EDP Sciences
Free access
Volume 419, Number 3, June I 2004
Page(s) 1095 - 1109
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20035785

A&A 419, 1095-1109 (2004)
DOI: 10.1051/0004-6361:20035785

Oxygen and magnesium abundance in the ultra-metal-poor giants CS 22949-037 and CS 29498-043: Challenges in models of atmospheres

G. Israelian1, N. Shchukina2, R. Rebolo1, 3, G. Basri4, J. I. González Hernández1 and T. Kajino5

1  Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
2  Main Astronomical Observatory, National Academy of Sciences, 03680 Kyiv-127, Ukraine
3  Consejo Superior de Investigaciones Científcas, Spain
4  Astronomy Department, University of California, Berkeley, California 94720, USA
5  National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan

(Received 2 December 2003 / Accepted 23 February 2004)

We report the results of a non-LTE Fe, O and Mg abundance analysis of the carbon-nitrogen-rich ultra-metal-poor giants CS 29498-043 and CS 22949-037. The abundance of oxygen has been derived from measurements of the oxygen triplet at 7771-5 Å in high resolution spectra obtained with KeckI/HIRES and the forbidden line [O I] 6300 Å detected in the TNG/SARG spectra of CS 29498-043. Detailed non-LTE analysis of Fe lines has provided reliable stellar parameters which, however, do not resolve the oxygen abundance conflict as derived from the O I 7771-5 Å triplet and the [O I] 6300 Å forbidden lines. We obtained the following oxygen abundance: for CS 22949-037 $\mbox{[O/Fe]} =3.13$, 1.95; and for CS 29498-043; [O/Fe] = 3.02, 2.49, based on the O I 7771-5 Å triplet and the [O I] 6300 Å forbidden line, respectively. A similar conflict appears to exist between the forbidden resonance line Mg I 4571 Å and several subordinate lines, such as Mg I 5172 and 5183 Å. Our analysis demonstrates the failure of standard plane-parallel atmosphere models to describe the physical conditions in the line-forming regions of these ultra-metal-poor giants.

Key words: Galaxy: evolution -- nuclear reactions, nucleosynthesis, abundances -- stars: abundances -- stars: late-type -- stars: population III -- stellar atmospheres

Offprint request: G. Israelian, gil@iac.es

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