Astronomy & Astrophysics

Issue		A&A Volume 486, Number 3, August II 2008
Page(s)		951 - 970
Section		Stellar atmospheres
DOI		10.1051/0004-6361:200809724
Published online		29 May 2008

A&A 486, 951-970 (2008)
DOI: 10.1051/0004-6361:200809724

A grid of MARCS model atmospheres for late-type stars

I. Methods and general properties

B. Gustafsson¹, B. Edvardsson¹, K. Eriksson¹, U. G. Jørgensen², Å. Nordlund², and B. Plez^{3, 1}

¹ Department of Physics and Astronomy, Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden
e-mail: [Bengt.Gustafsson;bg]@astro.uu.se
² Niels Bohr Institute for Astronomy, Physics and Geophysics, Copenhagen University, Blegdamsveg 17, Copenhagen Ø, 2100, Denmark
³ GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France

Received 5 March 2008 / Accepted 30 April 2008

Abstract
Context. In analyses of stellar spectra and colours, and for the analysis of integrated light from galaxies, a homogeneous grid of model atmospheres of late-type stars and corresponding flux spectra is needed.
Aims. We construct an extensive grid of spherically-symmetric models (supplemented with plane-parallel ones for the highest surface gravities), built on up-to-date atomic and molecular data, and make it available for public use.
Methods. The most recent version of the MARCS program is used.
Results. We present a grid of about 10⁴ model atmospheres for stars with 2500 K $\le$ $T_{\rm eff}$ $\le$ 8000 K, -1 $\le$ log g = log (GM/R²) $\le$ 5 (cgs) with various masses and radii, -5 $\le$ [Me/H] $\le$ +1, with [ $\alpha$ /Fe] = 0.0 and 0.4 and different choices of C and N abundances. This includes "CN-cycled" models with C/N = 4.07 (solar), 1.5 and 0.5, C/O ranging from 0.09 to (normally) 5.0 to also represent stars of spectral types R, S and N, and with 1.0 $\le$ $\xi_{\rm t}$ $\le$ 5 km s^-1. We also list thermodynamic quantities (T, $P_{\rm g}$ , $P_{\rm e}$ , $\rho$ , partial pressures of molecules, etc.) and provide them on the World Wide Web, as well as calculated fluxes in approximately 108 000 wavelength points. Underlying assumptions in addition to 1D stratification (spherical or plane-parallel) include hydrostatic equilibrium, mixing-length convection and local thermodynamic equilibrium. We discuss a number of general properties of the models, in particular in relation to the effects of changing abundances, of blanketing, and of sphericity. We illustrate positive and negative feedbacks between sphericity and molecular blanketing. We compare the models with those of other available grids and find excellent agreement with plane-parallel models of Castelli & Kurucz (if convection is treated consistently) within the overlapping parameter range. Although there are considerable departures from the spherically-symmetric NextGen models, the agreement with more recent PHOENIX models is gratifying.
Conclusions. The models of the grid show considerable regularities, but some interesting departures from general patterns occur for the coolest models due to the molecular opacities. We have tested a number of approximate "rules of thumb" concerning effects of blanketing and sphericity and often found them to be astonishingly accurate. Some interesting new phenomena have been discovered and explored, such as the intricate coupling between blanketing and sphericity, and the strong effects of carbon enhancement on metal-poor models. We give further details of line absorption data for molecules, as well as details of models and comparisons with observations in subsequent papers.

Key words: stars: atmospheres -- Sun: abundances -- stars: fundamental parameters -- stars: general -- stars: late-type -- stars: supergiants

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