Oxygen in the Very Early Galaxy

Abstract

Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derived\nfrom measurements of the oxygen triplet at 7771--5 A and OH lines in the near\nUV performed in high-resolution and high signal-to-noise spectra obtained with\nWHT/UES, KeckI/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE and\nthen corrected for NLTE effects following Thevenin and Idiart (1999). The new\noxygen abundances confirm previous findings for a progressive linear rise in\nthe oxygen-to-iron ratio with a slope -0.33+-0.02 from solar metallicity to\n[Fe/H] -3. A slightly higher slope would be obtained if the Fe NLTE corrections\nwere not considered. Below [Fe/H]= -2.5 our stars show [O/Fe] ratios as high as\n~ 1.17 (G64-12), which can be interpreted as evidence for oxygen overproduction\nin the very early epoch of the formation of the halo, possibly associated with\nsupernova events with very massive progenitor stars. We show that the arguments\nagainst this linear trend given by Fulbright and Kraft (1999), based on the LTE\nFe analysis of two metal-poor stars cannot be sustained when an NLTE analysis\nis performed. Using 1-D models our analysis of three oxygen indicators\navailable for BD +23 3130 gives consistent abundances within 0.16 dex and\naverage [O/Fe] ratio of 0.91.\n