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Astron. Astrophys. 346, L17-L20 (1999)

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Letter to the Editor

3D hydrodynamical model atmospheres of metal-poor stars

Evidence for a low primordial Li abundance

Martin Asplund 1, Åke Nordlund 2, Regner Trampedach 3 and Robert F. Stein 3

1 NORDITA, Blegdamsvej 17, DK-2100 Copenhagen O, Denmark
2 Astronomical Observatory, NBIfAFG, Juliane Maries Vej 30, DK-2100 Copenhagen O, Denmark
3 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48823, USA

Received 19 April 1999 / Accepted 6 May 1999

Abstract

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.

Key words: convection – stars: abundances – stars: atmospheres – stars: Population II – line: formation

Send offprint requests to: martin@nordita.dk

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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