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Author name code: hayek
ADS astronomy entries on 2022-09-14
author:"Hayek, Wolfgang"
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Title: The benchmark halo giant HD 122563: CNO abundances revisited
with three-dimensional hydrodynamic model stellar atmospheres
Authors: Collet, R.; Nordlund, Å.; Asplund, M.; Hayek, W.;
Trampedach, R.
2018MNRAS.475.3369C Altcode: 2017arXiv171208099C
We present an abundance analysis of the low-metallicity benchmark
red giant star HD 122563 based on realistic, state-of-the-art,
high-resolution, three-dimensional (3D) model stellar atmospheres
including non-grey radiative transfer through opacity binning with
4, 12, and 48 bins. The 48-bin 3D simulation reaches temperatures
lower by ∼300-500 K than the corresponding 1D model in the upper
atmosphere. Small variations in the opacity binning, adopted line
opacities, or chemical mixture can cool the photospheric layers by
a further ∼100-300 K and alter the effective temperature by ∼100
K. A 3D local thermodynamic equilibrium (LTE) spectroscopic analysis
of Fe I and Fe II lines gives discrepant results in terms of derived
Fe abundance, which we ascribe to non-LTE effects and systematic errors
on the stellar parameters. We also determine C, N, and O abundances by
simultaneously fitting CH, OH, NH, and CN molecular bands and lines
in the ultraviolet, visible, and infrared. We find a small positive
3D-1D abundance correction for carbon (+0.03 dex) and negative ones for
nitrogen (-0.07 dex) and oxygen (-0.34 dex). From the analysis of the [O
I] line at 6300.3 Å, we derive a significantly higher oxygen abundance
than from molecular lines (+0.46 dex in 3D and +0.15 dex in 1D). We rule
out important OH photodissociation effects as possible explanation for
the discrepancy and note that lowering the surface gravity would reduce
the oxygen abundance difference between molecular and atomic indicators.
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Title: The elemental composition of the Sun. I. The intermediate
mass elements Na to Ca
Authors: Scott, Pat; Grevesse, Nicolas; Asplund, Martin; Sauval,
A. Jacques; Lind, Karin; Takeda, Yoichi; Collet, Remo; Trampedach,
Regner; Hayek, Wolfgang
2015A&A...573A..25S Altcode: 2014arXiv1405.0279S
The chemical composition of the Sun is an essential piece of reference
data for astronomy, cosmology, astroparticle, space and geo-physics:
elemental abundances of essentially all astronomical objects are
referenced to the solar composition, and basically every process
involving the Sun depends on its composition. This article, dealing
with the intermediate-mass elements Na to Ca, is the first in a
series describing the comprehensive re-determination of the solar
composition. In this series we severely scrutinise all ingredients
of the analysis across all elements, to obtain the most accurate,
homogeneous and reliable results possible. We employ a highly
realistic 3D hydrodynamic model of the solar photosphere, which has
successfully passed an arsenal of observational diagnostics. For
comparison, and to quantify remaining systematic errors, we repeat
the analysis using three different 1D hydrostatic model atmospheres
(marcs, miss and Holweger & Müller 1974, Sol. Phys., 39, 19) and
a horizontally and temporally-averaged version of the 3D model (⟨ 3D
⟩). We account for departures from local thermodynamic equilibrium
(LTE) wherever possible. We have scoured the literature for the best
possible input data, carefully assessing transition probabilities,
hyperfine splitting, partition functions and other data for inclusion
in the analysis. We have put the lines we use through a very stringent
quality check in terms of their observed profiles and atomic data, and
discarded all that we suspect to be blended. Our final recommended
3D+NLTE abundances are: log ɛ<SUB>Na</SUB> = 6.21 ± 0.04, log
ɛ<SUB>Mg</SUB> = 7.59 ± 0.04, log ɛ<SUB>Al</SUB> = 6.43 ± 0.04,
log ɛ<SUB>Si</SUB> = 7.51 ± 0.03, log ɛ<SUB>P</SUB> = 5.41 ± 0.03,
log ɛ<SUB>S</SUB> = 7.13 ± 0.03, log ɛ<SUB>K</SUB> = 5.04 ± 0.05
and log ɛ<SUB>Ca</SUB> = 6.32 ± 0.03. The uncertainties include both
statistical and systematic errors. Our results are systematically
smaller than most previous ones with the 1D semi-empirical
Holweger & Müller model, whereas the ⟨ 3D ⟩ model returns
abundances very similar to the full 3D calculations. This analysis
provides a complete description and a slight update of the results
presented in Asplund et al. (2009, ARA&A, 47, 481) for Na to
Ca, and includes full details of all lines and input data used. <P
/>Tables 1-4 and Appendix A are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201424109/olm">http://www.aanda.org</A>
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Title: Accuracy tests of radiation schemes used in hot Jupiter global
circulation models
Authors: Amundsen, David S.; Baraffe, Isabelle; Tremblin, Pascal;
Manners, James; Hayek, Wolfgang; Mayne, Nathan J.; Acreman, David M.
2014A&A...564A..59A Altcode: 2014arXiv1402.0814S
The treatment of radiation transport in global circulation models (GCMs)
is crucial for correctly describing Earth and exoplanet atmospheric
dynamics processes. The two-stream approximation and correlated-k
method are currently state-of-the-art approximations applied in both
Earth and hot Jupiter GCM radiation schemes to facilitate the rapid
calculation of fluxes and heating rates. Their accuracy have been
tested extensively for Earth-like conditions, but verification of the
methods' applicability to hot Jupiter-like conditions is lacking in
the literature. We are adapting the UK Met Office GCM, the Unified
Model (UM), for the study of hot Jupiters, and present in this work
the adaptation of the Edwards-Slingo radiation scheme based on the
two-stream approximation and the correlated-k method. We discuss the
calculation of absorption coefficients from high-temperature line
lists and highlight the large uncertainty in the pressure-broadened
line widths. We compare fluxes and heating rates obtained with our
adapted scheme to more accurate discrete ordinate (DO) line-by-line
(LbL) calculations ignoring scattering effects. We find that, in
most cases, errors stay below 10% for both heating rates and fluxes
using ~10 k-coefficients in each band and a diffusivity factor D =
1.66. The two-stream approximation and the correlated-k method both
contribute non-negligibly to the total error. We also find that using
band-averaged absorption coefficients, which have previously been used
in radiative-hydrodynamical simulations of a hot Jupiter, may yield
errors of ~100%, and should thus be used with caution.
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Title: The Stagger-grid: A grid of 3D stellar atmosphere
models. II. Horizontal and temporal averaging and spectral line
formation
Authors: Magic, Z.; Collet, R.; Hayek, W.; Asplund, M.
2013A&A...560A...8M Altcode: 2013arXiv1307.3273M
<BR /> Aims: We study the implications of averaging methods with
different reference depth scales for 3D hydrodynamical model atmospheres
computed with the Stagger-code. The temporally and spatially averaged
(hereafter denoted as ⟨3D⟩) models are explored in the light
of local thermodynamic equilibrium (LTE) spectral line formation by
comparing spectrum calculations using full 3D atmosphere structures
with those from ⟨3D⟩ averages. <BR /> Methods: We explored methods
for computing mean ⟨3D⟩ stratifications from the Stagger-grid
time-dependent 3D radiative hydrodynamical atmosphere models by
considering four different reference depth scales (geometrical depth,
column-mass density, and two optical depth scales). Furthermore,
we investigated the influence of alternative averages (logarithmic,
enforced hydrostatic equilibrium, flux-weighted temperatures). For the
line formation we computed curves of growth for Fe i and Fe ii lines in
LTE. <BR /> Results: The resulting ⟨3D⟩ stratifications for the four
reference depth scales can be very different. We typically find that in
the upper atmosphere and in the superadiabatic region just below the
optical surface, where the temperature and density fluctuations are
highest, the differences become considerable and increase for higher
T<SUB>eff</SUB>, lower log g, and lower [Fe / H]. The differential
comparison of spectral line formation shows distinctive differences
depending on which ⟨3D⟩ model is applied. The averages over
layers of constant column-mass density yield the best mean ⟨3D⟩
representation of the full 3D models for LTE line formation, while
the averages on layers at constant geometrical height are the least
appropriate. Unexpectedly, the usually preferred averages over layers of
constant optical depth are prone to increasing interference by reversed
granulation towards higher effective temperature, in particular at
low metallicity. <P />Appendix A is available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>Mean
⟨3D⟩ models are 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/560/A8">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/560/A8</A>
as well as at <A
href="http://www.stagger-stars.net">http://www.stagger-stars.net</A>
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Title: VizieR Online Data Catalog: STAGGER-grid of 3D stellar
models. II. (Magic+, 2013)
Authors: Magic, Z.; Collet, R.; Hayek, W.; Asplund, M.
2013yCat..35600008M Altcode: 2013yCat..35609008M
More (updated) information and data available at:
http://www.stagger-stars.net/ <P />(5 data files).
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Title: The Stagger-grid: A grid of 3D stellar atmosphere
models. I. Methods and general properties
Authors: Magic, Z.; Collet, R.; Asplund, M.; Trampedach, R.; Hayek,
W.; Chiavassa, A.; Stein, R. F.; Nordlund, Å.
2013A&A...557A..26M Altcode: 2013arXiv1302.2621M
<BR /> Aims: We present the Stagger-grid, a comprehensive grid of
time-dependent, three-dimensional (3D), hydrodynamic model atmospheres
for late-type stars with realistic treatment of radiative transfer,
covering a wide range in stellar parameters. This grid of 3D models is
intended for various applications besides studies of stellar convection
and atmospheres per se, including stellar parameter determination,
stellar spectroscopy and abundance analysis, asteroseismology,
calibration of stellar evolution models, interferometry, and extrasolar
planet search. In this introductory paper, we describe the methods
we applied for the computation of the grid and discuss the general
properties of the 3D models as well as of their temporal and spatial
averages (here denoted ⟨3D⟩ models). <BR /> Methods: All our models
were generated with the Stagger-code, using realistic input physics for
the equation of state (EOS) and for continuous and line opacities. Our ~
220 grid models range in effective temperature, T<SUB>eff</SUB>, from
4000 to 7000 K in steps of 500 K, in surface gravity, log g, from 1.5
to 5.0 in steps of 0.5 dex, and metallicity, [Fe/H], from - 4.0 to +
0.5 in steps of 0.5 and 1.0 dex. <BR /> Results: We find a tight scaling
relation between the vertical velocity and the surface entropy jump,
which itself correlates with the constant entropy value of the adiabatic
convection zone. The range in intensity contrast is enhanced at lower
metallicity. The granule size correlates closely with the pressure
scale height sampled at the depth of maximum velocity. We compare the
⟨3D⟩ models with currently widely applied one-dimensional (1D)
atmosphere models, as well as with theoretical 1D hydrostatic models
generated with the same EOS and opacity tables as the 3D models, in
order to isolate the effects of using self-consistent and hydrodynamic
modeling of convection, rather than the classical mixing length theory
approach. For the first time, we are able to quantify systematically
over a broad range of stellar parameters the uncertainties of 1D
models arising from the simplified treatment of physics, in particular
convective energy transport. In agreement with previous findings,
we find that the differences can be rather significant, especially
for metal-poor stars. <P />Appendices A-C are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>Full
Table C.1 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/557/A26">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A26</A>
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Title: VizieR Online Data Catalog: STAGGER-grid of 3D stellar
models. I. (Magic+, 2013)
Authors: Magic, Z.; Collet, R.; Asplund, M.; Trampedach, R.; Hayek,
W.; Chiavassa, A.; Stein, R. F.; Nordlund, A.
2013yCat..35570026M Altcode: 2013yCat..35579026M
The 3D model atmospheres presented here were constructed with
a custom version of the Stagger-code, a state-of-the-art,
multipurpose, radiative-magnetohydrodynamics (R-MHD)
code originally developed by Nordlund & Galsgaard (1995,
http://www.astro.ku.dk/~kg/Papers/MHD_code.ps.gz), and continuously
improved over the years by its user community. <P />(1 data file).
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Title: Limb darkening laws for two exoplanet host stars derived
from 3D stellar model atmospheres. Comparison with 1D models and
HST light curve observations
Authors: Hayek, W.; Sing, D.; Pont, F.; Asplund, M.
2012A&A...539A.102H Altcode: 2012arXiv1202.0548H
We compare limb darkening laws derived from 3D hydrodynamical model
atmospheres and 1D hydrostatic MARCS models for the host stars of
two well-studied transiting exoplanet systems, the late-type dwarfs
<ASTROBJ>HD 209458</ASTROBJ> and <ASTROBJ>HD 189733</ASTROBJ>. The
surface brightness distribution of the stellar disks is calculated
for a wide spectral range using 3D LTE spectrum formation and opacity
sampling<SUP>⋆</SUP>. We test our theoretical predictions using
least-squares fits of model light curves to wavelength-integrated
primary eclipses that were observed with the Hubble Space Telescope
(HST). The limb darkening law derived from the 3D model of HD 209458 in
the spectral region between 2900 Å and 5700 Å produces significantly
better fits to the HST data, removing systematic residuals that were
previously observed for model light curves based on 1D limb darkening
predictions. This difference arises mainly from the shallower mean
temperature structure of the 3D model, which is a consequence of the
explicit simulation of stellar surface granulation where 1D models
need to rely on simplified recipes. In the case of HD 189733, the
model atmospheres produce practically equivalent limb darkening curves
between 2900 Å and 5700 Å, partly due to obstruction by spectral
lines, and the data are not sufficient to distinguish between the
light curves. We also analyze HST observations between 5350 Å and
10 500 Å for this star; the 3D model leads to a better fit compared
to 1D limb darkening predictions. The significant improvement of fit
quality for the HD 209458 system demonstrates the higher degree of
realism of 3D hydrodynamical models and the importance of surface
granulation for the formation of the atmospheric radiation field of
late-type stars. This result agrees well with recent investigations of
limb darkening in the solar continuum and other observational tests of
the 3D models. The case of HD 189733 is no contradiction as the model
light curves are less sensitive to the temperature stratification
of the stellar atmosphere and the observed data in the 2900-5700
Å region are not sufficient to distinguish more clearly between
the 3D and 1D limb darkening predictions. <P />Full theoretical
spectra for both stars 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/539/A102">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/539/A102</A>,
as well as at <A
href="http://www.astro.ex.ac.uk/people/sing">www.astro.ex.ac.uk/people/sing</A>.
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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.
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Title: VizieR Online Data Catalog: HD 209458 and HD 189733 theoretical
spectra (Hayek+, 2012)
Authors: Hayek, W.; Sing, D.; Pont, F.; Asplund, M.
2012yCat..35390102H Altcode: 2012yCat..35399102H
Theoretical spectrum computations for the G-type dwarf HD 209458 and
the K-type dwarf HD 189733 are presented, based on 3D hydrodynamical
models of the stellar atmospheres. Surface intensities were computed
in LTE using the SCATE spectrum formation code and cover the wavelength
region between about 910 Angstroem and 20 micron with constant sampling
of R=λ/δ_λ=20,000. The stellar disk was sampled at the disk center
(μ=cos(θ)=1.0) and at angles μ = 0.90, 0.80, 0.70, 0.60, 0.50,
0.40, 0.30, 0.25, 0.20, 0.15, 0.125, 0.1, 0.075, 0.05, 0.025, 0.01
towards the limb. Use integration weights 0.05, 0.1, 0.1, 0.1, 0.1,
0.1, 0.1, 0.075, 0.05, 0.05, 0.0375, 0.025, 0.025, 0.025, 0.025, 0.02,
0.0125 to obtain fluxes. <P />(2 data files).
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Title: On the effects of clouds and hazes in the atmospheres of hot
Jupiters: semi-analytical temperature-pressure profiles
Authors: Heng, Kevin; Hayek, Wolfgang; Pont, Frédéric; Sing, David K.
2012MNRAS.420...20H Altcode: 2011MNRAS.tmp.2117H; 2011arXiv1107.1390H
Motivated by the work of Guillot, we present a semi-analytical
formalism for calculating the temperature-pressure profiles in hot
Jovian atmospheres which includes the effects of clouds/hazes and
collision-induced absorption. Using the dual-band approximation,
we assume that stellar irradiation and thermal emission from the
hot Jupiter occur at distinct wavelengths ('shortwave' versus
'longwave'). For a purely absorbing cloud/haze, we demonstrate its
dual effect of cooling and warming the upper and lower atmosphere,
respectively, which modifies, in a non-trivial manner, the condition for
whether a temperature inversion is present in the upper atmosphere. The
warming effect becomes more pronounced as the cloud/haze deck resides at
greater depths. If it sits below the shortwave photosphere, the warming
effect becomes either more subdued or ceases altogether. If shortwave
scattering is present, its dual effect is to warm and cool the upper
and lower atmospheres, respectively, thus counteracting the effects
of enhanced longwave absorption by the cloud/haze. We make a tentative
comparison of a four-parameter model to the temperature-pressure data
points inferred from the observations of HD 189733b and estimate that
its Bond albedo is approximately 10 per cent. Besides their utility
in developing physical intuition, our semi-analytical models are a
guide for the parameter space exploration of hot Jovian atmospheres
via three-dimensional simulations of atmospheric circulation.
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Title: Effects of Scattering on the Temperature Stratification in
3D Model Atmospheres of Late-Type Stars
Authors: Collet, R.; Hayek, W.; Asplund, M.
2011ASPC..448..819C Altcode: 2011csss...16..819C
Three-dimensional (3D) radiative hydrodynamic model atmospheres
of metal-poor late-type stars predict cooler upper photospheric
stratifications than their one-dimensional (1D) counterparts. This
property of 3D model atmospheres affects the determination of
elemental abundances from temperature-sensitive spectral features, with
important consequences for galactic chemical evolution studies. In this
contribution, we investigate the impact of different approximations
of scattering in the solution of the radiative transfer equation on
the temperature stratification of 3D model atmospheres of metal-poor
red giants. We use the BIFROST code to construct 3D model atmospheres
of metal-poor red giants using three different approximations of
scattering. First, we self-consistently solve the radiative transfer
equation for the general case of a source function with a coherent
scattering term; second, we solve the radiative transfer equation
assuming a Planckian source function and neglecting altogether
the contribution of continuum scattering to extinction in the
optically thin layers; third, we assume a Planckian source function
and treat continuum scattering as pure absorption everywhere in the
simulation's domain. We find that the second approach produces very
similar temperature structures with cool upper photospheric layers as
when treating scattering correctly, and at a much lower computational
cost. In contrast, treating scattering as pure absorption leads to
significantly hotter and shallower temperature stratifications.
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Title: Hubble Space Telescope transmission spectroscopy of the
exoplanet HD 189733b: high-altitude atmospheric haze in the optical
and near-ultraviolet with STIS
Authors: Sing, D. K.; Pont, F.; Aigrain, S.; Charbonneau, D.; Désert,
J. -M.; Gibson, N.; Gilliland, R.; Hayek, W.; Henry, G.; Knutson,
H.; Lecavelier Des Etangs, A.; Mazeh, T.; Shporer, A.
2011MNRAS.416.1443S Altcode: 2011arXiv1103.0026S; 2011MNRAS.tmp.1159S
We present Hubble Space Telescope (HST) optical and near-ultraviolet
transmission spectra of the transiting hot Jupiter HD 189733b,
taken with the repaired Space Telescope Imaging Spectrograph (STIS)
instrument. The resulting spectra cover the range 2900-5700 Å and reach
per exposure signal-to-noise ratio levels greater than 11 000 within
a 500-Å bandwidth. We used time series spectra obtained during two
transit events to determine the wavelength dependence of the planetary
radius and measure the exoplanet's atmospheric transmission spectrum
for the first time over this wavelength range. Our measurements,
in conjunction with existing HST spectra, now provide a broad-band
transmission spectrum covering the full optical regime. The STIS
data also show unambiguous evidence of a large occulted stellar spot
during one of our transit events, which we use to place constraints on
the characteristics of the K dwarf's stellar spots, estimating spot
temperatures around T<SUB>eff</SUB>∼ 4250 K. With contemporaneous
ground-based photometric monitoring of the stellar variability,
we also measure the correlation between the stellar activity level
and transit-measured planet-to-star radius contrast, which is in good
agreement with predictions. We find a planetary transmission spectrum in
good agreement with that of Rayleigh scattering from a high-altitude
atmospheric haze as previously found from HST Advanced Camera for
Surveys. The high-altitude haze is now found to cover the entire optical
regime and is well characterized by Rayleigh scattering. These findings
suggest that haze may be a globally dominant atmospheric feature of
the planet which would result in a high optical albedo at shorter
optical wavelengths.
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Title: The Effect of Scattering on the Temperature Stratification
of 3D Model Atmospheres of Metal-Poor Red Giants
Authors: Collet, Remo; Hayek, Wolfgang; Asplund, Martin
2011IAUS..271..373C Altcode:
We study the effects of different approximations of scattering in 3D
radiation-hydrodynamics simulations on the photospheric temperature
stratification of metal-poor red giant stars. We find that assuming a
Planckian source function and neglecting the contribution of scattering
to extinction in optically thin layers provides a good approximation
of the effects of coherent scattering on the photospheric temperature
balance.
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Title: The stellar atmosphere simulation code Bifrost. Code
description and validation
Authors: Gudiksen, B. V.; Carlsson, M.; Hansteen, V. H.; Hayek, W.;
Leenaarts, J.; Martínez-Sykora, J.
2011A&A...531A.154G Altcode: 2011arXiv1105.6306G
Context. Numerical simulations of stellar convection and photospheres
have been developed to the point where detailed shapes of observed
spectral lines can be explained. Stellar atmospheres are very complex,
and very different physical regimes are present in the convection zone,
photosphere, chromosphere, transition region and corona. To understand
the details of the atmosphere it is necessary to simulate the whole
atmosphere since the different layers interact strongly. These physical
regimes are very diverse and it takes a highly efficient massively
parallel numerical code to solve the associated equations. <BR /> Aims:
The design, implementation and validation of the massively parallel
numerical code Bifrost for simulating stellar atmospheres from the
convection zone to the corona. <BR /> Methods: The code is subjected
to a number of validation tests, among them the Sod shock tube test,
the Orzag-Tang colliding shock test, boundary condition tests and
tests of how the code treats magnetic field advection, chromospheric
radiation, radiative transfer in an isothermal scattering atmosphere,
hydrogen ionization and thermal conduction. Results.Bifrost completes
the tests with good results and shows near linear efficiency scaling
to thousands of computing cores.
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Title: 3D LTE spectral line formation with scattering in red giant
stars
Authors: Hayek, W.; Asplund, M.; Collet, R.; Nordlund, Å.
2011A&A...529A.158H Altcode: 2011arXiv1108.3366H
<BR /> Aims: We investigate the effects of coherent isotropic continuum
scattering on the formation of spectral lines in local thermodynamic
equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model
atmospheres of red giant stars. <BR /> Methods: Detailed radiative
transfer with coherent and isotropic continuum scattering is computed
for 3D hydrodynamical and 1D hydrostatic models of late-type stellar
atmospheres using the SCATE code. Opacities are computed in LTE, while
a coherent and isotropic scattering term is added to the continuum
source function. We investigate the effects of scattering by comparing
continuum flux levels, spectral line profiles and curves of growth
for different species with calculations that treat scattering as
absorption. <BR /> Results: Rayleigh scattering is the dominant source
of scattering opacity in the continuum of red giant stars. Photons
may escape from deeper, hotter layers through scattering, resulting in
significantly higher continuum flux levels beneath a wavelength of λ
≲ 5000 Å. The magnitude of the effect is determined by the importance
of scattering opacity with respect to absorption opacity; we observe
the largest changes in continuum flux at the shortest wavelengths
and lowest metallicities; intergranular lanes of 3D models are more
strongly affected than granules. Continuum scattering acts to increase
the profile depth of LTE lines: continua gain more brightness than line
cores due to their larger thermalization depth in hotter layers. We
thus observe the strongest changes in line depth for high-excitation
species and ionized species, which contribute significantly to photon
thermalization through their absorption opacity near the continuum
optical surface. Scattering desaturates the line profiles, leading to
larger abundance corrections for stronger lines, which reach -0.5 dex
at 3000 Å for Fe ii lines in 3D with excitation potential χ = 2 eV at
[Fe/H] = -3.0. The corrections are less severe for low-excitation lines,
longer wavelengths, and higher metallicity. Velocity fields increase
the effects of scattering by separating emission from granules and
intergranular lanes in wavelength. 1D calculations exhibit similar
scattering abundance corrections for weak lines, but those for
strong lines are generally smaller compared to 3D models and depend
on the choice of microturbulence. <BR /> Conclusions: Continuum
scattering should be taken into account for computing realistic
spectral line profiles at wavelengths λ ≲ 4000 Å in metal-poor
giant stars. Profile shapes are strongly affected by velocity fields
and horizontal inhomogeneities, requiring a treatment based on 3D
hydrodynamical rather than classical 1D hydrostatic model atmospheres.
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Title: Three-dimensional surface convection simulations of metal-poor
stars. The effect of scattering on the photospheric temperature
stratification
Authors: Collet, R.; Hayek, W.; Asplund, M.; Nordlund, Å.; Trampedach,
R.; Gudiksen, B.
2011A&A...528A..32C Altcode: 2011arXiv1101.3265C
Context. Three-dimensional (3D) radiative hydrodynamic model atmospheres
of metal-poor late-type stars are characterized by cooler upper
photospheric layers than their one-dimensional counterparts. This
property of 3D model atmospheres can dramatically affect the
determination of elemental abundances from temperature-sensitive
spectral features, with profound consequences on galactic chemical
evolution studies. <BR /> Aims: We investigate whether the cool surface
temperatures predicted by 3D model atmospheres of metal-poor stars
can be ascribed to approximations in the treatment of scattering
during the modelling phase. <BR /> Methods: We use the Bifrost
code to construct 3D model atmospheres of metal-poor stars and test
three different ways to handle scattering in the radiative transfer
equation. As a first approach, we solve iteratively the radiative
transfer equation for the general case of a source function with
a coherent scattering term, treating scattering in a correct and
consistent way. As a second approach, we solve the radiative transfer
equation in local thermodynamic equilibrium approximation, neglecting
altogether the contribution of continuum scattering to extinction in the
optically thin layers; this has been the default mode in our previous
3D modelling as well as in present Stagger-Code models. As our third
and final approach, we treat continuum scattering as pure absorption
everywhere, which is the standard case in the 3D modelling by the
CO<SUP>5</SUP>BOLD collaboration. <BR /> Results: For all simulations,
we find that the second approach produces temperature structures
with cool upper photospheric layers very similar to the case in which
scattering is treated correctly. In contrast, treating scattering as
pure absorption leads instead to significantly hotter and shallower
temperature stratifications. The main differences in temperature
structure between our published models computed with the Stagger-
and Bifrost codes and those generated with the CO<SUP>5</SUP>BOLD
code can be traced to the different treatments of scattering. <BR />
Conclusions: Neglecting the contribution of continuum scattering to
extinction in optically thin layers provides a good approximation
to the full, iterative solution of the radiative transfer equation
in metal-poor stellar surface convection simulations, and at a much
lower computational cost. Our results also demonstrate that the cool
temperature stratifications predicted for metal-poor late-type stars
by previous models by our collaboration are not an artifact of the
approximated treatment of scattering.
---------------------------------------------------------
Title: Radiative transfer with scattering for domain-decomposed 3D
MHD simulations of cool stellar atmospheres. Numerical methods and
application to the quiet, non-magnetic, surface of a solar-type star
Authors: Hayek, W.; Asplund, M.; Carlsson, M.; Trampedach, R.; Collet,
R.; Gudiksen, B. V.; Hansteen, V. H.; Leenaarts, J.
2010A&A...517A..49H Altcode: 2010arXiv1007.2760H
<BR /> Aims: We present the implementation of a radiative
transfer solver with coherent scattering in the new BIFROST
code for radiative magneto-hydrodynamical (MHD) simulations of
stellar surface convection. The code is fully parallelized using
MPI domain decomposition, which allows for large grid sizes and
improved resolution of hydrodynamical structures. We apply the code
to simulate the surface granulation in a solar-type star, ignoring
magnetic fields, and investigate the importance of coherent scattering
for the atmospheric structure. <BR /> Methods: A scattering term
is added to the radiative transfer equation, requiring an iterative
computation of the radiation field. We use a short-characteristics-based
Gauss-Seidel acceleration scheme to compute radiative flux divergences
for the energy equation. The effects of coherent scattering are
tested by comparing the temperature stratification of three 3D
time-dependent hydrodynamical atmosphere models of a solar-type star:
without scattering, with continuum scattering only, and with both
continuum and line scattering. <BR /> Results: We show that continuum
scattering does not have a significant impact on the photospheric
temperature structure for a star like the Sun. Including scattering in
line-blanketing, however, leads to a decrease of temperatures by about
350 K below log<SUB>10</SUB> τ<SUB>5000</SUB> ⪉ -4. The effect is
opposite to that of 1D hydrostatic models in radiative equilibrium,
where scattering reduces the cooling effect of strong LTE lines in
the higher layers of the photosphere. Coherent line scattering also
changes the temperature distribution in the high atmosphere, where
we observe stronger fluctuations compared to a treatment of lines as
true absorbers.
---------------------------------------------------------
Title: Photon scattering 3D radiation-hydrodynamical simulations of
late-type stellar atmospheres
Authors: Hayek, Wolfgang Simon
2010PhDT.......323H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Photon Scattering in 3D Radiative MHD Simulations
Authors: Hayek, Wolfgang
2009AIPC.1171..309H Altcode:
Recent results from 3D time-dependent radiative hydrodynamic
simulations of stellar atmospheres are presented, which include
the effects of coherent scattering in the radiative transfer
treatment. Rayleigh scattering and electron scattering are accounted
for in the source function, requiring an iterative solution of the
transfer equation. Opacities and scattering coefficients are treated
in the multigroup opacity approximation. The impact of scattering
on the horizontal mean temperature structure is investigated, which
is an important diagnostic for model atmospheres, with implications
for line formation and stellar abundance measurements. <P />We find
that continuum scattering is not important for the atmosphere of a
metal-poor Sun with metailicity [Fe/H] = -3.0, similar to the previously
investigated photosphere at solar metallicity.
---------------------------------------------------------
Title: The Hamburg/ESO R-process enhanced star survey
(HERES). IV. Detailed abundance analysis and age dating of the
strongly r-process enhanced stars CS 29491-069 and HE 1219-0312
Authors: Hayek, W.; Wiesendahl, U.; Christlieb, N.; Eriksson, K.;
Korn, A. J.; Barklem, P. S.; Hill, V.; Beers, T. C.; Farouqi, K.;
Pfeiffer, B.; Kratz, K. -L.
2009A&A...504..511H Altcode: 2009arXiv0910.0707H
We report on a detailed abundance analysis of two strongly
r-process enhanced, very metal-poor stars newly discovered in
the HERES project, <ASTROBJ>CS 29491-069</ASTROBJ> ([Fe/H]=-2.51,
[r/Fe]=+1.1) and <ASTROBJ>HE 1219-0312</ASTROBJ> ([Fe/H]=-2.96,
[r/Fe]=+1.5). The analysis is based on high-quality VLT/UVES spectra
and MARCS model atmospheres. We detect lines of 15 heavy elements in
the spectrum of <ASTROBJ>CS 29491-069</ASTROBJ>, and 18 in <ASTROBJ>HE
1219-0312</ASTROBJ>; in both cases including the Th II 4019 Å
line. The heavy-element abundance patterns of these two stars are
mostly well-matched to scaled solar residual abundances not formed
by the s-process. We also compare the observed pattern with recent
high-entropy wind (HEW) calculations, which assume core-collapse
supernovae of massive stars as the astrophysical environment for the
r-process, and find good agreement for most lanthanides. The abundance
ratios of the lighter elements strontium, yttrium, and zirconium, which
are presumably not formed by the main r-process, are reproduced well by
the model. Radioactive dating for <ASTROBJ>CS 29491-069</ASTROBJ> with
the observed thorium and rare-earth element abundance pairs results in
an average age of 9.5 Gyr, when based on solar r-process residuals, and
17.6 Gyr, when using HEW model predictions. Chronometry seems to fail
in the case of <ASTROBJ>HE 1219-0312</ASTROBJ>, resulting in a negative
age due to its high thorium abundance. <ASTROBJ>HE 1219-0312</ASTROBJ>
could therefore exhibit an overabundance of the heaviest elements,
which is sometimes called an “actinide boost”. <P />Based on
observations collected at the European Southern Observatory, Paranal,
Chile (Proposal Number 170.D-0010). Table 8 is only available in
electronic form at http://www.aanda.org
---------------------------------------------------------
Title: Abundance analysis of the halo giant HD 122563 with
three-dimensional model stellar atmospheres
Authors: Collet, R.; Nordlund, Å.; Asplund, M.; Hayek, W.;
Trampedach, R.
2009MmSAI..80..719C Altcode: 2009arXiv0909.0690C
We present a preliminary local thermodynamic equilibrium (LTE) abundance
analysis of the template halo red giant HD122563 based on a realistic,
three-dimensional (3D), time-dependent, hydrodynamical model atmosphere
of the very metal-poor star. We compare the results of the 3D analysis
with the abundances derived by means of a standard LTE analysis based
on a classical, 1D, hydrostatic model atmosphere of the star. Due to
the different upper photospheric temperature stratifications predicted
by 1D and 3D models, we find large, negative, 3D-1D LTE abundance
differences for low-excitation OH and Fe I lines. We also find trends
with lower excitation potential in the derived Fe LTE abundances
from Fe I lines, in both the 1D and 3D analyses. Such trends may be
attributed to the neglected departures from LTE in the spectral line
formation calculations.
---------------------------------------------------------
Title: First 3D radiative transfer with scattering for
domain-decomposed MHD simulations
Authors: Hayek, W.
2008PhST..133a4006H Altcode:
This paper presents an implementation of the Gauss Seidel solver
for radiative transfer with scattering in the Oslo Stagger Code. It
fully supports MPI parallelism through domain decomposition of the
simulation box, enabling fast computation of radiative transfer at a
high resolution. Continuum and line opacities are treated with either a
multigroup method or opacity sampling. Line scattering probabilities are
estimated using the van Regemorter approximation for de-excitation rates
of electron collisions. A solar-type test simulation with continuum
and line scattering exhibits a steeper temperature gradient due to
decreased radiative heating above the optical surface when compared with
the strict local thermodynamic equilibrium (LTE) case. The classical
van Regemorter approximation may overestimate the importance of line
scattering, implying that the true temperature structure will be in
between the LTE case and the scattering case considered here. It is
demonstrated that continuum scattering is unimportant in the case of
the Sun.
