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

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

Corresponding author: G. Israelian, gil@iac.es

Received: 2 December 2003
Accepted: 23 February 2004

Abstract

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 , 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.