The oxygen abundance of the ultra-metal-poor star HE 0557-4840

John E. Norris*, N. Christlieb, M. S. Bessell, M. Asplund, K. Eriksson, A. J. Korn

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    16 Citations (Scopus)

    Abstract

    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 ± 0.4kms-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 HE0557-4840, 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.

    Original languageEnglish
    Article number150
    JournalAstrophysical Journal
    Volume753
    Issue number2
    DOIs
    Publication statusPublished - 10 Jul 2012

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