2004A&A...419.1095I

We report the results of a non-LTE Fe, O and Mg abundance analysis of the carbon-nitrogen-rich ultra-metal-poor giants CS29498-043 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 [OI] 6300Å detected in the TNG/SARG spectra of CS29498-043. 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 OI 7771-5Å triplet and the [OI] 6300Å forbidden lines. We obtained the following oxygen abundance: for CS22949-037 [O/Fe] =3.13, 1.95; and for CS29498-043; [O/Fe]=3.02, 2.49, based on the OI 7771-5Å triplet and the [OI] 6300Å forbidden line, respectively. A similar conflict appears to exist between the forbidden resonance line MgI 4571Å and several subordinate lines, such as MgI 5172 and 5183Å. 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.