using the ATLAS/SYNTHE LTE codes (Kurucz
1998) and resulted in a Pr abundance (log ɛ =0.40±0.1) that represents
a significant lowering from the current canonical value of the solar
photospheric abundance (log ɛ =0.71±0.08, Biemont et al. 1979,
Sol. Phys. 61, 17). We also tested the influence of 3D effects on
the determination of the Pr abundance using the CO<SUP>5</SUP>BOLD
radiation-hydrodynamics code (Freytag et al. 2002, AN 323, 213). Our
preliminary results show insignificant differences in the equivalent
widths of the Pr II lines between the 3D and 1D cases. Our new solar
abundance of Pr differs from that determined from a sample of two
carbonaceous condrite meteorites (log ɛ =0.78±0.03, Lodder 2003,
ApJ 591, 1220) and raises such questions as the appropriateness of
assuming the convergence of solar and meteoritic abundances and the
accuracy of modelling the solar spectrum.
---------------------------------------------------------
Title: Intrinsic spectral blueshifts in rapidly rotating stars?
Authors: Madsen, Søren; Dravins, Dainis; Ludwig, Hans-Günter;
Lindegren, Lennart
2003A&A...411..581M Altcode: 2003astro.ph..9346M
Spectroscopic radial velocities for several nearby open clusters
suggest that spectra of (especially earlier-type) rapidly rotating
stars are systematically blueshifted by 3 km s<SUP>-1</SUP> or more,
relative to the spectra of slowly rotating ones. Comparisons with
astrometrically determined radial motions in the Hyades suggests this
to be an absolute blueshift, relative to wavelengths naively expected
from stellar radial motion and gravitational redshift. Analogous
trends are seen also in most other clusters studied (Pleiades,
Coma Berenices, Praesepe, alpha Persei, IC 2391, NGC 6475, IC 4665,
NGC 1976 and NGC 2516). Possible mechanisms are discussed, including
photospheric convection, stellar pulsation, meridional circulation,
and shock-wave propagation, as well as effects caused by template
mismatch in determining wavelength displacements. For early-type
stars, a plausible mechanism is shock-wave propagation upward through
the photospheric line-forming regions. Such wavelength shifts thus
permit studies of certain types of stellar atmospheric dynamics and
- irrespective of their cause - may influence deduced open-cluster
membership (when selected from common velocity) and deduced cluster
dynamics (some types of stars might show fortuitous velocity patterns).
---------------------------------------------------------
Title: 3D Simulation of the Solar Granulation: A Comparison of two
Different Hydrodynamics Codes
Authors: Steffen, Matthias; Ludwig, Hans-Günter; Freytag, Bernd
2003ANS...324..174S Altcode: 2003ANS...324..P96S
No abstract at ADS
---------------------------------------------------------
Title: Convection and Small-scale Magnetic Fields in M-type
Atmospheres
Authors: Ludwig, Hans-Günter; Dorch, Søren Bertil Fabricius
2003ANS...324...65L Altcode: 2003ANS...324..I06L
No abstract at ADS
---------------------------------------------------------
Title: Modelling the Chromospheric Background Pattern of the
Non-magnetic Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Ludwig,
Hans-Günter; Holweger, Hartmut
2003ANS...324R..66W Altcode: 2003ANS...324..I07W
No abstract at ADS
---------------------------------------------------------
Title: Model Atmospheres and Spectra: The Role of Dust
Authors: Allard, France; Guillot, Tristan; Ludwig, Hans-Günter;
Hauschildt, Peter H.; Schweitzer, Andreas; Alexander, David R.;
Ferguson, Jason W.
2003IAUS..211..325A Altcode:
Brown dwarf atmospheres form molecules, then high temperature
condensates (corundum, titanates, silicates, and iron compounds),
and then low temperature condensates (ices) as they cool down over
time. These produce large opacities which govern entirely their
spectral energy distribution. Just as it is important to know
molecular opacities (TiO, H_2O, CH_4, etc.) with accuracy, it is
imperative to understand the interplay of processes (e.g. condensation,
sedimentation, coagulation, convection) that determines the radial
and size distribution of grains. Limiting case models have shown that
young, hot brown (L) dwarfs form dust mostly in equilibrium, while at
much cooler stages (late T dwarfs) all high temperature condensates
have sedimented out of their photospheres. But this process is gradual
and all intermediate classes of brown dwarfs can partly be understood
in terms of partial sedimentation of dust. With new models accounting
for these processes, we describe the effects they may have upon brown
dwarf spectral properties.
---------------------------------------------------------
Title: Challenges in the Solution of the Transfer Equation in Multi-D
Hydrodynamical Model Atmospheres for Cool Stars
Authors: Ludwig, H. -G.
2003ASPC..288..537L Altcode: 2003sam..conf..537L
My talk is intended to stimulate discussions about methods in radiative
transfer for hydrodynamical model atmospheres of late-type stars. I
will present a number a number of inherent problems, show how they
are presently tackled (if so), and ask for ideas for more efficient
approaches. The transparencies of my talk can be found in <P /><A
href="http://www.astro.lu.se/~hgl/public_files/tuebingen2001.ps">
Transparencies (4.3Mb)</A>
---------------------------------------------------------
Title: Acoustic Waves in the Solar Chromosphere - Numerical
Simulations with COBOLD
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2003IAUS..210P..C1W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: 3-D hydrodynamic simulations of the solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2003AN....324..410W Altcode:
We present first results of three-dimensional numerical simulations
of the non-magnetic solar chromosphere, computed with the radiation
hydrodynamics code CO<SUP>5</SUP>BOLD. Acoustic waves which are
excited at the top of the convection zone propagate upwards into the
chromosphere where the waves steepen into shocks. The interaction of
the waves leads to the formation of complex structures which evolve
on short time scales. Consequently, the model chromosphere is highly
dynamical, inhomogeneous, and thermally bifurcated.
---------------------------------------------------------
Title: Energy Transport, Overshoot, and Mixing in the Atmospheres
of Very Cool Stars
Authors: Ludwig, H. -G.
2003IAUS..210..113L Altcode: 2002astro.ph..8583L
We constructed hydrodynamical model atmospheres for mid M-type
main-, as well as pre-main-sequence objects. Despite the complex
chemistry encountered in such cool atmospheres a reasonably accurate
representation of the radiative transfer is possible. The detailed
treatment of the interplay between radiation and convection in the
hydrodynamical models allows to study processes usually not accessible
within the framework conventional model atmospheres. In particular,
we determined the efficiency of the convective energy transport,
and the efficiency of mixing by convective overshoot. The convective
transport efficiency expressed in terms of an equivalent mixing-length
parameter amounts to values around ~2 in the optically thick, and ~2.8
in the optically thin regime. The thermal structure of the formally
convectively stable layers is little affected by convective overshoot
and wave heating, i.e. stays close to radiative equilibrium. Mixing
by convective overshoot shows an exponential decline with geometrical
distance from the Schwarzschild stability boundary. The scale height of
the decline varies with gravitational acceleration roughly as g^(-1/2),
with 0.5 pressure scale heights at log(g)=5.0.
---------------------------------------------------------
Title: Numerical simulations of surface convection in a late M-dwarf
Authors: Ludwig, H. -G.; Allard, F.; Hauschildt, P. H.
2002A&A...395...99L Altcode: 2002astro.ph..8584L
Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD)
simulations of time-dependent compressible convection, we have studied
the dynamics and thermal structure of the convective surface layers of a
prototypical late-type M-dwarf (T<SUB>eff</SUB>approx 2800 K, log g=5.0,
solar chemical composition). The RHD models predict stellar granulation
qualitatively similar to the familiar solar pattern. Quantitatively,
the granular cells show a convective turn-over time scale of ~100 s,
and a horizontal scale of 80 km; the relative intensity contrast of
the granular pattern amounts to 1.1%, and root-mean-square vertical
velocities reach 240 m s<SUP>-1</SUP> at maximum. Deviations from
radiative equilibrium in the higher, formally convectively stable
atmospheric layers are found to be insignificant allowing a reliable
modeling of the atmosphere with 1D standard model atmospheres. A
mixing-length parameter of α<SUB>MLT = 2.1</SUB> provides the
best representation of the average thermal structure of the RHD
model atmosphere while alternative values are found when fitting the
asymptotic entropy encountered in deeper layers of the stellar envelope
(α<SUB>MLT = 1.5</SUB>), or when matching the vertical velocity
(α<SUB>MLT = 3.5</SUB>). The close correspondence between RHD and
standard model atmospheres implies that presently existing discrepancies
between observed and predicted stellar colors in the M-dwarf regime
cannot be traced back to an inadequate treatment of convection in the
1D standard models. The RHD models predict a modest extension of the
convectively mixed region beyond the formal Schwarzschild stability
boundary which provides hints for the distribution of dust grains in
cooler (brown dwarf) atmospheres.
---------------------------------------------------------
Title: A simulation of solar convection at supergranulation scale
Authors: Rieutord, M.; Ludwig, H. -G.; Roudier, T.; Nordlund, .;
Stein, R.
2002NCimC..25..523R Altcode: 2001astro.ph.10208R
We present here numerical simulations of surface solar convection
which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of
315$\times315\times$82, which is used to investigate the dynamics of
scales larger than granulation. No structure resembling supergranulation
is present; possibly higher Reynolds numbers (i.e. higher numerical
resolution), or magnetic fields, or greater depth are necessary. The
results also show interesting aspects of granular dynamics which are
briefly presented, like extensive p-mode ridges in the k-$\omega$
diagram and a ringlike distribution of horizontal vorticity around
granules. At large scales, the horizontal velocity is much larger
than the vertical velocity and the vertical motion is dominated by
p-mode oscillations.
---------------------------------------------------------
Title: Small-scale magnetic fields on late-type M-dwarfs
Authors: Dorch, S. B. F.; Ludwig, H. -G.
2002AN....323..402D Altcode: 2002astro.ph..5458D
We performed kinematic studies of the evolution of small-scale magnetic
fields in the surface layers of M-dwarfs. We solved the induction
equation for a prescribed velocity field, magnetic Reynolds number
\ReM, and boundary conditions in a Cartesian box, representing a
volume comprising the optically thin stellar atmosphere and the
uppermost part of the optically thick convective envelope. The
velocity field is spatially and temporally variable, and stems
from detailed radiation-hydrodynamics simulations of convective
flows in a proto-typical late-type M-dwarf (Teff =2800pun {K}, logg
=5.0, solar chemical composition, spectral type ~M6). We find dynamo
action for ReM >= 400. Growth time scales of the magnetic field are
comparable to the convective turn-over time scale (~ 150pun {sec}). The
convective velocity field concentrates the magnetic field in sheets
and tubular structures in the inter-granular downflows. Scaling from
solar conditions suggests that field strengths as high as 20\pun{kG}
might be reached locally. Perhaps surprisingly, \ReM\ is of order
unity in the surface layers of cooler M-dwarfs, rendering the dynamo
inoperative. In all studied cases we find a rather low spatial filling
factor of the magnetic field.
---------------------------------------------------------
Title: Are granules good tracers of solar surface velocity fields?
Authors: Rieutord, M.; Roudier, T.; Ludwig, H. -G.; Nordlund, Å.;
Stein, R.
2001A&A...377L..14R Altcode: 2001astro.ph..8284R
Using a numerical simulation of compressible convection with radiative
transfer mimicking the solar photosphere, we compare the velocity
field derived from granule motions to the actual velocity field of
the plasma. We thus test the idea that granules may be used to trace
large-scale velocity fields at the sun's surface. Our results show that
this is indeed the case provided the scale separation is sufficient. We
thus estimate that neither velocity fields at scales less than 2500
km nor time evolution at scales shorter than 0.5 hr can be faithfully
described by granules. At larger scales the granular motions correlate
linearly with the underlying fluid motions with a slope of ≲2 reaching
correlation coefficients up to ~ 0.9.
---------------------------------------------------------
Title: Radiation-hydrodynamics Simulations of Surface Convection in
a Late M-dwarf
Authors: Ludwig, Hans-Günter; Hauschildt, Peter
2001AGM....18..P15L Altcode:
Based on detailed 2D and 3D numerical radiation hydrodynamics
calculations of time-dependent compressible convection, we have
studied the dynamics and thermal structure of the convective surface
layers of a prototypical late-type M-dwarf at T<SUB>eff</SUB>=2800 K,
and log g=5.0 with solar chemical composition. The thermal structure
of the atmosphere is dominated by molecular absorption, the formation
of dust grains is not important for the chosen model parameters. In
the multi dimensional models the radiative transfer is treated
by a multi-group (4 groups) approach which provides a simplified,
nevertheless rather realistic treatment of the complex radiative energy
transport. The equation of state includes the important contribution
of H<SUB>2</SUB> molecule formation. Our models predict a convective
pattern at the surface of an late M-dwarf qualitatively similar to
solar granulation. Quantitatively, the convective turn-over timescale
amounts to ≈ 100 s, a typical horizontal scale of convective cells
to 80 km, and a relative intensity contrast of the granular pattern to
1.1 %. The efficiency of convective energy transport corresponds to
an effective mixing-length parameter between 1.5 to 2.1 depending on
the thermal property which is represented. The models provide mixing
timescales due to atmospheric overshoot which can be extrapolated to
lower effective temperatures where dust grains are present and mixed
into optically thin layers.
---------------------------------------------------------
Title: Acoustic Energy Generated by Convection: 3-D Numerical
Simulations for the Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Holweger, Hartmut; Ludwig,
Hans-Günter; Steffen, Matthias
2001AGM....18..P01W Altcode:
Dissipation of acoustic waves may be an efficient heating mechanism for
the lower and middle chromosphere of the quiet Sun. The basic idea is
that turbulent motions at the top of the solar convection zone generate
acoustic waves which propagate upwards and dissipate in the lower and
middle chromosphere, transporting energy into the higher layers. But
still the question remains if this amount of energy is sufficient to
explain the observed temperature increase without invoking magnetic
fields. With a new version of the COBOLD radiation hydrodynamics code
we are able to compute 3-D models extending all the way from the upper
convection zone to the middle chromosphere. First 3-D simulations reveal
a complex, inhomogenous and highly dynamical structure of the lower and
middle chromosphere which evolves on rather short timescales. On small
spatial dimensions very cool regions are present next to a "network"
of hotter matter. The code is being developed further to provide a
more detailed analysis and comparison with observations.
---------------------------------------------------------
Title: The effects of numerical resolution on hydrodynamical surface
convection simulations and spectral line formation
Authors: Asplund, M.; Ludwig, H. -G.; Nordlund, Å.; Stein, R. F.
2000A&A...359..669A Altcode: 2000astro.ph..5319A
The computationally demanding nature of radiative-hydrodynamical
simulations of stellar surface convection warrants an investigation
of the sensitivity of the convective structure and spectral synthesis
to the numerical resolution and dimension of the simulations, which
is presented here. With too coarse a resolution the predicted spectral
lines tend to be too narrow, reflecting insufficient Doppler broadening
from the convective motions, while at the currently highest affordable
resolution the line shapes have converged essentially perfectly to
the observed profiles. Similar conclusions are drawn from the line
asymmetries and shifts. Due to the robustness of the pressure and
temperature structures with respect to the numerical resolution, strong
Fe lines with pronounced damping wings and H i lines are essentially
immune to resolution effects, and can therefore be used for improved
T_eff and log g determinations even at very modest resolutions. In
terms of abundances, weak Fe i and Fe ii lines show a very small
dependence ( =~ 0.02 dex) while for intermediate strong lines with
significant non-thermal broadening the sensitivity increases (<~ 0.10
dex). Problems arise when using 2D convection simulations to describe
an inherent 3D phenomenon, which translates to inaccurate atmospheric
velocity fields and temperature and pressure structures. In 2D the
theoretical line profiles tend to be too shallow and broad compared with
the 3D calculations and observations, in particular for intermediate
strong lines. In terms of abundances, the 2D results are systematically
about 0.1 dex lower than for the 3D case for Fe i lines. Furthermore,
the predicted line asymmetries and shifts are much inferior in 2D
with discrepancies amounting to ~ 200 m s<SUP>-1</SUP>. Given these
shortcomings and computing time considerations it is better to use
3D simulations of even modest resolution than high-resolution 2D
simulations.
---------------------------------------------------------
Title: The atmospheric dynamics in 2D and 3D simulations of stellar
surface convection
Authors: Ludwig, Hans-Guenter; Nordlund, Ake
2000ASSL..254...37L Altcode: 2000stas.conf...37L
No abstract at ADS
---------------------------------------------------------
Title: A calibration of the mixing-length for solar-type stars based
on hydrodynamical simulations. I. Methodical aspects and results
for solar metallicity
Authors: Ludwig, Hans-Günter; Freytag, Bernd; Steffen, Matthias
1999A&A...346..111L Altcode: 1998astro.ph.11179L
Based on detailed 2D numerical radiation hydrodynamics (RHD)
calculations of time-dependent compressible convection, we have
studied the dynamics and thermal structure of the convective surface
layers of solar-type stars. The RHD models provide information about
the convective efficiency in the superadiabatic region at the top of
convective envelopes and predict the asymptotic value of the entropy of
the deep, adiabatically stratified layers (Fig. \ref{f:sstarhd}). This
information is translated into an effective mixing-length parameter
\alpha<SUB>MLT</SUB> suitable to construct standard stellar
structure models. We validate the approach by a detailed comparison
to helioseismic data. The grid of RHD models for solar metallicity
comprises 58 simulation runs with a helium abundance of Y=0.28 in the
range of effective temperatures 4300pun {K}<=T<SUB>eff</SUB><=
7100pun {K} and gravities 2.54<={log g}<= 4.74. We find a
moderate, nevertheless significant variation of \alpha<SUB>MLT</SUB>
between about 1.3 for F-dwarfs and 1.75 for K-subgiants with a
dominant dependence on T<SUB>eff</SUB> (Fig. \ref{f:mlp}). In the close
neighbourhood of the Sun we find a plateau where \alpha<SUB>MLT</SUB>
remains almost constant. The internal accuracy of the calibration
of \alpha<SUB>MLT</SUB> is estimated to be +/- 0.05 with a possible
systematic bias towards lower values. An analogous calibration of
the convection theory of Canuto &\ Mazzitelli (1991, 1992; CMT)
gives a different temperature dependence but a similar variation of
the free parameter (Fig. \ref{f:mlpcm}). For the first time, values
for the gravity-darkening exponent beta are derived independently of
mixing-length theory: beta = 0.07... 0.10. We show that our findings
are consistent with constraints from stellar stability considerations
and provide compact fitting formulae for the calibrations.
---------------------------------------------------------
Title: Treatment of the Superadiabatic Convection in Low-Mass
Metal-Poor Stars from Realistic Hydrodynamics Simulations: Application
to Globular Clusters Isochrones
Authors: Freytag, B.; Salaris, M.; Ludwig, H. -G.
1999ASPC..173..201F Altcode: 1999sstt.conf..201F
No abstract at ADS
---------------------------------------------------------
Title: A Calibration of the Mixing-Length for Solar-Type Stars Based
on Hydrodynamical Models of Stellar Surface Convection
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
1999ASPC..173..225F Altcode: 1999sstt.conf..225F
No abstract at ADS
---------------------------------------------------------
Title: Convection and the Eclipsing Binary AI Phoenicis: How Well
Can We Constrain the Mixing-Length parameter from Stellar Modelling?
Authors: Ludwig, H. -G.; Salaries, M.
1999ASPC..173..229L Altcode: 1999sstt.conf..229L
No abstract at ADS
---------------------------------------------------------
Title: Balmer Line Formation in Convective Stellar Atmospheres
Authors: Steffen, M.; Ludwig, H. -G.
1999ASPC..173..217S Altcode: 1999sstt.conf..217S
No abstract at ADS
---------------------------------------------------------
Title: New peculiar CO data of the shell around IRC +10 216
Authors: Groenewegen, M. A. T.; Ludwig, H. -G.
1998A&A...339..489G Altcode: 1998astro.ph..9010G
A CO(1-0) on-source spectrum of the well-known carbon star IRC +10 216
taken with the IRAM 30m telescope in June 1996 shows excess emission
between -18.3 and -14.3 km s(-1) at the red wing of the underlying
profile. The excess emission is confirmed in January 1997 but is absent
in April 1997 and June 1998 IRAM spectra. Such a transient feature has
not been seen before in this star or any other AGB star. In April 1997
we mapped the circumstellar shell out to 110\arcsec. Both the J = 1-0
and J = 2-1 spectra show “spikes” or components which vary in strength
with position in the envelope. One of these components corresponds to
the velocity interval mentioned above. An immediate conclusion is that
the circumstellar shell is not spherically symmetric, contrary to what
was believed based on lower spectral resolution data. We are probably
seeing emission from a complex geometrical structure. Neither a disk
structure nor a double-wind structure seem to be able to explain the
observations. The on-source transient behaviour of the red excess
emission can reasonably well be explained by a single large ( ~ 5 x
10(13) cm) blob, that expands due to internal motion.
---------------------------------------------------------
Title: Lithium Depletion in the Sun: A Study of Mixing Based on
Hydrodynamical Simulations
Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig,
H. -G.; Steffen, M.
1998SSRv...85..105B Altcode: 1998astro.ph..6310B
Based on radiation hydrodynamics modeling of stellar convection zones,
a diffusion scheme has been devised describing the downward penetration
of convective motions beyond the Schwarzschild boundary (overshoot)
into the radiative interior. This scheme of exponential diffusive
overshoot has already been successfully applied to AGB stars. Here
we present an application to the Sun in order to determine the time
scale and depth extent of this additional mixing, i.e. diffusive
overshoot at the base of the convective envelope. We calculated the
associated destruction of lithium during the evolution towards and on
the main-sequence. We found that the slow-mixing processes induced by
the diffusive overshoot may lead to a substantial depletion of lithium
during the Sun's main-sequence evolution.
---------------------------------------------------------
Title: Lithium Depletion in the Sun: A Study of Mixing Based on
Hydrodynamical Simulations
Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig,
H. -G.; Steffen, M.
1998sce..conf..105B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An improved calibration of the mixing-length based on
simulations of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
1998IAUS..185..115L Altcode:
Based on detailed 2D numerical radiation hydrodynamics (RHD)
calculations of time-dependent compressible convection, we have studied
the dynamics and thermal structure of the convective surface layers
of stars in the range of effective temperatures and gravities between
4500 pun{K} <= Teff <= 7100 pun{K} and 2.54 <= logg <=
4.74. Although our hydrodynamical models describe only the shallow,
strongly superadiabatic layers at the top of the convective stellar
envelope, we demonstrate that they provide information about the value
of the entropy of the deeper, adiabatically stratified regions. This
quantity can be translated into an effective mixing-length parameter
suitable for constructing standard stellar structure models. We
show that a hydrodynamically calibrated envelope model for the Sun
closely matches the known adiabat and corresponding depth of the solar
convection zone. We determined the dependence of the mixing-length
parameter on Teff, log g, and chemical composition obtaining a moderate
variation over the range studied. We note that the recent description
of convection by Canuto & Mazzitelli extended by including a
variable amount of overshoot does not lead to a smaller variation of
the controlling parameter. We discuss the consistency of our results
with findings derived in the context of the tentative detection of
solar-like oscillations in eta Bootis.
---------------------------------------------------------
Title: A calibration of mixing length theory based on RHD simulations
of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
1997ASSL..225...59L Altcode: 1997scor.proc...59L
Radiation hydrodynamics (RHD) models provide detailed information
about the dynamics, thermal structure, and convective efficiency
of the superadiabatic region at the top of solar-type convection
zones, and allow an extrapolation of the entropy (s*) in their deep,
adiabatic layers. For the Sun we find a close agreement between s*
inferred from our RHD models and an empirical determination of s*
from helioseismology. In the framework of mixing length theory (MLT),
s* is translated to an effective mixing-length parameter (alpha c)
appropriate to construct global stellar models. The calibration based
on our present set of 2D RHD models shows a moderate variation of
alpha c across the domain of the HRD investigated so far.
---------------------------------------------------------
Title: Is stellar granulation turbulence?
Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R.
1997A&A...328..229N Altcode:
We show that power spectra of granulation images or velocity fields
cannot be compared meaningfully with spectra from theoretical models
based on turbulent cascades. The small scale power in these images
is due almost entirely to the sharp edges between granules and
intergranular lanes, not to turbulence in the usual sense. This is
demonstrated with a number of experiments with result from numerical
simulations, and with simpler synthetic data with power spectra similar
to that of granulation. The reason for the seemingly laminar behavior of
the granulation flow, in spite of the high Reynolds numbers involved,
is the influence of stratification on the local ratio of turbulence
to bulk flow. The rapid expansion of upflows, their deep origin and
near-adiabatic stratification lead to low levels of turbulence in
the overturning fluid at the surface. Higher levels of turbulence
are expected in the converging flows near downdrafts, but mostly at
scales that are below current observational resolution limits, and
contributing relatively little to the total convective flux and to
spectral line broadening.
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Title: A precision-controlled solar model with realistic
subatmospheric stratification
Authors: Schlattl, H.; Weiss, A.; Ludwig, H. -G.; Bernkopf, J.
1997ASSL..225...65S Altcode: 1997scor.proc...65S
The Garching stellar evolution code has been modified to allow the
calculation of an up-to-date solar model. The modifications are of
numerical and physical nature. Concerning the first aspect, the spatial
as well as the temporal discretization of the PDEs can now be done with
a controlled and guaranteed accuracy of up to $\approx 10^{-5}$. With
regard to the second aspect, latest input physics tables, diffusion
and synthetic atmospheres used as outer boundary conditions at $\tau
\approx 20$ have been implemented. In addition, a spatially variable
mixing length has been employed to reproduce the T-P-stratification
of the outer convective layers as calculated by 2D-hydrodynamical
models. We find that the changes in the subatmospheric regions lead
to a further improvement of the solar model.
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Title: Two mass-losing carbon stars in the Galactic halo
Authors: Groenewegen, M. A. T.; Oudmaijer, R. D.; Ludwig, H. -G.
1997MNRAS.292..686G Altcode:
Observations of two mass-losing carbon stars in the Galactic halo,
IRAS08546+1732 and 12560+1656, are presented. These objects were
discovered serendipitously, and stand out from the usual carbon stars
at high galactic latitudes in that they have optical and IRAS colours
consistent with current mass-loss. New optical spectra, CO observations,
and modelling of the spectral energy distribution and of the CO lines
are presented. Luminosities are derived using a period-luminosity
relation. From the dust modelling IRAS08546+1732 is found to be at
20 kpc from the Sun (11.3 kpc from the Galactic plane) and has a
mass-loss rate of 3.3x10^-6 Msolar yr^-1. From the CO non-detection
we deduce that it is probably oxygen-deficient, corroborating earlier
work. IRAS12560+1656 is found to be at 8.0 kpc from the Sun (7.8 kpc
from the Galactic plane) and has a mass-loss rate of 1.3x10^-6 Msolar
yr^-1. The detection of the ^12CO J=2-1 transition in the spectrum of
IRAS12560+1656 after an integration time of 10h makes it probably the
longest ^12CO integration on a stellar object. The detection itself
makes the star one of the most distant stellar objects detected in
the CO line. The outflow velocity of 3.2 km s^-1 is very low, and the
stellar velocity is +88 km s^-1 with respect to the LSR. Modelling of
the CO line implies an oxygen abundance of 0.7 dex below solar. We
examine existing data on the `faint high-latitude carbon stars' and
identify two additional distant, mass-losing, N-type AGB stars. The
nature of halo carbon stars is discussed, and suggestions on how to
find more mass-losing halo AGB stars are presented.
---------------------------------------------------------
Title: A solar model with improved subatmospheric stratification.
Authors: Schlattl, H.; Weiss, A.; Ludwig, H. -G.
1997A&A...322..646S Altcode:
The calculated intermediate and high degree p-mode frequencies of
standard solar models show greater disagreement with the observations
than low degree modes. This leads to the conclusion that the
subatmospheric structure of the models has to be improved. We are
presenting solar models with up-to-date input physics and controlled
numerical accuracy of 10^-5^. For the outer boundary condition we use
synthetic atmospheres fitted to the interior solution at τ=~20. In
addition, a spatially variable mixing length parameter is employed
to reproduce the pressure-temperature stratification of the outer
convective zone calculated by 2D-hydrodynamical models we used for
comparison. With this changed subatmospheric structure we could improve
the agreement between predicted and observed p-mode frequencies in
our solar models.
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Title: On the Scale of Photospheric Convection
Authors: Freytag, B.; Holweger, H.; Steffen, M.; Ludwig, H. -G.
1997svlt.work..316F Altcode:
No abstract at ADS
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Title: Hydrodynamical models of stellar convection. The role of
overshoot in DA white dwarfs, A-type stars, and the Sun.
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
1996A&A...313..497F Altcode:
Based on two-dimensional numerical radiation hydrodynamics
simulations of time-dependent compressible convection, we have
studied the structure and dynamics of a variety of shallow stellar
surface convection zones. Our present grid of models includes
detailed simulations of surface convection in solar-type stars,
main-sequence A-type stars and cool DA white dwarfs, as well as
numerical experiments to study convection and overshoot at the base of
the solar convection zone. Taking into account a realistic equation of
state (including the effects of ionization) and adopting an elaborate
treatment of non-local radiative transfer (with appropriate grey
or frequency-dependent opacities), our simulations are designed to
represent specific stellar objects characterized by T_eff_, logg, and
chemical composition. Contrary to solar-type stars, the A-type stars and
cool DA white dwarfs investigated here have shallow convection zones
which fit into the computational domain together with thick stable
buffer layers on top and below, thus permitting a study of convective
overshoot under genuine conditions. We find that convective motions
extend well beyond the boundary of the convectively unstable region,
with vertical velocities decaying exponentially with depth in the
deeper parts of the lower overshoot region, as expected for linear
g^-^-modes. Even though convective velocities are reduced by orders of
magnitude, they are still able to counteract molecular diffusion. For
a quantitative description of convective mixing in the far overshoot
layers we have derived a depth dependent diffusion coefficient from
the numerical simulations. In combination with otherwise independent
1D diffusion calculations for a trace element, this allows the
determination of the "effective depth" of the overshoot region. For
a typical main-sequence A-type star (T_eff_=7943#1, logg=4.34) the
mass in the overshoot region exceeds the mass in the unstable region
by approximately a factor 10. The amount of overshoot in cool DA white
dwarfs (around T_eff_=12200#1) is even larger: the convectively mixed
mass is increased by roughly a factor 100.
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Title: Overtures to the pulsational instability of ZZ Ceti variables.
Authors: Gautschy, A.; Ludwig, H. -G.; Freytag, B.
1996A&A...311..493G Altcode: 1995astro.ph..8002G
Results of nonradial, nonadiabatic pulsation calculations on
hydrogen-rich white dwarf models are presented. In contrast to earlier
attempts, the modeling builds on hydrodynamically simulated convective
surface layers supplemented with standard interior models. Based on
our stellar models and despite of various simple attempts to couple
convection and pulsation we could not reproduce theoretically the
presently adopted location of the observed blue edge of the ZZ Ceti
variables. When the convective efficiency is high enough we found a
sensitive dependence of the stability properties of the g modes on
the pulsational treatment of shear within the convection zone.
---------------------------------------------------------
Title: Synthetic spectra computed from hydrodynamical model
atmospheres of DA white dwarfs.
Authors: Steffen, M.; Ludwig, H. -G.; Freytag, B.
1995A&A...300..473S Altcode:
From detailed 2-dimensional numerical radiation hydrodynamics
calculations of time-dependent compressible convection we have obtained
the thermal structure of the convective surface layers of DA white
dwarfs with effective temperatures near the blue edge of the ZZ Ceti
instability strip. Synthetic line profiles of H<SUB>beta</SUB>_ and the
red wing of L<SUB>alpha</SUB>_ (including the very temperature sensitive
satellite absorption features) computed from two representative
inhomogeneous hydrodynamical models (T_eff_=12200K, 12600K; log g=8.0)
are compared with the spectra resulting from different plane-parallel
model atmospheres. We find that it is possible to represent a given
inhomogeneous atmosphere by a spectroscopically equivalent 1D model,
constructed to have the same frequency-integrated radiative flux as
the respective 2D hydrodynamical model at all depths. Synthetic spectra
computed from this representative 1D model are almost indistinguishable
from the horizontally averaged 2D synthetic spectra of the corresponding
inhomogeneous model. We conclude that in the investigated range of
effective temperature (probably even for the whole range of convective
DAs), spectroscopic analysis based on appropriate 1D atmospheres is
almost unaffected by systematic errors associated with non-linear flux
variations due to the substantial thermal inhomogeneities generated
by photospheric convection in these stars. This work provides the
basis for a well defined comparison between 2D or 3D hydrodynamical
convection models and 1D standard mixing length models.
---------------------------------------------------------
Title: Hydrodynamical model atmospheres: convection and line formation
in the Sun
Authors: Ludwig, H. -G.; Steffen, M.
1995IAUS..176P.235L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Mixing-Length Parameter for Solar-Type Convection Zones
Inferred from Hydrodynamical Models of the Surface Layers
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.; Wagenhuber, J.
1995LIACo..32..213L Altcode: 1995sews.book..213L
No abstract at ADS
---------------------------------------------------------
Title: Numerical Simulations of Convection and Overshoot in the
Envelope of DA White Dwarfs
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter
1995LNP...443...88F Altcode: 1995whdw.conf...88F
We present results of realistic 21) numerical radiation hydrodynamics
calculations, simulating the surface convection zones of DA white
dwarfs in the range of effective temperatures from 14 200 K down
to 11400 K. Comparison with mixing length theory (MLT) yields a
conflicting picture: The dynamics of convection is not governed by
up- and downflowing bubbles which dissolve after travelling some
characteristic distance - but by the formation, advection, merging,
and disruption of fast narrow downdrafts in a slowly upstreaming
surrounding. MLT tremendously underestimates the depth of the region
where material is mixed. Nevertheless, it turns out that a mixing
length model with α = 1.5 gives a good fit of the photospheric
temperature structure (T <SUB>eff</SUB> = 12 600 K) and that a 1D
temperature stratification suffices to reproduce the mean spectrum
of the 2D simulations, indicating that the photospheric temperature
inhomogeneities are negligible for spectroscopic analysis. In deeper
layers the temperature stratification of our hydrodynamical models
corresponds to larger values of α. Introducing our envelope models into
nonadiabatic pulsation calculations results in a blue edge of the ZZ
Ceti instability strip near T <SUB>eff</SUB> = 12 400 K at log g = 8.0.
---------------------------------------------------------
Title: Spectroscopic Effects of T-Inhomogeneities in the Atmospheres
of DA White Dwarfs
Authors: Ludwig, Hans-Günter; Steffen, Matthias
1995LNP...443..128L Altcode: 1995whdw.conf..128L
No abstract at ADS
---------------------------------------------------------
Title: Grasping at the Hot End of ZZ Ceti Variability
Authors: Gautschy, Alfred; Ludwig, Hans-Günter
1995LNP...443..295G Altcode: 1995whdw.conf..295G
No abstract at ADS
---------------------------------------------------------
Title: Numerical simulations of convection at the surface of a ZZ
Ceti white dwarf.
Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M.
1994A&A...284..105L Altcode:
We applied two-dimensional hydrodynamics and non-grey radiative transfer
calculations to the surface layers of a hydrogen-rich white dwarf
(spectral type DA) with T_eff_=12600K and log(g)=8.0, corresponding to
a position in the HR-diagram slightly cooler than the hot boundary of
the ZZ Ceti instability strip. In our simulations the entire convection
zone including the overshoot layers is embedded in the computational box
so that we obtain a complete and detailed model of convection for this
representative object. We address the important question to what extent
models based on mixing length theory (MLT) are able to predict the
physical properties of convection. We find a rapidly (timescale ~100ms)
evolving flow pattern with fast concentrated downdrafts surrounded by
slow broad upflows of warmer material. Convection carries up to 30%
of the total flux and excites internal gravity waves by dynamical
processes associated with the merging of downdrafts. The mean entropy
gradient is reversed with respect to MLT predictions in the deeper
layers of the convection zone. Strong overshoot occurs at its upper
and lower boundary. A synthetic spectrum calculated from the mean
photospheric temperature stratification can be fitted satisfactorily
with a MLT model adopting α=1.5. At greater depth the temperature
profile approaches a model with α=4. The total depth of the convective
layers is rather small compared to values suggested by studies of the
excitation mechanism for the pulsations of DAs.
---------------------------------------------------------
Title: Numerical simulation of overshoot at the base of the solar
convection zone
Authors: Ludwig, H. -G.
1994smf..conf..149L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Numerical simulations of convection at the surface of a ZZ
Ceti white dwarf.
Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M.
1993AGAb....9..147L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: First numerical simulations of convection at the surface of
a ZZ Ceti white dwarf
Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M.
1993ASIC..403..471L Altcode: 1993wdao.conf..471L
No abstract at ADS
---------------------------------------------------------
Title: Results from 2-D Numerical Simulations of Solar Granules
Authors: Steffen, M.; Gigas, D.; Holweger, H.; Krüss, A.; Ludwig,
H. -G.
1990IAUS..138..213S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Non-grey radiative transfer in numerical simulations of
photospheric convection.
Authors: Ludwig, H. -G.; Steffen, M.; Rauch, T.
1990AGAb....5...38L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A numerical simulation study of solar granular convection in
cells of different horizontal dimension
Authors: Steffen, M.; Ludwig, H. -G.; Kruess, A.
1989A&A...213..371S Altcode:
The properties of a series of granular convection cells of different
horizontal scale, obtained from two-dimensional numerical simulations
of the solar granulation, are examined. For model diameters below
approximately 2000 km, steady state solutions are obtained. It
is shown that the properties of granular convection systematically
depend on the horizontal scale of the granules. Results indicate that
the larger granular structures have the strongest influence on the
visible layers of the solar photosphere, while the smaller granules
become increasingly more difficult to observe as their activity
progressively disappears below the surface. It is demonstrated that
details of numerical results can be largely understood in terms of
'buoyancy braking' and radiative transfer.
---------------------------------------------------------
Title: Results from 2-D Numerical Simulations of the Solar Granulation
Authors: Steffen, M.; Gigas, D.; Holweger, H.; Krüß, A.; Ludwig,
H. -G.
1989AGAb....3...12S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Struktur solarer Konvektionszellen unterschiedlicher
Durchmesser.
Authors: Steffen, M.; Ludwig, H. G.; Krüss, A.
1988AGAb....1...10S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Bestimmung von Stationskoordinaten aus der Analyse langer
Bahnen.
Authors: Reigber, C.; Ludwig, H.
1976VeBKI..35..126R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Die Arbeiten des Sonderforschungsbereiches 78
Satellitengeodäsie der Technischen Universität München im Jahre
1974.
Authors: Sigl, R.; Kaniuth, K.; Bauch, A.; Dichtl, G.; Nagel, E.;
Schmidt, H. F.; Ilk, K. H.; Ludwig, H.; Schneider, M.; Reigber, C.
1975VeBKI..33.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Einige Bemerkungen zur Bestimmung und Verwendung geozentrischer
Koordinaten.
Authors: Sigl, R.; Ludwig, H.
1973VeBKI..31..149S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Die Arbeiten des Sonderforschungsbereiches 78
Satellitengeodäsie im Jahre 1972.
Authors: Sigl, R.; Dichtl, G.; Ilk, K. H.; Lelgemann, D.; Wilson,
P.; Ludwig, H.; Nagel, E.; Reigber, C.; Schneider, M.; Schmidt, F.
1973VeBKI..30.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Anwendung der Hammersteinschen Methode der unendlich vielen
Variablen auf Probleme der Satellitengeodäsie und Himmelsmechanik.
Authors: Sigl, R.; Schneider, M.; Reigber, C.; Ludwig, H.
1970adhm.book.....S Altcode:
No abstract at ADS
