The Oxygen Abundance of the Ultra-Metal-Poor Star He 0557–4840

We present a high-resolution ultraviolet (UV) spectrum of the ultra-metal-poor (UMP) carbon-enhanced red giant HE 0557−4840 ( T eff /log g /[Fe/H] = 4900/2.2/−4.8). Combining these data with earlier observations, the radial velocity is 212.0 ± 0.4 km s −1 , with no evidence of variability during 2006 February to 2007 December. One-dimensional (1D) LTE model-atmosphere analysis of UV Fe and CH lines confirms the iron and carbon abundances obtained previously ([Fe/H] = −4.8 and [C/Fe] 1D = +1.7), and places a more stringent limit on nitrogen abundance of [N/Fe] 1D < +1.0. Analysis of the UV OH lines yields [O/Fe] 1D = +2.3 ± 0.4. When corrections are made for three-dimensional (3D) effects we obtain [C/Fe] 3D = +1.1, [N/Fe] 3D < +0.1, and [O/Fe] 3D = +1.4. Comparison of the abundances of HE 0557−4840 with those of supernova models of Nomoto et al. and Joggerst et al. suggests that none is able to explain fully the observed abundance pattern. For HE 0557−4840, the Frebel et al. transition discriminant D trans (= log(10 [C/H] + 0.3 × 10 [O/H] ) = −3.4 ± 0.2, consistent with fine-structure transitions of C ii and O i being a major cooling mechanism of star-forming regions at the earliest times. Of the four stars known to have [Fe/H] ≲ −4.3, three are strongly carbon and oxygen enhanced. If the suggestion by Caffau et al. that SDSS J102915+172927 ([Fe/H] = −4.7) does not belong to the class of C-rich, O-rich, UMP stars is supported by future similar discoveries, one will need to consider multiple channels for the production of stars having [Fe/H] ≲ −4.3.