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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 ofthe carbon-nitrogen-rich ultra-metal-poor giants CS 29498-043 and CS22949-037. The abundance of oxygen has been derived from measurements ofthe oxygen triplet at 7771-5 Å in high resolution spectra obtainedwith KeckI/HIRES and the forbidden line [O I] 6300 Å detected inthe TNG/SARG spectra of CS 29498-043. Detailed non-LTE analysis of Felines has provided reliable stellar parameters which, however, do notresolve the oxygen abundance conflict as derived from the O I 7771-5Å triplet and the [O I] 6300 Å forbidden lines. We obtainedthe 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 7771-5Å triplet and the [O I] 6300 Å forbidden line, respectively. A similar conflict appears to exist between the forbidden resonanceline Mg C I 4571 Å and several subordinate lines, such as Mg I5172 and 5183 Å. Our analysis demonstrates the failure ofstandard plane-parallel atmosphere models to describe the physicalconditions in the line-forming regions of these ultra-metal-poor giants.
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