Oxygen and Magnesium Abundance in the Ultra-Metal-Poor Giants CS 22949-037 and CS 29498-043: Challenges in Models of Atmospheres

We report the results of a non-LTE Fe, O and Mg abundance analysis of the carbon-nitrogen-rich ultrametal-poor giants CS 29498–043 and CS22949–037. 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 [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.