Author name code: trampedach
ADS astronomy entries on 2022-09-14
author:"Trampedach, Regner"
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Title: Granulation, Metallicity, and Red Giants
Authors: Perillo, Amber; Trampedach, Regner; Mathur, Savita; Delsanti,
Vincent; Garcia, Rafael; Breton, Sylvain
Bibcode: 2022AAS...24020807P
Altcode:
We explore the metallicity dependence of granulation parameters for
thousands of red giants observed with Kepler, and find a significant
dependence of granulation amplitudes with metallicity. NASA's
Kepler mission has observed almost 200,000 stars during its nominal
operation, including more than 20,000 red giants with solar-like
oscillations. We fitted the power spectra of light curves of
thousands of those red giants, carried out with the new Apollinaire
code, which describes the granulation part with two Harvey-law
components. We perform a regression of the granulation parameters,
amplitude and timescale, against the atmospheric parameters, effective
temperature, surface gravity, and metallicity. The latter are obtained
from the APOKASC collaboration (between the APOGEE spectroscopic survey
and the Kepler Asteroseismic Scientific Consortium). In the regression
we also account for the dilution of the granulation signal by the
number of granules on the stellar disk, which amounts to a scaling
by radius. For that purpose, we use radii from GAIA DR2 as well as
seismic radii. Investigating red clump and red giant branch stars
separately, we find curious differences in the behavior of granulation,
which is not expected from our current understanding of convection
as supported by 3-D simulations. We are still exploring the reasons
behind these differences. Ideas and suggestions are most welcome.
Title: Effective Rossby numbers of stellar convective envelopes from
3D simulations
Authors: Trampedach, R.
Bibcode: 2022fysr.confE..57T
Altcode:
The Rossby number, the ratio of inertial to Coriolis forces,
of convective envelopes is a deciding parameter in the magnetic
braking formulation of van Saders et al. (2016) and Metcalfe et
al. (2016). What is needed is a bulk quantity, but the Rossby number is
inherently a local quantity, since those forces depend on location in
the star. Using results from the grid of 3D convection simulations by
Trampedach et al. (2013) the convective velocity scales and convection
zone depths are calibrated for 1D stellar envelope models carefully
matched to each simulation. This velocity structure is then used for a
volume integration of the Rossby number modulo a rigid body rotation
rate. The results will be compared to the current standard choice,
which is somewhat arbitrary.
Title: The SAPP pipeline for the determination of stellar abundances
and atmospheric parameters of stars in the core program of the
PLATO mission
Authors: Gent, Matthew Raymond; Bergemann, Maria; Serenelli, Aldo;
Casagrande, Luca; Gerber, Jeffrey M.; Heiter, Ulrike; Kovalev, Mikhail;
Morel, Thierry; Nardetto, Nicolas; Adibekyan, Vardan; Silva Aguirre,
Víctor; Asplund, Martin; Belkacem, Kevin; del Burgo, Carlos; Bigot,
Lionel; Chiavassa, Andrea; Rodríguez Díaz, Luisa Fernanda; Goupil,
Marie-Jo; González Hernández, Jonay I.; Mourard, Denis; Merle,
Thibault; Mészáros, Szabolcs; Marshall, Douglas J.; Ouazzani,
Rhita-Maria; Plez, Bertrand; Reese, Daniel; Trampedach, Regner;
Tsantaki, Maria
Bibcode: 2022A&A...658A.147G
Altcode: 2021arXiv211106666G
We introduce the SAPP (Stellar Abundances and atmospheric Parameters
Pipeline), the prototype of the code that will be used to determine
parameters of stars observed within the core program of the PLATO
space mission. The pipeline is based on the Bayesian inference and
provides effective temperature, surface gravity, metallicity, chemical
abundances, and luminosity. The code in its more general version has a
much wider range of potential applications. It can also provide masses,
ages, and radii of stars and can be used with stellar types not targeted
by the PLATO core program, such as red giants. We validate the code on
a set of 27 benchmark stars that includes 19 FGK-type dwarfs, 6 GK-type
subgiants, and 2 red giants. Our results suggest that combining various
observables is the optimal approach, as this allows the degeneracies
between different parameters to be broken and yields more accurate
values of stellar parameters and more realistic uncertainties. For the
PLATO core sample, we obtain a typical uncertainty of 27 (syst.) ±
37 (stat.) K for Teff, 0.00 ± 0.01 dex for log g, 0.02
± 0.02 dex for metallicity [Fe/H], −0.01 ± 0.03 R⊙
for radii, −0.01 ± 0.05 M⊙ for stellar masses, and
−0.14 ± 0.63 Gyr for ages. We also show that the best results
are obtained by combining the νmax scaling relation with
stellar spectra. This resolves the notorious problem of degeneracies,
which is particularly important for F-type stars.
Title: R-Matrix Calculations of Plasma Opacities
Authors: Pradhan, Anil; Nahar, Sultana; Zhao, Lianshui; Eissner,
Werner; Trampedach, Regner; Mendoza, Claudio
Bibcode: 2020APS..DMPK01021P
Altcode:
A renewed effort is in progress to implement the R-Matrix (RM)
methodology developed for the Opacity Project to compute astrophysical
opacities. The coupled channel (CC) calculations should be of higher
accuracy than the distorted wave (DW) approximation heretofore
employed for opacities calculations, and would precisely incorporate
autoionization and coupling effects. The resulting energy distribution
of the RM opacity spectrum at solar interior conditions is found
to be significantly different than the DW, and mean opacities
are higher than other opacity models. Results are compared with
available experimental data as well as other theoretical models. A
new treatment of plasma broadening of autoionizing resonances is
described, as well as an improved Equation-of-State. Specific features
of bound-free photoionization cross sections relevant to plasma opacity
are illustrated. Convergence of CC wavefunction expansion with respect
to the large number of target ion levels included in the calculations,
and completeness using ''top-up'' DW atomic data, is discussed. Future
plans include extensive opacity calculations for iron and oxygen that
are generally of higher abundance in stellar interiors than other
metals. National Science Fioundation, Department of Energy.
Title: Better Physics for Modelling Stars and their Oscillations
Authors: Trampedach, R.; Houdek, G.; Däppen, W.
Bibcode: 2020svos.conf..317T
Altcode:
Our interpretation of stellar observations can only be as good
as our stellar models and the strong constraints provided by
asteroseismology demand very good models indeed. We have approached
modelling improvements from three angles: Including effects of
realistic 3D convection on the structure of stellar surface layers,
including non-adiabatic effects of that convection on oscillations,
and finally improving and modernising the equation of state for stellar
plasmas. We present a review of our progress on all three fronts.
Title: Non-adiabatic Helioseismology via 3D Convection Simulations
Authors: Trampedach, Regner
Bibcode: 2020ASSP...57..145T
Altcode:
Eigen-functions of p-modes have been extracted from a 3D simulation of
a deep convective solar atmosphere. The part of the eigen-functions
that are out of phase with respect to the density wave, is fitted to
an analytical expression in depth and frequency and then applied to
a non-adiabatic oscillation calculation. The resulting damping widths
and frequency shifts are compared with helioseismic observations.
Title: VizieR Online Data Catalog: High-precision radial velocities
for HD 221416 (Huber+, 2019)
Authors: Huber, D.; Chaplin, W. J.; Chontos, A.; Kjeldsen, H.;
Christensen-Dalsgaard, J.; Bedding, T. R.; Ball, W.; Brahm, R.;
Espinoza, N.; Henning, T.; Jordan, A.; Sarkis, P.; Knudstrup, E.;
Albrecht, S.; Grundahl, F.; Andersen, M. F.; Palle, P. L.; Crossfield,
I.; Fulton, B.; Howard, A. W.; Isaacson, H. T.; Weiss, L. M.; Handberg,
R.; Lund, M. N.; Serenelli, A. M.; Rorsted Mosumgaard, J.; Stokholm,
A.; Bieryla, A.; Buchhave, L. A.; Latham, D. W.; Quinn, S. N.;
Gaidos, E.; Hirano, T.; Ricker, G. R.; Vanderspek, R. K.; Seager,
S.; Jenkins, J. M.; Winn, J. N.; Antia, H. M.; Appourchaux, T.; Basu,
S.; Bell, K. J.; Benomar, O.; Bonanno, A.; Buzasi, D. L.; Campante,
T. L.; Celik Orhan, Z.; Corsaro, E.; Cunha, M. S.; Davies, G. R.;
Deheuvels, S.; Grunblatt, S. K.; Hasanzadeh, A.; di Mauro, M. P.;
Garcia, R. A.; Gaulme, P.; Girardi, L.; Guzik, J. A.; Hon, M.; Jiang,
C.; Kallinger, T.; Kawaler, S. D.; Kuszlewicz, J. S.; Lebreton, Y.; Li,
T.; Lucas, M.; Lundkvist, M. S.; Mann, A. W.; Mathis, S.; Mathur, S.;
Mazumdar, A.; Metcalfe, T. S.; Miglio, A.; Monteiro, M. J. P. F. G.;
Mosser, B.; Noll, A.; Nsamba, B.; Ong, J. M. J.; Ortel, S.; Pereira,
F.; Ranadive, P.; Regulo, C.; Rodrigues, T. S.; Roxburgh, I. W.;
Aguirre, V. S.; Smalley, B.; Schofield, M.; Sousa, S. G.; Stassun,
K. G.; Stello, D.; Tayar, J.; White, T. R.; Verma, K.; Vrard, M.;
Yildiz, M.; Baker, D.; Bazot, M.; Beichmann, C.; Bergmann, C.;
Bugnet, L.; Cale, B.; Carlino, R.; Cartwright, S. M.; Christiansen,
J. L.; Ciardi, D. R.; Creevey, O.; Dittmann, J. A.; Do Nascimento,
J. -D., Jr.; van Eylen, V.; Furesz, G.; Gagne, J.; Gao, P.; Gazeas,
K.; Giddens, F.; Hall, O. J.; Hekker, S.; Ireland, M. J.; Latouf,
N.; Lebrun, D.; Levine, A. M.; Matzko, W.; Natinsky, E.; Page, E.;
Plavchan, P.; Mansouri-Samani, M.; McCauliff, S.; Mullally, S. E.;
Orenstein, B.; Soto, A. G.; Paegert, M.; van Saders, J. L.; Schnaible,
C.; Soderblom, D. R.; Szabo, R.; Tanner, A.; Tinney, C. G.; Teske,
J.; Thomas, A.; Trampedach, R.; Wright, D.; Yuan, T. T.; Zohrabi, F.
Bibcode: 2019yCat..51570245H
Altcode:
We obtained high-resolution spectra of HD 221416 using several
facilities within the TESS Follow-up Observation Program (TFOP),
including HIRES (Vogt et al. 1994SPIE.2198..362V) on the 10 m telescope
at Keck Observatory (Maunakea, Hawai'i); the Hertzsprung SONG Telescope
at Teide Observatory (Tenerife; Grundahl et al. 2017ApJ...836..142G);
HARPS (Mayor et al. 2003Msngr.114...20M), FEROS (Kaufer et
al. 1999Msngr..95....8K), Coralie (Queloz et al. 2001Msngr.105....1Q),
and FIDEOS (Vanzi et al. 2018MNRAS.477.5041V) on the MPG/ESO 3.6 m, 2.2
m, 1.2 m, and 1 m telescopes at La Silla Observatory (Chile); Veloce
(Gilbert et al. 2018SPIE10702E..0YG) on the 3.9 m Anglo-Australian
Telescope at Siding Spring Observatory (Australia); TRES (Furesz 2008,
PhD thesis Univ. Szeged) on the 1.5 m Tillinghast reflector at the
F. L. Whipple Observatory (Mt. Hopkins, Arizona); and iSHELL (Rayner
et al. 2012SPIE.8446E..2CR) on the NASA IRTF Telescope (Maunakea,
Hawai'i). All spectra used in this paper were obtained between 2018
November 11 and December 30 and have a minimum spectral resolution of
R~44000. (1 data file).
Title: Damping rates and frequency corrections of Kepler LEGACY stars
Authors: Houdek, G.; Lund, M. N.; Trampedach, R.;
Christensen-Dalsgaard, J.; Handberg, R.; Appourchaux, T.
Bibcode: 2019MNRAS.487..595H
Altcode: 2019MNRAS.tmp.1157H; 2019arXiv190413170H
Linear damping rates and modal frequency corrections of radial
oscillation modes in selected LEGACY main-sequence stars are
estimated by means of a non-adiabatic stability analysis. The
selected stellar sample covers stars observed by Kepler with a large
range of surface temperatures and surface gravities. A non-local,
time-dependent convection model is perturbed to assess stability
against pulsation modes. The mixing-length parameter is calibrated
to the surface-convection-zone depth of a stellar model obtained from
fitting adiabatic frequencies to the LEGACY observations, and two of
the non-local convection parameters are calibrated to the corresponding
LEGACY linewidth measurements. The remaining non-local convection
parameters in the 1D calculations are calibrated so as to reproduce
profiles of turbulent pressure and of the anisotropy of the turbulent
velocity field of corresponding 3D hydrodynamical simulations. The
atmospheric structure in the 1D stability analysis adopts a
temperature-optical-depth relation derived from 3D hydrodynamical
simulations. Despite the small number of parameters to adjust, we
find good agreement with detailed shapes of both turbulent pressure
profiles and anisotropy profiles with depth, and with damping rates
as a function of frequency. Furthermore, we find the absolute modal
frequency corrections, relative to a standard adiabatic pulsation
calculation, to increase with surface temperature and surface gravity.
Title: Supergranulation on the Sun and stars: A simple model for
its length scale
Authors: Rast, Mark; Trampedach, Regner
Bibcode: 2019AAS...23412205R
Altcode:
Turbulent convection in stellar envelopes is critical to heat transport
and dynamo activity. Modeling it well it has proven surprisingly
difficult, and recent solar and stellar observations have raised
questions about our understanding of the dynamics of both the deep solar
convection and the mean structure of the upper layers of convective
stellar envelopes. In particular, the amplitude of low wavenumber
convective motions in both local area radiative magnetohydrodynamic
and global spherical shell magnetohydrodynamic simulations of the Sun
appear to be too high. In global simulations this results in weaker
than needed rotational constraint of the motions and consequent
non solar-like differential rotation profiles. In deep local area
simulations it yields strong horizontal flows in the photosphere
on scales much larger than the observed supergranulation, leaving
the origin of the solar supergranular scale enigmatic. The problems
are not confined to the Sun. Models of stellar convection show too
sharp a transition to the interior adiabatic gradient, leading to a
mismatch between computed and observed oscillation frequencies. We
suggest that there is a common solution to these problems: convective
motions in stellar envelopes are even more nonlocal than numerical
models suggest. Small scale photospherically driven motions dominate
convective transport even at depth, descending through a very nearly
adiabatic interior (more nearly adiabatic in the mean than numerical
models achieve). To test this, we develop a simple model that reproduces
the mean thermodynamic stratification of three dimensional hydrodynamic
stellar envelope models. It can recover the mean thermodynmaic states of
the full models knowing only the filling factor and entropy fluctuations
of the granular downflows in their photospheres. The supergranular scale
of convection is then determined by the depth to which the presence
of granular downflows alters the otherwise adiabatically stratified
background. The supergranular scale of convection is then determined
by the depth to which the presence of granular downflows alters the
otherwise adiabatically stratified background.
Title: A Hot Saturn Orbiting an Oscillating Late Subgiant Discovered
by TESS
Authors: Huber, Daniel; Chaplin, William J.; Chontos, Ashley; Kjeldsen,
Hans; Christensen-Dalsgaard, Jørgen; Bedding, Timothy R.; Ball,
Warrick; Brahm, Rafael; Espinoza, Nestor; Henning, Thomas; Jordán,
Andrés; Sarkis, Paula; Knudstrup, Emil; Albrecht, Simon; Grundahl,
Frank; Fredslund Andersen, Mads; Pallé, Pere L.; Crossfield, Ian;
Fulton, Benjamin; Howard, Andrew W.; Isaacson, Howard T.; Weiss,
Lauren M.; Handberg, Rasmus; Lund, Mikkel N.; Serenelli, Aldo M.;
Rørsted Mosumgaard, Jakob; Stokholm, Amalie; Bieryla, Allyson;
Buchhave, Lars A.; Latham, David W.; Quinn, Samuel N.; Gaidos, Eric;
Hirano, Teruyuki; Ricker, George R.; Vanderspek, Roland K.; Seager,
Sara; Jenkins, Jon M.; Winn, Joshua N.; Antia, H. M.; Appourchaux,
Thierry; Basu, Sarbani; Bell, Keaton J.; Benomar, Othman; Bonanno,
Alfio; Buzasi, Derek L.; Campante, Tiago L.; Çelik Orhan, Z.; Corsaro,
Enrico; Cunha, Margarida S.; Davies, Guy R.; Deheuvels, Sebastien;
Grunblatt, Samuel K.; Hasanzadeh, Amir; Di Mauro, Maria Pia; García,
Rafael A.; Gaulme, Patrick; Girardi, Léo; Guzik, Joyce A.; Hon, Marc;
Jiang, Chen; Kallinger, Thomas; Kawaler, Steven D.; Kuszlewicz, James
S.; Lebreton, Yveline; Li, Tanda; Lucas, Miles; Lundkvist, Mia S.;
Mann, Andrew W.; Mathis, Stéphane; Mathur, Savita; Mazumdar, Anwesh;
Metcalfe, Travis S.; Miglio, Andrea; Monteiro, Mário J. P. F. G.;
Mosser, Benoit; Noll, Anthony; Nsamba, Benard; Ong, Jia Mian Joel;
Örtel, S.; Pereira, Filipe; Ranadive, Pritesh; Régulo, Clara;
Rodrigues, Thaíse S.; Roxburgh, Ian W.; Silva Aguirre, Victor;
Smalley, Barry; Schofield, Mathew; Sousa, Sérgio G.; Stassun,
Keivan G.; Stello, Dennis; Tayar, Jamie; White, Timothy R.; Verma,
Kuldeep; Vrard, Mathieu; Yıldız, M.; Baker, David; Bazot, Michaël;
Beichmann, Charles; Bergmann, Christoph; Bugnet, Lisa; Cale, Bryson;
Carlino, Roberto; Cartwright, Scott M.; Christiansen, Jessie L.;
Ciardi, David R.; Creevey, Orlagh; Dittmann, Jason A.; Do Nascimento,
Jose-Dias, Jr.; Van Eylen, Vincent; Fürész, Gabor; Gagné, Jonathan;
Gao, Peter; Gazeas, Kosmas; Giddens, Frank; Hall, Oliver J.; Hekker,
Saskia; Ireland, Michael J.; Latouf, Natasha; LeBrun, Danny; Levine,
Alan M.; Matzko, William; Natinsky, Eva; Page, Emma; Plavchan,
Peter; Mansouri-Samani, Masoud; McCauliff, Sean; Mullally, Susan E.;
Orenstein, Brendan; Garcia Soto, Aylin; Paegert, Martin; van Saders,
Jennifer L.; Schnaible, Chloe; Soderblom, David R.; Szabó, Róbert;
Tanner, Angelle; Tinney, C. G.; Teske, Johanna; Thomas, Alexandra;
Trampedach, Regner; Wright, Duncan; Yuan, Thomas T.; Zohrabi, Farzaneh
Bibcode: 2019AJ....157..245H
Altcode: 2019arXiv190101643H
We present the discovery of HD 221416 b, the first transiting planet
identified by the Transiting Exoplanet Survey Satellite (TESS) for
which asteroseismology of the host star is possible. HD 221416 b
(HIP 116158, TOI-197) is a bright (V = 8.2 mag), spectroscopically
classified subgiant that oscillates with an average frequency of
about 430 μHz and displays a clear signature of mixed modes. The
oscillation amplitude confirms that the redder TESS bandpass compared
to Kepler has a small effect on the oscillations, supporting the
expected yield of thousands of solar-like oscillators with TESS 2
minute cadence observations. Asteroseismic modeling yields a robust
determination of the host star radius (R ⋆ = 2.943 ±
0.064 R ⊙), mass (M ⋆ = 1.212 ± 0.074 M
⊙), and age (4.9 ± 1.1 Gyr), and demonstrates that it has
just started ascending the red-giant branch. Combining asteroseismology
with transit modeling and radial-velocity observations, we show that
the planet is a “hot Saturn” (R p = 9.17 ± 0.33 R
⊕) with an orbital period of ∼14.3 days, irradiance
of F = 343 ± 24 F ⊕, and moderate mass (M p
= 60.5 ± 5.7 M ⊕) and density (ρ p = 0.431
± 0.062 g cm-3). The properties of HD 221416 b show that
the host-star metallicity-planet mass correlation found in sub-Saturns
(4-8 R ⊕) does not extend to larger radii, indicating that
planets in the transition between sub-Saturns and Jupiters follow a
relatively narrow range of densities. With a density measured to ∼15%,
HD 221416 b is one of the best characterized Saturn-size planets to
date, augmenting the small number of known transiting planets around
evolved stars and demonstrating the power of TESS to characterize
exoplanets and their host stars using asteroseismology.
Title: Stellar Physics and Galactic Archaeology using Asteroseismology
in the 2020's
Authors: Huber, Daniel; Basu, Sarbani; Beck, Paul; Bedding, Timothy R.;
Buzasi, Derek; Cantiello, Matteo; Chaplin, William J.; Christiansen,
Jessie L.; Cunha, Katia; Egeland, Ricky; Fuller, Jim; Garcia,
Rafael A.; Gies, Douglas R.; Guzik, Joyce; Hekker, Saskia; Hermes,
JJ; Jackiewicz, Jason; Johnson, Jennifer; Kawaler, Steve; Metcalfe,
Travis; Mosser, Benoit; Ness, Melissa; Pinsonneault, Marc; Piro,
Anthony L.; Aguirre, Victor Silva; Soderblom, David; Stassun, Keivan;
Tayar, Jamie; ten Brummelaar, Theo; Roettenbacher, Rachael; Trampedach,
Regner; van Belle, Gerard; van Saders, Jennifer; Stello, Dennis
Bibcode: 2019BAAS...51c.488H
Altcode: 2019astro2020T.488H; 2019arXiv190308188H
Asteroseismology is the only observational tool in astronomy that can
probe the interiors of stars, and is a benchmark method for deriving
fundamental properties of stars and exoplanets. In this white paper,
we describe key science questions and necessary facilities to continue
the asteroseismology revolution into the 2020's.
Title: 3D Spectral Synthesis for Large-Scale Stellar Surveys,
Asteroseismology and Galactic Archaeology
Authors: Trampedach, Regner
Bibcode: 2019atp..prop..217T
Altcode:
Much of our understanding of our Galaxy's formation and evolution
relies on accurate knowledge of stellar abundances of the chemical
elements. The general amount of metals, for example, tells us the
number of stellar generations between the Big Bang and the formation
of a particular star; Particular abundance patterns can reveal the
enrichment of star-forming clouds by nearby super novae and stellar
winds, or the interior dynamics of stars. The nascent field of
Galactic archaeology puts the strongest demands on our knowledge of
stellar abundances. This is the pursuit of the history of the Galaxy,
including its star formation history, galactic mergers, and coupling
to the growth history of our central black hole. Such studies are
based on the idea that distinct populations of stars in our Galaxy
have kinematic and chemical fingerprints, that uniquely identifies
them. We live in an unprecedented era for Galactic Archaeology. For
the first time, representative large scale surveys for millions (and
even billions) of stars allow a precise probing of the Milky Way
structure and evolution. In this respect, ESA's astrometry mission,
Gaia, is currently revolutionizing our knowledge of the structure
and kinematics of the Milky Way. Gaia has revealed populations
of stars with common origins, such as merger events or a shared
birth place --- populations that now, after thousands of orbits,
are scattered throughout the Galaxy. A special population being
searched for, is that of our stellar siblings, born out of the same
giant molecular cloud as the Sun. Accurate abundances are crucial
for discriminating between such populations, but the accuracy is
currently limited by the 1D stellar atmosphere models they are based
on. These models employ simplistic prescriptions for convection that
has free parameters, adjusted to reproduce the depths and widths of
spectral lines, compromising the reliability of abundances based on 1D
atmospheres. Furthermore, the assumptions behind 1D prescriptions for
convection are known to break down in the exact layers where the light
we see from stars, form. In stark contrast, realistic 3D, hydrodynamic,
atmosphere simulations, exhibit convection as an emergent property,
with large temperature fluctuations and velocities. At the surface,
the temperatures and velocities are correlated in a way that produce
blue-shifted, slightly C-shaped spectral lines, in robust agreement
with observed stellar spectra. With such realistic synthetic lines,
blends by other weaker lines become obvious and can be accounted
for, whereas in 1D analysis such blends go unnoticed. Missed blends
systematically increase inferred abundances. 3D convection simulations
are computationally expensive, however, and there is no trivial way of
interpolating between simulations in a grid, as can be easily done in
grids of 1D atmosphere models. This means a single simulation is both
expensive, and also only relevant for the few stars that are similar
enough. This project aims at alleviating this obstacle, and make
grids of 3D simulations as useful as grids of 1D atmosphere models,
but with much higher fidelity and realism. This work will match the
wealth of high-quality spectroscopic, ground-based surveys, with an
equally high-quality analysis of the observations to produce robust
and reliable abundances and surface-temperatures and -gravities. This
will apply to major surveys like RAVE, GALAH and SDSS/APOGEE, as
well as the targeted SAGA and APOKASC surveys, that support NASA's
planet-hunting and asteroseismology mission TESS, and the past Kepler/K2
missions. Combining this with the now ubiquitous Gaia parallaxes
will provide unprecedentedly strong constraints on all fundamental
parameters of stars, advancing our understanding of stellar structure,
the Milky Way's evolution and our place in it.
Title: The Dark Side of the Sun
Authors: Trampedach, R.
Bibcode: 2018ASPC..515...29T
Altcode: 2018arXiv180403746T
Is the Sun likely to have a more opaque interior than previously
thought? The solar oxygen (or abundance) problem can be solved with
higher interior opacities, reconciling abundance analyses based on 3D
convective atmospheres with the helioseismic structure of the solar
interior. This has been known for more than a decade, but last year
we learned that the absorption by just iron may contribute 7% more to
the solar opacity at the bottom of the convection zone than predicted
by any opacity calculation so far, and by OP05 in particular. I find
that artificial changes to the absorption (calibrated against the
iron experiment) by other elements in a solar mixture give an opacity
increase of a shape and magnitude that can restore agreement between
modern abundance analysis and helioseismology. This suggests that
improved opacity calculations will solve the solar oxygen problem.
Title: A Modelers' Opacity Wish List
Authors: Trampedach, R.
Bibcode: 2018ASPC..515..319T
Altcode: 2018arXiv180404123T
At the Workshop on Astrophysical Opacities, several attendees voiced
their interest in a list of absorption data that are missing from or
inadequate in current models of astrophysical objects. This wish list
by modelers is meant as motivation and inspiration for experimentalists
and theoreticians alike.
Title: Current State of Astrophysical Opacities: A White Paper
Authors: Lynas-Gray, A. E.; Basu, S.; Bautista, M. A.; Colgan, J.;
Mendoza, C.; Tennyson, J.; Trampedach, R.; Turck-Chièze, S.
Bibcode: 2018ASPC..515..301L
Altcode: 2018arXiv180406804L
Availability of reliable atomic and molecular opacity tables is
essential in a wide variety of astronomical modeling: the solar and
stellar interiors, stellar and planetary atmospheres, stellar evolution,
pulsating stars, and protoplanetary disks, to name a few. With the
advancement of powerful research techniques—such as helioseismology
and asteroseismology, solar neutrino-flux measurements, exoplanet survey
satellites, three-dimensional hydrodynamic atmospheric simulations
(including non-LTE and granulation effects), high-performance computing
of atomic and molecular data, and innovative plasma experiments—the
accuracy and completeness of opacity tables is being taken to an
unprecedented level. The goal of the second Workshop on Astrophysical
Opacities was to gather opacity data producers and consumers from both
the atomic and molecular sectors to contribute to solving outstanding
problems and to develop more effective and integrated interfaces. In
this review we attempt to summarize the discussion at the workshop
and propose future directions for opacity research.
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.
Bibcode: 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.
Title: Improving 1D Stellar Models with 3D Atmospheres
Authors: Mosumgaard, Jakob Rørsted; Silva Aguirre, Víctor; Weiss,
Achim; Christensen-Dalsgaard, Jørgen; Trampedach, Regner
Bibcode: 2017EPJWC.16003009M
Altcode: 2016arXiv161007323R; 2017EPJWC.16003009R; 2016arXiv161007323M
Stellar evolution codes play a major role in present-day astrophysics,
yet they share common issues. In this work we seek to remedy some
of those by the use of results from realistic and highly detailed 3D
hydrodynamical simulations of stellar atmospheres. We have implemented
a new temperature stratification extracted directly from the 3D
simulations into the Garching Stellar Evolution Code to replace the
simplified atmosphere normally used. Secondly, we have implemented
the use of a variable mixing-length parameter, which changes as a
function of the stellar surface gravity and temperature - also derived
from the 3D simulations. Furthermore, to make our models consistent,
we have calculated new opacity tables to match the atmospheric
simulations. Here, we present the modified code and initial results
on stellar evolution using it.
Title: An opaque Sun? The potential for future, higher opacities to
solve the solar abundance problem
Authors: Trampedach, Regner
Bibcode: 2017EPJWC.16002005T
Altcode:
Last year Bailey et al. announced their measurement of iron opacity that
increases the Rosseland mean at the base of the solar convection zone
by 7%. I ask what happens if the absorption by other elements is also
stronger than predicted so far. Artificially increasing the absorption
by other elements, proportional to the number of bound electrons in
the absorber (reflecting our remaining ignorance of atomic physics)
gives an opacity increase for a solar model, that has the potential to
solve the long-standing solar abundance problem. Conclusion: Opacities
are the likely source of the solar abundance problem, and the solar
abundances are likely closer to those of Asplund et al. (2009) than
to the various classic sets of abundances.
Title: Metallicity effect on stellar granulation detected from
oscillating red giants in open clusters
Authors: Corsaro, E.; Mathur, S.; García, R. A.; Gaulme, P.;
Pinsonneault, M.; Stassun, K.; Stello, D.; Tayar, J.; Trampedach,
R.; Jiang, C.; Nitschelm, C.; Salabert, D.
Bibcode: 2017A&A...605A...3C
Altcode: 2017arXiv170707474C
Context. The effect of metallicity on the granulation activity in
stars, and hence on the convective motions in general, is still
poorly understood. Available spectroscopic parameters from the
updated APOGEE-Kepler catalog, coupled with high-precision photometric
observations from NASA's Kepler mission spanning more than four years
of observation, make oscillating red giant stars in open clusters
crucial testbeds.
Aims: We aim to determine the role of
metallicity on the stellar granulation activity by discriminating
its effect from that of different stellar properties such as surface
gravity, mass, and temperature. We analyze 60 known red giant stars
belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811,
spanning a metallicity range from [Fe/H] ≃ - 0.09 to 0.32. The
parameters describing the granulation activity of these stars and
their frequency of maximum oscillation power, νmax, are
studied while taking into account different masses, metallicities,
and stellar evolutionary stages. We derive new scaling relations for
the granulation activity, re-calibrate existing ones, and identify the
best scaling relations from the available set of observations.
Methods: We adopted the Bayesian code DIAMONDS for the analysis of the
background signal in the Fourier spectra of the stars. We performed a
Bayesian parameter estimation and model comparison to test the different
model hypotheses proposed in this work and in the literature.
Results: Metallicity causes a statistically significant change in the
amplitude of the granulation activity, with a dependency stronger than
that induced by both stellar mass and surface gravity. We also find
that the metallicity has a significant impact on the corresponding time
scales of the phenomenon. The effect of metallicity on the time scale
is stronger than that of mass.
Conclusions: A higher metallicity
increases the amplitude of granulation and meso-granulation signals and
slows down their characteristic time scales toward longer periods. The
trend in amplitude is in qualitative agreement with predictions from
existing 3D hydrodynamical simulations of stellar atmospheres from
main sequence to red giant stars. We confirm that the granulation
activity is not sensitive to changes in the stellar core and that it
only depends on the atmospheric parameters of stars.
Title: The asteroseismic surface effect from a grid of 3D convection
simulations - I. Frequency shifts from convective expansion of
stellar atmospheres
Authors: Trampedach, Regner; Aarslev, Magnus J.; Houdek, Günter;
Collet, Remo; Christensen-Dalsgaard, Jørgen; Stein, Robert F.;
Asplund, Martin
Bibcode: 2017MNRAS.466L..43T
Altcode: 2016arXiv161102638T
We analyse the effect on adiabatic stellar oscillation frequencies
of replacing the near-surface layers in 1D stellar structure models
with averaged 3D stellar surface convection simulations. The main
difference is an expansion of the atmosphere by 3D convection,
expected to explain a major part of the asteroseismic surface effect,
a systematic overestimation of p-mode frequencies due to inadequate
surface physics. We employ pairs of 1D stellar envelope models and 3D
simulations from a previous calibration of the mixing-length parameter,
α. That calibration constitutes the hitherto most consistent matching
of 1D models to 3D simulations, ensuring that their differences are not
spurious, but entirely due to the 3D nature of convection. The resulting
frequency shift is identified as the structural part of the surface
effect. The important, typically non-adiabatic, modal components of
the surface effect are not included in this analysis, but relegated to
future papers. Evaluating the structural surface effect at the frequency
of maximum mode amplitude, νmax , we find shifts from δν =
-0.8 μHz for giants at log g = 2.2 to - 35 μHz for a (Teff
= 6901 K, log g = 4.29) dwarf. The fractional effect δν(νmax
)/νmax , ranges from -0.1 per cent for a cool dwarf
(4185 K, 4.74) to -6 per cent for a warm giant (4962 K, 2.20).
Title: On the surface physics affecting solar oscillation frequencies
Authors: Houdek, G.; Trampedach, R.; Aarslev, M. J.;
Christensen-Dalsgaard, J.
Bibcode: 2017MNRAS.464L.124H
Altcode: 2016arXiv160906129H
Adiabatic oscillation frequencies of stellar models, computed with the
standard mixing-length formulation for convection, increasingly deviate
with radial order from observations in solar-like stars. Standard
solar models overestimate adiabatic frequencies by as much as ∼ 20
μHz. In this Letter, we address the physical processes of turbulent
convection that are predominantly responsible for the frequency
differences between standard models and observations, also called
`surface effects'. We compare measured solar frequencies from the
Michelson Doppler Imager instrument on the SOlar and Heliospheric
Observatory spacecraft with frequency calculations that include
3D hydrodynamical simulation results in the equilibrium model,
non-adiabatic effects, and a consistent treatment of the turbulent
pressure in both the equilibrium and stability computations. With the
consistent inclusion of the above physics in our model computation,
we are able to reproduce the observed solar frequencies to ≲3 μHz
without the need of any additional ad hoc functional corrections.
Title: Accessing the Full Potential of Asteroseismology with Kepler,
K2 and TESS: Reconciling Modeling Conflicts Between Classic and
Seismic Observables
Authors: Trampedach, Regner
Bibcode: 2017atp..prop..149T
Altcode:
Asteroseismic analyses of stars often yield conflicting sets of
solutions to the observations. The dichotomies arise from tensions
between classical observables, such as photometry, spectroscopy and
abundance analysis, versus the seismic observables of dozens of p-
and g-mode frequencies. This in turn is a tension between stellar
atmosphere models and interior structure models. F-stars and sub-giants
seem most prone to this, but sub-giants, thanks to their mixed g-
and p-modes also have the promise of showing us stellar interiors
in unprecedented detail. Methods have been developed to resolve
such conflicts, but only do so by injecting unwanted bias into the
analysis. The proposed project will instead address the root cause
of the problem, which is a conflict between the modeling of classic
and seismic observables. The solar abundance problem points to the
same tension between atmospheric and interior models, and is likely
to also apply to other stars. One solution, made more probable by
recent experiments on iron under solar conditions, is an increase of
interior opacity. This will be addressed by a new opacity calculation
by A. Pradhan's group at Ohio State Univ. (outside this project),
based on a new equation of state being developed by the PI. These will
form the atomic physics foundation of the project proposed here. The
atmospheric structure, producing the classic observables, is greatly
affected by the 3D nature of convection in late-type stars. The proposed
project will therefore center around the calculation of a new grid
of realistic 3D simulations of deep stellar atmospheres. It will span
from M dwarfs to early A stars on the main sequence, and up to giants
with logg=0. The grid will initially span metallicities of [Fe/H] =
[-1.0, -0.5, 0.0, +0.2], with some additional simulations to explore
dependencies on helium content. The grid will employ the new equation
of state and opacities mentioned above, and a number of improvements
to the hydrodynamics. We will also change the file format to make the
files more immediately accessible, as we plan to give the community
access to the full simulation results and supporting code. To perform
asteroseismic analysis the atmosphere simulations need to be connected
with interior and evolution models. These models will use the same
new atomic physics as the 3D atmospheres, the photospheric transition
resulting from the 3D radiative transfer in the convecting simulations,
the convective expansion of the atmosphere, and a mixing-length
parameter that reproduce the bottom of the simulations. The seismic
modeling will build on the PI's current efforts to evaluate the
components of the asteroseismic surface effect, arising from direct
interactions between convection and oscillation modes. The result
will be an unprecedented level of consistency and quality in all the
steps of the modeling, and an elimination of the classic break between
atmosphere and interior models. The Asteroseismic Modeling Portal
(AMP), by team-member T. Metcalfe, will combine these changes to the
stellar structure and seismic modeling to perform searches for the
best fit models to both seismic and classic observations of stars. The
result will be robust and unambiguous, matching the quality of Kepler
and K2 observations. Such an improved focus of Kepler/K2 will enable
investigations into more subtle phenomena, and possibly break some
of the current degeneracy between stellar parameters we try to solve
for. Our results will be applied to a sample of particularly difficult,
or interesting Kepler and K2 targets. Our project will advance the
science goals of NASA's astrophysics program, and in particular that
of the K2 mission and the soon to be launched TESS mission. The same
physics applies to the Sun, and we will therefore also support the
science goals of the heliophysics program and the SOHO and SDO missions.
Title: The Surface of Stellar Models - Now with more 3D simulations!
Authors: Trampedach, Regner; Christensen-Dalsgaard, Jørgen; Asplund,
Martin; Stein, Robert F.; Nordlund, Åke
Bibcode: 2015EPJWC.10106064T
Altcode:
We have constructed a grid of 3D hydrodynamic simulations of deep
convective and line-blanketed atmospheres. We have developed a
new consistent method for computing and employing T(τ) relations
from these simulations, as surface boundary conditions for 1D
stellar structure models. These 1D models have, in turn, had their
mixing-length, α, calibrated against the averaged structure of
each of the simulations. Both α and T(τ) vary significantly with
Teff and log g.
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
Bibcode: 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 ɛNa = 6.21 ± 0.04, log
ɛMg = 7.59 ± 0.04, log ɛAl = 6.43 ± 0.04,
log ɛSi = 7.51 ± 0.03, log ɛP = 5.41 ± 0.03,
log ɛS = 7.13 ± 0.03, log ɛK = 5.04 ± 0.05
and log ɛCa = 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. Tables 1-4 and Appendix A are available in electronic form at http://www.aanda.org
Title: The Role of Turbulent Pressure as a Coherent Pulsational
Driving Mechanism: The Case of the δ Scuti Star HD 187547
Authors: Antoci, V.; Cunha, M.; Houdek, G.; Kjeldsen, H.; Trampedach,
R.; Handler, G.; Lüftinger, T.; Arentoft, T.; Murphy, S.
Bibcode: 2014ApJ...796..118A
Altcode: 2014arXiv1411.0931A
HD 187547 was the first candidate that led to the suggestion
that solar-like oscillations are present in δ Scuti stars. Longer
observations, however, show that the modes interpreted as solar-like
oscillations have either very long mode lifetimes, longer than
960 days, or are coherent. These results are incompatible with the
nature of "pure" stochastic excitation as observed in solar-like
stars. Nonetheless, one point is certain: the opacity mechanism alone
cannot explain the oscillation spectrum of HD 187547. Here we present
new theoretical investigations showing that convection dynamics can
intrinsically excite coherent pulsations in the chemically peculiar δ
Scuti star HD 187547. More precisely, it is the perturbations of the
mean Reynold stresses (turbulent pressure) that drives the pulsations
and the excitation takes place predominantly in the hydrogen ionization
zone.
Title: Improvements to stellar structure models, based on a grid of 3D
convection simulations - II. Calibrating the mixing-length formulation
Authors: Trampedach, Regner; Stein, Robert F.; Christensen-Dalsgaard,
Jørgen; Nordlund, Åke; Asplund, Martin
Bibcode: 2014MNRAS.445.4366T
Altcode: 2014arXiv1410.1559T
We perform a calibration of the mixing length of convection in stellar
structure models against realistic 3D radiation-coupled hydrodynamics
simulations of convection in stellar surface layers, determining
the adiabat deep in convective stellar envelopes. The mixing-length
parameter α is calibrated by matching averages of the 3D simulations
to 1D stellar envelope models, ensuring identical atomic physics
in the two cases. This is done for a previously published grid of
solar-metallicity convection simulations, covering from 4200 to 6900 K
on the main sequence, and from 4300 to 5000 K for giants with log g =
2.2. Our calibration results in an α varying from 1.6 for the warmest
dwarf, which is just cool enough to admit a convective envelope, and
up to 2.05 for the coolest dwarfs in our grid. In between these is a
triangular plateau of α ∼ 1.76. The Sun is located on this plateau
and has seen little change during its evolution so far. When stars
ascend the giant branch, they largely do so along tracks of constant
α, with α decreasing with increasing mass.
Title: Properties of 42 Solar-type Kepler Targets from the
Asteroseismic Modeling Portal
Authors: Metcalfe, T. S.; Creevey, O. L.; Doğan, G.; Mathur, S.;
Xu, H.; Bedding, T. R.; Chaplin, W. J.; Christensen-Dalsgaard, J.;
Karoff, C.; Trampedach, R.; Benomar, O.; Brown, B. P.; Buzasi, D. L.;
Campante, T. L.; Çelik, Z.; Cunha, M. S.; Davies, G. R.; Deheuvels,
S.; Derekas, A.; Di Mauro, M. P.; García, R. A.; Guzik, J. A.;
Howe, R.; MacGregor, K. B.; Mazumdar, A.; Montalbán, J.; Monteiro,
M. J. P. F. G.; Salabert, D.; Serenelli, A.; Stello, D.; Ste&şacute;
licki, M.; Suran, M. D.; Yıldız, M.; Aksoy, C.; Elsworth, Y.;
Gruberbauer, M.; Guenther, D. B.; Lebreton, Y.; Molaverdikhani, K.;
Pricopi, D.; Simoniello, R.; White, T. R.
Bibcode: 2014ApJS..214...27M
Altcode: 2014arXiv1402.3614M
Recently the number of main-sequence and subgiant stars exhibiting
solar-like oscillations that are resolved into individual mode
frequencies has increased dramatically. While only a few such data
sets were available for detailed modeling just a decade ago, the
Kepler mission has produced suitable observations for hundreds of
new targets. This rapid expansion in observational capacity has been
accompanied by a shift in analysis and modeling strategies to yield
uniform sets of derived stellar properties more quickly and easily. We
use previously published asteroseismic and spectroscopic data sets
to provide a uniform analysis of 42 solar-type Kepler targets from
the Asteroseismic Modeling Portal. We find that fitting the individual
frequencies typically doubles the precision of the asteroseismic radius,
mass, and age compared to grid-based modeling of the global oscillation
properties, and improves the precision of the radius and mass by about
a factor of three over empirical scaling relations. We demonstrate
the utility of the derived properties with several applications.
Title: Improvements to stellar structure models, based on a grid of
3D convection simulations - I. T(τ) relations
Authors: Trampedach, Regner; Stein, Robert F.; Christensen-Dalsgaard,
Jørgen; Nordlund, Åke; Asplund, Martin
Bibcode: 2014MNRAS.442..805T
Altcode: 2014arXiv1405.0236T
Relations between temperature, T, and optical depth, τ, are often
used for describing the photospheric transition from optically thick
to optically thin in stellar structure models. We show that this is
well justified, but also that currently used T(τ) relations are often
inconsistent with their implementation. As an outer boundary condition
on the system of stellar structure equations, T(τ) relations have an
undue effect on the overall structure of stars. In this age of precision
asteroseismology, we need to re-assess both the method for computing
and for implementing T(τ) relations, and the assumptions they rest
on. We develop a formulation for proper and consistent evaluation
of T(τ) relations from arbitrary 1D or 3D stellar atmospheres,
and for their implementation in stellar structure and evolution
models. We extract radiative T(τ) relations, as described by our
new formulation, from 3D simulations of convection in deep stellar
atmospheres of late-type stars from dwarfs to giants. These simulations
employ realistic opacities and equation of state, and account for
line blanketing. For comparison, we also extract T(τ) relations from
1DMARCSmodel atmospheres using the same formulation. T(τ) relations
from our grid of 3D convection simulations display a larger range of
behaviours with surface gravity, compared with those of conventional
theoretical 1D hydrostatic atmosphere models based on the mixing-length
theory for convection. The 1D atmospheres show little dependence on
gravity. 1D atmospheres of main-sequence stars also show an abrupt
transition to the diffusion approximation at τ ≃ 2.5, whereas the
3D simulations exhibit smooth transitions that occur at the same depth
for M ≃ 0.8 M⊙, and higher in the atmosphere for both
more and less massive main-sequence stars. Based on these results,
we recommend no longer using scaled solar T(τ) relations. Files with
T(τ) relations for our grid of simulations are made available to the
community, together with routines for interpolating in this irregular
grid. We also provide matching tables of atmospheric opacity, for
consistent implementation in stellar structure models.
Title: VizieR Online Data Catalog: T(tau) relations code (Trampedach+,
2014)
Authors: Trampedach, R.; Stein, R. F.; Christensen-Dalsgaard, J.;
Nordlund, A.; Asplund, M.
Bibcode: 2014yCat..74420805T
Altcode:
Radiative T({tau})-relations, in the form of generalised Hopf functions,
computed from a grid of 37, solar metallicity, realistic, 3D convection
simulations with radiative transfer. (6 data files).
Title: Models of solar surface dynamics: impact on eigenfrequencies
and radius
Authors: Piau, L.; Collet, R.; Stein, R. F.; Trampedach, R.; Morel,
P.; Turck-Chièze, S.
Bibcode: 2014MNRAS.437..164P
Altcode: 2013arXiv1309.7179P; 2013MNRAS.tmp.2547P
We study the effects of different descriptions of the solar surface
convection on the eigenfrequencies of p modes. 1D evolution calculations
of the whole Sun and 3D hydrodynamic and magnetohydrodynamic simulations
of the current surface are performed. These calculations rely on
realistic physics. Averaged stratifications of the 3D simulations are
introduced in the 1D solar evolution or in the structure models. The
eigenfrequencies obtained are compared to those of 1D models relying
on the usual phenomenologies of convection and to observations of the
Michelson Doppler Imager instrument aboard the Solar Heliospheric
Observatory (SoHO). We also investigate how the magnetic activity
could change the eigenfrequencies and the solar radius, assuming that,
3 Mm below the surface, the upgoing plasma advects a 1.2 kG horizontal
field. All models and observed eigenfrequencies are fairly close below 3
mHz. Above 3 mHz the eigenfrequencies of the phenomenological convection
models are above the observed eigenfrequencies. The frequencies
of the models based on the 3D simulations are slightly below the
observed frequencies. Their maximum deviation is ≈3 μHz at 3 mHz
but drops below 1 μHz at 4 mHz. Replacing the hydrodynamic by the
magnetohydrodynamic simulation increases the eigenfrequencies. The shift
is negligible below 2.2 mHz and then increases linearly with frequency
to reach ≈1.7 μHz at 4 mHz. The impact of the simulated activity
is a 14 mas shrinking of the solar layers near the optical depth unity.
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.
Bibcode: 2013A&A...557A..26M
Altcode: 2013arXiv1302.2621M
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).
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, Teff, 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.
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. Appendices A-C are available in electronic
form at http://www.aanda.orgFull
Table C.1 is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/557/A26
Title: Light bosons in the photosphere and the solar abundance problem
Authors: Vincent, A. C.; Scott, P.; Trampedach, R.
Bibcode: 2013MNRAS.432.3332V
Altcode: 2012arXiv1206.4315V; 2013MNRAS.tmp.1360V
Spectroscopy is used to measure the elemental abundances in the outer
layers of the Sun, whereas helioseismology probes the interior. It is
well known that current spectroscopic determinations of the chemical
composition are starkly at odds with the metallicity implied by
helioseismology. We investigate whether the discrepancy may be due to
conversion of photons to a new light boson in the solar photosphere. We
examine the impact of particles with axion-like interactions with
the photon on the inferred photospheric abundances, showing that
resonant axion-photon conversion is not possible in the region of the
solar atmosphere in which line formation occurs. Although non-resonant
conversion in the line-forming regions can in principle impact derived
abundances, constraints from axion-photon conversion experiments rule
out the couplings necessary for these effects to be detectable. We show
that this extends to hidden photons and chameleons (which would exhibit
similar phenomenological behaviour), ruling out known theories of new
light bosons as photospheric solutions to the solar abundance problem.
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.
Bibcode: 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. (1 data file).
Title: How realistic are solar model atmospheres?
Authors: Pereira, T. M. D.; Asplund, M.; Collet, R.; Thaler, I.;
Trampedach, R.; Leenaarts, J.
Bibcode: 2013A&A...554A.118P
Altcode: 2013arXiv1304.4932P
Context. Recently, new solar model atmospheres have been developed
to replace classical 1D local thermodynamical equilibrium (LTE)
hydrostatic models and used to for example derive the solar chemical
composition.
Aims: We aim to test various models against key
observational constraints. In particular, a 3D model used to derive
the solar abundances, a 3D magnetohydrodynamical (MHD) model (with an
imposed 10 mT vertical magnetic field), 1D NLTE and LTE models from
the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical
model of Holweger & Müller.
Methods: We confronted the
models with observational diagnostics of the temperature profile:
continuum centre-to-limb variations (CLVs), absolute continuum fluxes,
and the wings of hydrogen lines. We also tested the 3D models for the
intensity distribution of the granulation and spectral line shapes.
Results: The predictions from the 3D model are in excellent agreement
with the continuum CLV observations, performing even better than
the Holweger & Müller model (constructed largely to fulfil such
observations). The predictions of the 1D theoretical models are worse,
given their steeper temperature gradients. For the continuum fluxes,
predictions for most models agree well with the observations. No
model fits all hydrogen lines perfectly, but again the 3D model comes
ahead. The 3D model also reproduces the observed continuum intensity
fluctuations and spectral line shapes very well.
Conclusions:
The excellent agreement of the 3D model with the observables reinforces
the view that its temperature structure is realistic. It outperforms
the MHD simulation in all diagnostics, implying that recent claims
for revised abundances based on MHD modelling are premature. Several
weaknesses in the 1D hydrostatic models (theoretical and semi-empirical)
are exposed. The differences between the PHOENIX LTE and NLTE models
are small. We conclude that the 3D hydrodynamical model is superior
to any of the tested 1D models, which gives further confidence in the
solar abundance analyses based on it.
Title: A Grid of Three-dimensional Stellar Atmosphere Models of Solar
Metallicity. I. General Properties, Granulation, and Atmospheric
Expansion
Authors: Trampedach, Regner; Asplund, Martin; Collet, Remo; Nordlund,
Åke; Stein, Robert F.
Bibcode: 2013ApJ...769...18T
Altcode: 2013arXiv1303.1780T
Present grids of stellar atmosphere models are the workhorses in
interpreting stellar observations and determining their fundamental
parameters. These models rely on greatly simplified models of
convection, however, lending less predictive power to such models of
late-type stars. We present a grid of improved and more reliable stellar
atmosphere models of late-type stars, based on deep, three-dimensional
(3D), convective, stellar atmosphere simulations. This grid is to be
used in general for interpreting observations and improving stellar
and asteroseismic modeling. We solve the Navier Stokes equations in
3D and concurrent with the radiative transfer equation, for a range
of atmospheric parameters, covering most of stellar evolution with
convection at the surface. We emphasize the use of the best available
atomic physics for quantitative predictions and comparisons with
observations. We present granulation size, convective expansion of the
acoustic cavity, and asymptotic adiabat as functions of atmospheric
parameters.
Title: Accessing the Full Potential of Asteroseismology with Kepler
using Realistic 3D Simulations of Stellar Convection
Authors: Trampedach, Regner
Bibcode: 2013atp..prop..188T
Altcode:
Asteroseismic analysis of stars is frequently hampered by the
unknown surface effect: a systematic frequency shift of acoustic
modes between observed and theoretical frequencies from 1D stellar
structure models. If ignored, the surface effect will lead to the
wrong set of physical parameters for the target star. This bias will
also depend on the available modeset, as the surface effect increases
with frequency. Well constrained low- frequency modes are needed for
an unaffected reference, but are often not available. For F stars the
surface effect seems to be of similar magnitude as the large frequency
separation (the difference between modes of consecutive radial order and
same spherical degree), severely limiting the possibility of unambiguous
mode identification. Two rather different F-star models can be found
to match the frequencies similarly well, and conflicting schemes have
been proposed for discriminating between them. This situation worsens
for evolved stars displaying avoided crossings between p-modes,
and g-modes from the interior radiative zone. In such cases mode
identification is even more challenging ― correct identification is
however, also rewarded with frequencies that are highly sensitive to
the structure of the core (and hence the age), which can therefore be
exceedingly well constrained. The central objective of the proposed
project is to evaluate the surface effect and implement appropriate
corrections in an asteroseismic data-reduction pipeline. The surface
effect is composed of a number of inherently 3D convective effects,
which will be evaluated from a grid of realistic 3D hydrodynamical
simulations of deep convective atmospheres. New formalisms need to
be developed for this, and the results will be made available to
the community. The theoretical findings will be validated against,
in particular, Kepler observations. This project is of fundamental
importance to all asteroseismic research and will advance the science
goals of NASA’s astrophysics program, and in particular that of the
Kepler mission and the recently approved TESS mission. This analysis
will remove the largest systematic effect in asteroseismology and
make it possible to realize the full potential for mode-frequencies
to strongly constrain stellar models and the properties of stars.
Title: On the Amplitude of Convective Velocities in the Deep Solar
Interior
Authors: Miesch, Mark S.; Featherstone, Nicholas A.; Rempel, Matthias;
Trampedach, Regner
Bibcode: 2012ApJ...757..128M
Altcode: 2012arXiv1205.1530M
We obtain lower limits on the amplitude of convective velocities
in the deep solar convection zone (CZ) based only on the observed
properties of the differential rotation and meridional circulation
together with simple and robust dynamical balances obtained from
the fundamental magnetohydrodynamics equations. The linchpin of the
approach is the concept of gyroscopic pumping whereby the meridional
circulation across isosurfaces of specific angular momentum is linked
to the angular momentum transport by the convective Reynolds stress. We
find that the amplitude of the convective velocity must be at least
30 m s-1 in the upper CZ (r ~ 0.95R) and at least 8 m
s-1 in the lower CZ (r ~ 0.75R) in order to be consistent
with the observed mean flows. Using the base of the near-surface shear
layer as a probe of the rotational influence, we are further able to
show that the characteristic length scale of deep convective motions
must be no smaller than 5.5-30 Mm. These results are compatible with
convection models but suggest that the efficiency of the turbulent
transport assumed in advection-dominated flux-transport dynamo models
is generally not consistent with the mean flows they employ.
Title: Investigating the Properties of Granulation in the Red Giants
Observed by Kepler
Authors: Mathur, S.; Hekker, S.; Trampedach, R.; Ballot, J.; Kallinger,
T.; Buzasi, D.; García, R. A.; Huber, D.; Jiménez, A.; Mosser, B.;
Bedding, T. R.; Elsworth, Y.; Régulo, C.; Stello, D.; Chaplin, W. J.;
De Ridder, J.; Hale, S. J.; Kinemuchi, K.; Kjeldsen, H.; Mullally,
F.; Thompson, S. E.
Bibcode: 2012ASPC..462..375M
Altcode: 2011arXiv1110.0117M
More than 1000 red giants have been observed by NASA/Kepler mission
during a nearly continuous period of ∼ 13 months. The resulting
high-frequency resolution (< 0.03 μHz) allows us to study the
granulation parameters of these stars. The granulation pattern results
from the convection motions leading to upward flows of hot plasma
and downward flows of cooler plasma. We fitted Harvey-like functions
to the power spectra, to retrieve the timescale and amplitude of
granulation. We show that there is an anti-correlation between both of
these parameters and the position of maximum power of acoustic modes,
while we also find a correlation with the radius, which agrees with the
theory. We finally compare our results with 3D models of the convection.
Title: Calibrating Convective Properties of Solar-like Stars in the
Kepler Field of View
Authors: Bonaca, Ana; Tanner, Joel D.; Basu, Sarbani; Chaplin,
William J.; Metcalfe, Travis S.; Monteiro, Mário J. P. F. G.; Ballot,
Jérôme; Bedding, Timothy R.; Bonanno, Alfio; Broomhall, Anne-Marie;
Bruntt, Hans; Campante, Tiago L.; Christensen-Dalsgaard, Jørgen;
Corsaro, Enrico; Elsworth, Yvonne; García, Rafael A.; Hekker, Saskia;
Karoff, Christoffer; Kjeldsen, Hans; Mathur, Savita; Régulo, Clara;
Roxburgh, Ian; Stello, Dennis; Trampedach, Regner; Barclay, Thomas;
Burke, Christopher J.; Caldwell, Douglas A.
Bibcode: 2012ApJ...755L..12B
Altcode: 2012arXiv1207.2765B
Stellar models generally use simple parameterizations to treat
convection. The most widely used parameterization is the so-called
mixing-length theory where the convective eddy sizes are described
using a single number, α, the mixing-length parameter. This is a free
parameter, and the general practice is to calibrate α using the known
properties of the Sun and apply that to all stars. Using data from
NASA's Kepler mission we show that using the solar-calibrated α is not
always appropriate, and that in many cases it would lead to estimates
of initial helium abundances that are lower than the primordial
helium abundance. Kepler data allow us to calibrate α for many other
stars and we show that for the sample of stars we have studied, the
mixing-length parameter is generally lower than the solar value. We
studied the correlation between α and stellar properties, and we find
that α increases with metallicity. We therefore conclude that results
obtained by fitting stellar models or by using population-synthesis
models constructed with solar values of α are likely to have large
systematic errors. Our results also confirm theoretical expectations
that the mixing-length parameter should vary with stellar properties.
Title: A Uniform Asteroseismic Analysis of 22 Solar-type Stars
Observed by Kepler
Authors: Mathur, S.; Metcalfe, T. S.; Woitaszek, M.; Bruntt, H.;
Verner, G. A.; Christensen-Dalsgaard, J.; Creevey, O. L.; Doǧan, G.;
Basu, S.; Karoff, C.; Stello, D.; Appourchaux, T.; Campante, T. L.;
Chaplin, W. J.; García, R. A.; Bedding, T. R.; Benomar, O.; Bonanno,
A.; Deheuvels, S.; Elsworth, Y.; Gaulme, P.; Guzik, J. A.; Handberg,
R.; Hekker, S.; Herzberg, W.; Monteiro, M. J. P. F. G.; Piau, L.;
Quirion, P. -O.; Régulo, C.; Roth, M.; Salabert, D.; Serenelli, A.;
Thompson, M. J.; Trampedach, R.; White, T. R.; Ballot, J.; Brandão,
I. M.; Molenda-Żakowicz, J.; Kjeldsen, H.; Twicken, J. D.; Uddin,
K.; Wohler, B.
Bibcode: 2012ApJ...749..152M
Altcode: 2012arXiv1202.2844M
Asteroseismology with the Kepler space telescope is providing not
only an improved characterization of exoplanets and their host stars,
but also a new window on stellar structure and evolution for the
large sample of solar-type stars in the field. We perform a uniform
analysis of 22 of the brightest asteroseismic targets with the highest
signal-to-noise ratio observed for 1 month each during the first year
of the mission, and we quantify the precision and relative accuracy
of asteroseismic determinations of the stellar radius, mass, and age
that are possible using various methods. We present the properties
of each star in the sample derived from an automated analysis of the
individual oscillation frequencies and other observational constraints
using the Asteroseismic Modeling Portal (AMP), and we compare them to
the results of model-grid-based methods that fit the global oscillation
properties. We find that fitting the individual frequencies typically
yields asteroseismic radii and masses to ~1% precision, and ages to
~2.5% precision (respectively, 2, 5, and 8 times better than fitting
the global oscillation properties). The absolute level of agreement
between the results from different approaches is also encouraging,
with model-grid-based methods yielding slightly smaller estimates of
the radius and mass and slightly older values for the stellar age
relative to AMP, which computes a large number of dedicated models
for each star. The sample of targets for which this type of analysis
is possible will grow as longer data sets are obtained during the
remainder of the mission.
Title: Granulation in Red Giants: Observations by the Kepler Mission
and Three-dimensional Convection Simulations
Authors: Mathur, S.; Hekker, S.; Trampedach, R.; Ballot, J.; Kallinger,
T.; Buzasi, D.; García, R. A.; Huber, D.; Jiménez, A.; Mosser, B.;
Bedding, T. R.; Elsworth, Y.; Régulo, C.; Stello, D.; Chaplin, W. J.;
De Ridder, J.; Hale, S. J.; Kinemuchi, K.; Kjeldsen, H.; Mullally,
F.; Thompson, S. E.
Bibcode: 2011ApJ...741..119M
Altcode: 2011arXiv1109.1194M
The granulation pattern that we observe on the surface of the Sun is
due to hot plasma rising to the photosphere where it cools down and
descends back into the interior at the edges of granules. This is the
visible manifestation of convection taking place in the outer part of
the solar convection zone. Because red giants have deeper convection
zones than the Sun, we cannot a priori assume that their granulation is
a scaled version of solar granulation. Until now, neither observations
nor one-dimensional analytical convection models could put constraints
on granulation in red giants. With asteroseismology, this study can
now be performed. We analyze ~1000 red giants that have been observed
by Kepler during 13 months. We fit the power spectra with Harvey-like
profiles to retrieve the characteristics of the granulation (timescale
τgran and power P gran). We search for a
correlation between these parameters and the global acoustic-mode
parameter (the position of maximum power, νmax) as
well as with stellar parameters (mass, radius, surface gravity
(log g), and effective temperature (T eff)). We show
that τeffvpropν-0.89 max and
P granvpropν-1.90 max, which
is consistent with the theoretical predictions. We find that
the granulation timescales of stars that belong to the red clump
have similar values while the timescales of stars in the red giant
branch are spread in a wider range. Finally, we show that realistic
three-dimensional simulations of the surface convection in stars,
spanning the (T eff, log g) range of our sample of red
giants, match the Kepler observations well in terms of trends.
Title: Near-surface Convection in Solar-like Stars
Authors: Trampedach, Regner
Bibcode: 2011iasa.confE...7T
Altcode:
No abstract at ADS
Title: Using simulations of solar surface convection as boundary
conditions on global simulations
Authors: Trampedach, Regner; Augustson, Kyle
Bibcode: 2011IAUS..271..403T
Altcode:
Direct numerical simulations of convective stellar envelopes, are
divided between two different physical regimes, that are rather
difficult to reconcile - at least with the computational power of
present-day computers. This paper outlines an attempt at bridging the
gap between surface and interior simulations of convection.
Title: The Mass Mixing Length in Convective Stellar Envelopes
Authors: Trampedach, Regner; Stein, Robert F.
Bibcode: 2011ApJ...731...78T
Altcode: 2011arXiv1102.1102T
The scale length over which convection mixes mass in a star can
be calculated as the inverse of the vertical derivative of the
unidirectional (up or down) mass flux. This is related to the mixing
length in the mixing length theory of stellar convection. We give
the ratio of mass mixing length to pressure scale height for a grid
of three-dimensional surface convection simulations, covering from
4300 K to 6900 K on the main sequence, and up to giants at log g =
2.2, all for solar composition. These simulations also confirm what is
already known from solar simulations that convection does not proceed by
discrete convective elements, but rather as a continuous, slow, smooth,
warm upflow and turbulent, entropy deficient, fast down drafts. This
convective topology also results in mixing on a scale comparable to
the classic mixing length formulation, and is simply a consequence of
mass conservation on flows in a stratified atmosphere.
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.
Bibcode: 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.
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.
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
CO5BOLD collaboration.
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 CO5BOLD
code can be traced to the different treatments of scattering.
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: Modeling the Near-Surface Shear Layer: Diffusion Schemes
Studied With CSS
Authors: Augustson, Kyle; Rast, Mark; Trampedach, Regner; Toomre, Juri
Bibcode: 2011JPhCS.271a2070A
Altcode: 2010arXiv1012.4781A
As we approach solar convection simulations that seek to model the
interaction of small-scale granulation and supergranulation and even
larger scales of convection within the near-surface shear layer (NSSL),
the treatment of the boundary conditions and minimization of sub-grid
scale diffusive processes become increasingly crucial. We here assess
changes in the dynamics and the energy flux balance of the flows
established in rotating spherical shell segments that capture much
of the NSSL with the Curved Spherical Segment (CSS) code using two
different diffusion schemes. The CSS code is a new massively parallel
modeling tool capable of simulating 3-D compressible MHD convection with
a realistic solar stratification in rotating spherical shell segments.
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.
Bibcode: 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: Convection in stellar models
Authors: Trampedach, R.
Bibcode: 2010Ap&SS.328..213T
Altcode: 2010Ap&SS.tmp...85T
Convection has two main effects of interest to stellar modelers:
It changes the stratification compared to a radiative or conductive
region, and it mixes the matter efficiently. This was recognized
early on, and a rudimentary description of convection was soon worked
out—this is the familiar mixing length formulation. This paper will
compare and contrast this much used formulation with results from
realistic 3D simulations of interacting convection and radiation in
the surface layers of stars. Due to space limitations, this review
will be concerned with convective envelopes, only.
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.
Bibcode: 2010A&A...517A..49H
Altcode: 2010arXiv1007.2760H
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.
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.
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 log10 τ5000 ⪉ -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: 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.
Bibcode: 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: 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.
Bibcode: 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: Comparing 3D Solar Model Atmospheres with Observations:
Hydrogen Lines and Centre-to-limb Variations
Authors: Pereira, Tiago M. D.; Asplund, Martin; Trampedach, Regner
Bibcode: 2008psa..conf..313P
Altcode:
Three dimensional hydrodynamical stellar model atmospheres
represent a major step forward in stellar spectroscopy. Making use
of radiative-hydrodynamical convection simulations that contain no
adjustable free parameters, the model atmospheres provide a robust
and realistic treatment of convection. These models have been applied
to several lines in the Sun and other stars, yielding an excellent
agreement with observations (e.g., Asplund et al. (2000) [1]).
Title: Beyond 1D: spectral line formation with 3D hydrodynamical
model atmospheres of red giants
Authors: Collet, R.; Asplund, M.; Trampedach, R.
Bibcode: 2008MmSAI..79..649C
Altcode: 2007arXiv0711.3186C
We present the results of realistic, 3D, hydrodynamical, simulations
of surface convection in red giant stars with varying effective
temperatures and metallicities. We use the convection simulations as
time-dependent, hydrodynamical, model atmospheres to compute spectral
line profiles for a number of ions and molecules under the assumption
of local thermodynamic equilibrium (LTE). We compare the results with
the predictions of line formation calculations based on 1D, hydrostatic,
model stellar atmospheres in order to estimate the impact of 3D models
on the derivation of elemental abundances. We find large negative
3D-1D LTE abundance corrections (typically -0.5 to -1 dex) for weak
low-excitation lines from molecules and neutral species in the very
low metallicity cases. Finally, we discuss the extent of departures
from LTE in the case of neutral iron spectral line formation.
Title: A New Stellar Atmosphere Grid-in 3D
Authors: Trampedach, Regner
Bibcode: 2007AIPC..948..141T
Altcode:
The main limitation of atmosphere models of late-type stars is the,
at best, sketchy treatment of convection. Because the top of the outer
convection zone of such stars is located in the photosphere, convection
has a large effect on both the atmospheric structure and the emergent
spectrum. I present the first results from a grid of 3D, non-grey,
radiation-coupled, convection simulations for a range of atmospheric
parameters. These results include calibration of the mixing-length,
T-τ relations and granulation spectra.
Title: Three-dimensional hydrodynamical simulations of surface
convection in red giant stars. Impact on spectral line formation
and abundance analysis
Authors: Collet, R.; Asplund, M.; Trampedach, R.
Bibcode: 2007A&A...469..687C
Altcode: 2007astro.ph..3652C
Aims:We investigate the impact of realistic three-dimensional (3D)
hydrodynamical model atmospheres of red giant stars at different
metallicities on the formation of spectral lines of a number of ions
and molecules.
Methods: We carry out realistic, ab initio, 3D,
hydrodynamical simulations of surface convection at the surface of red
giant stars with varying effective temperatures and metallicities. We
use the convection simulations as time-dependent hydrodynamical model
stellar atmospheres to calculate spectral lines of a number of ions (Li
I, O I, Na I, Mg I, Ca I, Fe I, and Fe II) and molecules (CH, NH, and
OH) under the assumption of local thermodynamic equilibrium (LTE). We
carry out a differential comparison of the line strengths computed
in 3D with the results of analogous line formation calculations for
classical, 1D, hydrostatic, plane-parallel marcs model atmospheres in
order to estimate the impact of 3D models on the derivation of elemental
abundances.
Results: The temperature and density inhomogeneities
and correlated velocity fields in 3D models, as well as the differences
between the mean 3D stratifications and corresponding 1D model
atmospheres significantly affect the predicted strengths of spectral
lines. Under the assumption of LTE, the low atmospheric temperatures
encountered in 3D model atmospheres of very metal-poor giant stars
cause spectral lines from neutral species and molecules to appear
stronger than within the framework of 1D models. As a consequence,
elemental abundances derived from these lines using 3D models are
significantly lower than according to 1D analyses. In particular,
the differences between 3D and 1D abundances of C, N, and O derived
from CH, NH, and OH weak low-excitation lines are found to be in the
range -0.5 dex to -1.0 dex for the the red giant stars at [Fe/H]=-3
considered here. At this metallicity, large negative corrections
(about -0.8 dex) are also found, in LTE, for weak low-excitation Fe
I lines. We caution, however, that the neglected departures from LTE
might be significant for these and other elements and comparable to
the effects due to stellar granulation.
Title: Excitation of solar-like oscillations across the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
Bibcode: 2007A&A...463..297S
Altcode: 2006astro.ph.11762S
Aims:We extend semi-analytical computations of excitation rates for
solar oscillation modes to those of other solar-like oscillating stars
to compare them with recent observations
Methods: Numerical
3D simulations of surface convective zones of several solar-type
oscillating stars are used to characterize the turbulent spectra
as well as to constrain the convective velocities and turbulent
entropy fluctuations in the uppermost part of the convective zone of
such stars. These constraints, coupled with a theoretical model for
stochastic excitation, provide the rate P at which energy is injected
into the p-modes by turbulent convection. These energy rates are
compared with those derived directly from the 3D simulations.
Results: The excitation rates obtained from the 3D simulations
are systematically lower than those computed from the semi-analytical
excitation model. We find that Pmax, the P maximum, scales as
(L/M)s where s is the slope of the power law and L and M are
the mass and luminosity of the 1D stellar model built consistently
with the associated 3D simulation. The slope is found to depend
significantly on the adopted form of χ_k, the eddy time-correlation;
using a Lorentzian, χ_k^L, results in s=2.6, whereas a Gaussian,
χ_k^G, gives s=3.1. Finally, values of V_max, the maximum in the mode
velocity, are estimated from the computed power laws for P_max and we
find that Vmax increases as (L/M)sv. Comparisons
with the currently available ground-based observations show that the
computations assuming a Lorentzian χk yield a slope, sv,
closer to the observed one than the slope obtained when assuming a
Gaussian. We show that the spatial resolution of the 3D simulations
must be high enough to obtain accurate computed energy rates.
Title: And We Thought We Knew What the Sun Was Made Of
Authors: Trampedach, R.
Bibcode: 2006ASPC..354..103T
Altcode:
Recent abundance analysis of the Sun, using 3D convection simulations
instead of the usual 1D solar atmosphere models, have resulted
in markedly lower metal abundances. This seems to be in strong
conflict with helioseismology, and I will explore some of the ways
to reconciliation.
Title: A Synoptic Comparison of the Mihalas-Hummer-Däppen and OPAL
Equations of State
Authors: Trampedach, R.; Däppen, W.; Baturin, V. A.
Bibcode: 2006ApJ...646..560T
Altcode: 2006astro.ph..4352T
A detailed comparison is carried out between two popular equations of
state (EOSs), the Mihalas-Hummer-Däppen (MHD) and OPAL equations of
state, which have found widespread use in solar and stellar modeling
during the past two decades. They are parts of two independent efforts
to recalculate stellar opacities: the international Opacity Project (OP)
and the Livermore-based OPAL project. We examine the difference between
the two EOSs in a broad sense, over the whole applicable ϱ-T range,
and for three different chemical mixtures. Such a global comparison
highlights both their differences and their similarities. We find that
omitting a questionable hard-sphere correction, τ, to the Coulomb
interaction in the MHD formulation, greatly improves the agreement
between the MHD and OPAL EOSs. We also find signs of differences
that could stem from quantum effects not yet included in the MHD EOS,
and differences in the ionization zones that are probably caused by
differences in the mechanisms for pressure ionization. Our analysis
not only gives a clearer perception of the limitations of each EOS for
astrophysical applications, but also serves as guidance for future work
on the physical issues behind the differences. The outcome should be
an improvement of both EOSs.
Title: The Chemical Compositions of the Extreme Halo Stars
HE 0107-5240 and HE 1327-2326 Inferred from Three-dimensional
Hydrodynamical Model Atmospheres
Authors: Collet, R.; Asplund, M.; Trampedach, R.
Bibcode: 2006ApJ...644L.121C
Altcode: 2006astro.ph..5219C
We investigate the impact of realistic three-dimensional (3D)
hydrodynamical model stellar atmospheres on the determination of
elemental abundances in the carbon-rich, hyper-iron-poor stars HE
0107-5240 and HE 1327-2326. We derive the chemical compositions of the
two stars by means of a detailed 3D analysis of spectral lines under
the assumption of local thermodynamic equilibrium (LTE). The lower
temperatures of the line-forming regions of the hydrodynamical models
cause changes in the predicted spectral line strengths. In particular,
we find the 3D abundances of C, N, and O to be lower by about -0.8 dex
(or more) than estimated from a 1D analysis. The 3D abundances of iron
peak elements are also decreased but by smaller factors (about -0.2
dex). We caution, however, that the neglected non-LTE effects might
actually be substantial for these metals. We finally discuss possible
implications for studies of early Galactic chemical evolution.
Title: 3D Hydrodynamical Simulations of Convection in Red-Giants
Stellar Atmospheres
Authors: Collet, R.; Asplund, M.; Trampedach, R.
Bibcode: 2006cams.book..306C
Altcode:
We present preliminary results of 3D hydrodynamical simulations
of surface convection in red giants stars. We investigate the main
differences between static 1D and 3D time-dependent model stellar
atmospheres of red giants for a range of metallicities between solar
and [Fe/H] = -3 focusing in particular on the impact of 3D spectral
line formation on the derivation of stellar abundances.
Title: Excitation of Solar-like Oscillations: From PMS to MS Stellar
Models
Authors: Samadi, R.; Goupil, M. -J.; Alecian, E.; Baudin, F.;
Georgobiani, D.; Trampedach, R.; Stein, R.; Nordlund, Å.
Bibcode: 2005JApA...26..171S
Altcode:
The amplitude of solar-like oscillations results from a balance between
excitation and damping. As in the sun, the excitation is attributed
to turbulent motions that stochastically excite the p modes in the
upper-most part of the convective zone. We present here a model for the
excitation mechanism. Comparisons between modeled amplitudes and helio
and stellar seismic constraints are presented and the discrepancies
discussed. Finally the possibility and the interest of detecting
such stochastically excited modes in pre-main sequence stars are
also discussed.
Title: Excitation of P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
Bibcode: 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: 3D hydrodynamical model stellar atmospheres of metal-poor
red giants
Authors: Collet, Remo; Asplund, Martin; Trampedach, Regner
Bibcode: 2005IAUS..228..247C
Altcode:
We investigate the main differences between static 1D and 3D
time-dependent model stellar atmospheres of red giants at very low
metallicities. We focus in particular on the impact of 3D LTE spectral
line formation on the derivation of elemental abundances for the
extremely metal-poor ([Fe/H] ≈-5.3) red giant HE 0107-5240.
Title: Excitation rates of p modes: mass luminosity relation across
the HR diagram
Authors: Samadi, R.; Georgobiani, D.; Trampedach, R.; Goupil, M. J.;
Stein, R. F.; Nordlund, Å.
Bibcode: 2004sf2a.conf..323S
Altcode: 2004astro.ph.10043S
We compute the rates P at which energy is injected into the p modes for
a set of 3D simulations of outer layers of stars. We found that Pmax
- the maximum in P - scales as (L/M)^s where s is the slope of the
power law, L and M are the luminosity and the mass of the 1D stellar
models associated with the simulations. The slope is found to depend
significantly on the adopted representation for the turbulent eddy-time
correlation function, chi_k. According to the expected performances
of COROT, it will likely be possible to measure Pmax as a function
of L/M and to constrain the properties of stellar turbulence as the
turbulent eddy time-correlation.
Title: 3D-simulation of the outer convection-zone of an A-star
Authors: Trampedach, Regner
Bibcode: 2004IAUS..224..155T
Altcode: 2004astro.ph.11254T
The convection code of Nordlund & Stein has been used to evaluate
the 3D, radiation-coupled convection in a stellar atmosphere with
Teff = 7300K, log g = 4.3 and [Fe/H]= 0.0, corresponding
to a main-sequence A9 star. I present preliminary comparisons between
the 3D-simulation and a conventional 1D stellar structure calculation,
and elaborate on the consequences of the differences.
Title: Oscillation Power Spectra of the Sun and of CEN a: Observations
Versus Models
Authors: Samadi, R.; Goupil, M. J.; Baudin, F.; Georgobiani, D.;
Trampedach, R.; Stein, R.; Nordlund, A.
Bibcode: 2004ESASP.559..615S
Altcode: 2004astro.ph..9325S; 2004soho...14..615S
Hydrodynamical, 3D simulations of the outer layers of the Sun and Alpha
Cen A are used to obtain constraints on the properties of turbulent
convection in such stars. These constraints enable us to compute -
on the base of a theoretical model of stochastic excitation - the
rate P at which p modes are excited by turbulent convection in those
two stars. Results are then compared with solar seismic observations
and recent observations of Alpha Cen A. For the Sun, a good agreement
between observations and computed P is obtained. For Alpha Cen A a
large discrepancy is obtained which origin cannot be yet identified:
it can either be caused by the present data quality which is not
sufficient for our purpose or by the way the intrinsic amplitudes and
the life-times of the modes are determined or finally attributed to
our present modelling. Nevertheless, data with higher quality or/and
more adapted data reductions will likely provide constraints on the
p-mode excitation mechanism in Alpha Cen A.
Title: Improved phenomenological equation of state in the chemical
picture
Authors: Trampedach, Regner
Bibcode: 2004AIPC..731...99T
Altcode: 2004astro.ph.11315T
I present an overview of an equation of state, being developed in
the chemical picture, and based on the very successful MHD equation
of state. The flexibility of the chemical picture combined with the
free-energy minimization procedure, makes it rather straight-forward,
albeit laborious, to include new effects in the model free-energy,
simply by adding new terms. The most notable additions to the
original MHD equation of state, are relativistic effects, quantum
effects, improved higher order Coulomb terms and a long list of
molecules other than the H 2 and H2+ treated
so far.
Title: Ingredients for accurate simulations of convection in stellar
envelopes
Authors: Trampedach, Regner
Bibcode: 2004PhDT.........6T
Altcode:
I present the ingredients for high precision, 3D hydrodynamical
simulations of convection in stellar atmospheres, as well as a number
of applications. I have developed a new scheme for evaluating radiative
transfer, an improved equation of state and I have investigated a number
of directions for improving the numerical stability of the convection
simulations. The equation of state (EOS) used for the simulations,
is updated by including post-Holtsmark micro-field distributions and
relativistic electron-degeneracy as previously published. I have further
included quantum effects, higher-order Coulomb interactions and improved
treatment of extended particles. These processes (except relativistic
degeneracy) have a significant effect in the solar convection zone,
and most of them peak at a depth of only 10 Mm. I also include a range
of astrophysically significant molecules, besides H2 and
the H+2 -ion. This FOS will be used directly
in the convection simulations, providing the thermodynamic state of
the plasma, and as a foundation for a new calculation of opacities
for stellar atmospheres and interiors. A new scheme for evaluating
radiative transfer in dynamic and multi-dimensional stellar atmosphere
calculations is developed. The idea being, that if carefully chosen,
very few wavelengths can reproduce the full radiative transfer
solution. This method is based on a calibration against a full solution
to a 1D reference atmosphere, and is therefore not relevant for static
1D stellar atmosphere modeling. The first tests of the method are very
promising, and reveal that the new method is an improvement over the
former opacity binning technique. The range of convective fluctuations
is spanned more accurately and not only the radiative heating,
but also the first three angular moments of the specific intensity,
can be evaluated reliably. Work on implementing the method in the
convection-code, is in progress. These developments will be employed
in the future for a number of detailed simulations of primary targets
for the upcoming, space-based, astero-seismology missions, and will
include a Cen A and B, η Boo. Procyon and β Hyi. Work on a 10 Mm deep
solar simulation was severely hampered by numerical instabilities, but
investigating the issue has revealed a number of potential solutions
that will be tested in the near future. The work on individual stars
will soon be superseded by an effort to compute a grid of convection
simulations in Teff, log g and metallicity, [Fe/H], in the
spirit of present-day, grids of conventional atmosphere models.
Title: Excitation of Radial P-Modes in the Sun and Stars
Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig,
Hans-Günter; Nordlund, Åke
Bibcode: 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: Understanding the convective Sun
Authors: Trampedach, Regner; Georgobiani, Dali; Stein, Robert F.;
Nordlund, Åke
Bibcode: 2003ESASP.517..195T
Altcode: 2003soho...12..195T
Hydrodynamical simulations of the surface layers of the Sun, has greatly
improved our understanding and interpretation of solar observations. I
review some past successes in matching spectral lines, improving the
agreement with high-degree p-mode frequencies and matching the depth of
the solar convection zone without adjustable convection-parameters. Our
solar simulations contain p-modes, and are used for studying the
asymmetry of p-mode peaks and to calibrate the conversion between the
observed velocity proxies and the actual velocities.
Title: Radiative Transfer with Very Few Wavelengths
Authors: Trampedach, R.; Asplund, M.
Bibcode: 2003ASPC..293..209T
Altcode: 2003tdse.conf..209T
Our aim is to develop an opacity sampling scheme suitable for 3D
hydrodynamical simulations of convective stellar atmospheres. This
paper presents a feasibility test for the concept.
Title: Solar and Stellar Oscillations
Authors: Stein, Robert; Nordlund, Aake; Georgobiani, Dali; Trampedach,
Regner; Ludwig, Hans-Guenther
Bibcode: 2003IAUJD..12E..41S
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: 3-D simulations of stellar parameters
Authors: Trampedach, R.
Bibcode: 2001JAD.....7R...8T
Altcode:
I propose to use 3-dimensional simulations of convection in stellar
atmospheres, as a basis for analysis and determination of the
fundamental parameters for the MONS primary target stars.
Title: 3-D simulations of stellar parameters
Authors: Trampedach, R.
Bibcode: 2001fcm..book...59T
Altcode:
I propose to use 3-dimensional simulations of convection in stellar
atmospheres, as a basis for analysis and determination of the
fundamental parameters for the MONS primary target stars.
Title: Line formation in solar granulation. II. The photospheric
Fe abundance
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.; Stein, R. F.
Bibcode: 2000A&A...359..743A
Altcode: 2000astro.ph..5321A
The solar photospheric Fe abundance has been determined using realistic
ab initio 3D, time-dependent, hydrodynamical model atmospheres. The
study is based on the excellent agreement between the predicted
and observed line profiles directly rather than equivalent widths,
since the intrinsic Doppler broadening from the convective motions and
oscillations provide the necessary non-thermal broadening. Thus, three
of the four hotly debated parameters (equivalent widths, microturbulence
and damping enhancement factors) in the center of the recent solar Fe
abundance dispute regarding Fe i lines no longer enter the analysis,
leaving the transition probabilities as the main uncertainty. Both Fe i
(using the samples of lines of both the Oxford and Kiel studies) and
Fe ii lines have been investigated, which give consistent results:
log epsilon_FeI = 7.44 +/- 0.05 and log epsilon_FeII = 7.45 +/-
0.10. Also the wings of strong Fe i lines return consistent abundances,
log epsilon_FeII = 7.42 +/- 0.03, but due to the uncertainties inherent
in analyses of strong lines we give this determination lower weight
than the results from weak and intermediate strong lines. In view of
the recent slight downward revision of the meteoritic Fe abundance
log epsilon_Fe = 7.46 +/- 0.01, the agreement between the meteoritic
and photospheric values is very good, thus appearingly settling the
debate over the photospheric Fe abundance from Fe i lines.
Title: Line formation in solar granulation. I. Fe line shapes,
shifts and asymmetries
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.; Allende Prieto,
C.; Stein, R. F.
Bibcode: 2000A&A...359..729A
Altcode: 2000astro.ph..5320A
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations
of the solar granulation have been applied to Fe i and Fe ii line
formation. In contrast to classical analyses based on hydrostatic 1D
model atmospheres the procedure contains no adjustable free parameters
but the treatment of the numerical viscosity in the construction
of the 3D, time-dependent, inhomogeneous model atmosphere and the
elemental abundance in the 3D spectral synthesis. However, the numerical
viscosity is introduced purely for numerical stability purposes and is
determined from standard hydrodynamical test cases with no adjustments
allowed to improve the agreement with the observational constraints
from the solar granulation. The non-thermal line broadening is mainly
provided by the Doppler shifts arising from the convective flows in
the solar photosphere and the solar oscillations. The almost perfect
agreement between the predicted temporally and spatially averaged
line profiles for weak Fe lines with the observed profiles and the
absence of trends in derived abundances with line strengths, seem to
imply that the micro- and macroturbulence concepts are obsolete in
these 3D analyses. Furthermore, the theoretical line asymmetries and
shifts show a very satisfactory agreement with observations with an
accuracy of typically 50-100 m s-1 on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.
Title: Convective contributions to the frequencies of solar
oscillations
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Nordlund, Å.;
Stein, R. F.; Trampedach, R.
Bibcode: 1999A&A...351..689R
Altcode: 1998astro.ph..3206R
Differences between observed and theoretical eigenfrequencies of the Sun
have characteristics which identify them as arising predominantly from
properties of the oscillations in the vicinity of the solar surface:
in the super-adiabatic, convective boundary layer and above. These
frequency differences may therefore provide useful information about
the structure of these regions, precisely where the theory of solar
structure is most uncertain. In the present work we use numerical
simulations of the outer part of the Sun to quantify the influence
of turbulent convection on solar oscillation frequencies. Separating
the influence into effects on the mean model and effects on the
physics of the modes, we find that the main model effects are due
to the turbulent pressure that provides additional support against
gravity, and thermal differences between average 3-D models and 1-D
models. Surfaces of constant pressure in the visible photosphere are
elevated by about 150 km, relative to a standard envelope model. As a
result, the turning points of high-frequency modes are raised, while
those of the low-frequency modes remain essentially unaffected. The
corresponding gradual lowering of the mode frequencies accounts for
most of the frequency difference between observations and standard
solar models. Additional effects are expected to come primarily from
changes in the physics of the modes, in particular from the modulation
of the turbulent pressure by the oscillations.
Title: 3D hydrodynamical model atmospheres of metal-poor
stars. Evidence for a low primordial Li abundance
Authors: Asplund, Martin; Nordlund, Åke; Trampedach, Regner; Stein,
Robert F.
Bibcode: 1999A&A...346L..17A
Altcode: 1999astro.ph..5059A
Realistic 3-dimensional (3D), radiative hydrodynamical surface
convection simulations of the metal-poor halo stars HD 140283 and
HD 84937 have been performed. Due to the dominance of adiabatic
cooling over radiative heating very low atmospheric temperatures are
encountered. The lack of spectral lines in these metal-poor stars
thus causes much steeper temperature gradients than in classical 1D
hydrostatic model atmospheres where the temperature of the optically
thin layers is determined by radiative equilibrium. The modified
atmospheric structures cause changes in the emergent stellar spectra. In
particular, the primordial Li abundances may have been overestimated
by 0.2-0.35 dex with 1D model atmospheres. However, we caution that
our result assumes local thermodynamic equilibrium (LTE), while the
steep temperature gradients may be prone to e.g. over-ionization.
Title: Confrontation of Stellar Surface Convection Simulations with
Stellar Spectroscopy
Authors: Asplund, M.; Nordlund, Å.; Trampedach, R.
Bibcode: 1999ASPC..173..221A
Altcode: 1999sstt.conf..221A
No abstract at ADS
Title: Stellar Evolution with a Variable Mixing-Length Parameter
Authors: Trampedach, R.; Stein, R. F.; Christensen-Dalsgaard, J.;
Nordlund, Å.
Bibcode: 1999ASPC..173..233T
Altcode: 1999sstt.conf..233T
No abstract at ADS
Title: Stellar background power spectra from hydrodynamical
simulations of stellar atmospheres
Authors: Trampedach, R.; Christensen-Dalsgaard, J.; Nordlund, A.;
Stein, R. F.
Bibcode: 1998mons.proc...59T
Altcode:
The non-p-mode contribution to the temporal irradiance or velocity
spectra of the Sun has for a long time been considered as noise,
but in recent years it has gradually been appreciated as the signal of
granulation. Accordingly these spectra are now referred to as background
spectra. We hope that further analysis of these background spectra
will serve two purposes: to provide information about convection in
other stars; and, as the background still constitutes a noise source
when looking for p- and in particular g-modes of solar type stars,
to provide us with stricter limits as to what is observable. Based on
hydrodynamical simulations of convection in the atmospheres of the Sun,
alpha Cen A and Procyon, we calculate irradiance and velocity spectra
and infer a few properties of these spectra. Due to the limited
horizontal extent of the simulations (covering 6-8 granules each)
we only get a signal from the granulation, whereas effects of meso-
and supergranulation are missing in our signal. At the high-frequency
end we are limited by the horizontal resolution of the simulations.
Title: Tests of Convective Frequency Effects with SOI/MDI High-Degree
Data
Authors: Rosenthal, C. S.; Christensen-Dalsgaard, J.; Kosovichev,
A. G.; Nordlund, A. A.; Reiter, J.; Rhodes, E. J., Jr.; Schou, J.;
Stein, R. F.; Trampedach, R.
Bibcode: 1998ESASP.418..521R
Altcode: 1998astro.ph..7066R; 1998soho....6..521R
Advances in hydrodynamical simulations have provided new insight into
the effects of convection on the frequencies of solar oscillations. As
more accurate observations become available, this may lead to an
improved understanding of the dynamics of convection and the interaction
between convection and pulsation (Rosenthal et al. 1999). Recent
high-resolution observations from the SOI/MDI instrument on the
SOHO spacecraft have provided the so-far most-detailed observations
of high-degree modes of solar oscillations, which are particularly
sensitive to the near-surface properties of the Sun. Here we present
preliminary results of a comparison between these observations and
frequencies computed for models based on realistic simulations of
near-surface convection. Such comparisons may be expected to help
in identifying the causes for the remaining differences between the
observed frequencies and those of solar models.
Title: Near-surface constraints on the structure of stellar convection
zones
Authors: Trampedach, R.; Christensen-Dalsgaard, J.; Nordlund, A.;
Stein, R.
Bibcode: 1997ASSL..225...73T
Altcode: 1997scor.proc...73T
By simulating the convection in the upper layers of six different stars
and matching these simulations to 1D-mixing length models using the
same input physics, we have been able to infer the behaviour of the
mixing-length parameter, $\alpha$, as the stellar parameters changes.
Title: Is stellar granulation turbulence?
Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R.
Bibcode: 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.