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 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 Angstrom in high resolution spectra obtained with KeckI/HIRES and the forbidden line [O I] 6300 Angstrom 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 Angstrom triplet and the [O I] 6300 Angstrom 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 Angstrom triplet and the [O I] 6300 Angstrom forbidden line, respectively. A similar conflict appears to exist between the forbidden resonance line Mg 14571 Angstrom and several subordinate lines, such as Mg 15172 and 5183 Angstrom. 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.