2012ApJ...753..150N

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, 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] + 0.3x10^[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.