The impact of non-LTE effects and granulation inhomogeneities on the derived iron and oxygen abundances in metal-poor halo stars

Details The impact of non-LTE effects and granulation inhomogeneities on the derived iron and oxygen abundances in metal-poor halo stars The impact of non-LTE effects and granulation inhomogeneities on the derived iron and oxygen abundances in metal-poor halo stars

2004-10-20

arxiv.org/abs/astro-ph/0410475v1

Astrophys.J. 618 (2005) 939-952

Astronomy and Astrophysics

Astrophysics

The Astrophysical Journal (in press). (35 pages, 5 figures and 4 tables)

Scientific paper

10.1086/426012

This paper presents the results of a detailed theoretical investigation of the impact of non-LTE effects and of granulation inhomogeneities on the derived iron and oxygen abundances in the metal-poor halo subgiant HD140283. Our analysis is based on both the `classical' one-dimensional (1D) stellar atmosphere models and on the new generation of three-dimensional (3D) hydrodynamical models. We find that the non-LTE effects on the iron abundance derived from Fe {\sc i} lines are very important, amounting to ${\sim}0.9$ dex and to ${\sim}0.6$ dex in the 3D and 1D cases, respectively. On the other hand, we find that non-LTE and 3D effects have to be taken into account for a reliable determination of the iron abundance from weak Fe {\sc ii} lines, because the significant overexcitation of their upper levels in the granular regions tend to produce emission features. As a result such Fe {\sc ii} lines are weaker than in LTE and the abundance correction amounts to ${\sim}0.4$ dex for the 3D case. We derive also the oxygen-to-iron abundance ratio in the metal-poor star HD140283 by using the O {\sc i} triplet at 7772--5 \AA and the forbidden [O {\sc i}] line at 6300 \AA. Interestingly, when both non-LTE and 3D effects are taken into account there still remains significant discrepancies in the iron abundances derived from Fe {\sc i} and Fe {\sc ii} lines, as well as in the oxygen abundances inferred from the O {\sc i} and [O {\sc i}] lines. We conclude that the metalicity of this type of metal-poor stars could be significantly larger than previously thought, which may have far-reaching implications in stellar astrophysics.

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