Oxygen in the Very Early Galaxy

Oxygen abundances in a sample of ultra-metal-poor subdwarfs have been derivedfrom measurements of the oxygen triplet at 7771--5 A and OH lines in the nearUV performed in high-resolution and high signal-to-noise spectra obtained withWHT/UES, KeckI/HIRES, and VLT/UVES. Our Fe abundances were derived in LTE andthen corrected for NLTE effects following Thevenin and Idiart (1999). The newoxygen abundances confirm previous findings for a progressive linear rise inthe oxygen-to-iron ratio with a slope -0.33+-0.02 from solar metallicity to[Fe/H] -3. A slightly higher slope would be obtained if the Fe NLTE correctionswere not considered. Below [Fe/H]= -2.5 our stars show [O/Fe] ratios as high as~ 1.17 (G64-12), which can be interpreted as evidence for oxygen overproductionin the very early epoch of the formation of the halo, possibly associated withsupernova events with very massive progenitor stars. We show that the argumentsagainst this linear trend given by Fulbright and Kraft (1999), based on the LTEFe analysis of two metal-poor stars cannot be sustained when an NLTE analysisis performed. Using 1-D models our analysis of three oxygen indicatorsavailable for BD +23 3130 gives consistent abundances within 0.16 dex andaverage [O/Fe] ratio of 0.91.Comment: 45 pages, 11 figures, accepted for publication in The Astrophysical Journa