Oxygen abundances in metal-poor subgiants as determined from [OI], OI and OH lines

The debate on the oxygen abundances of metal-poor stars has its origin in contradictory results obtained using different abundance indicators. To achieve a better understanding of the problem we have acquired high quality spectra with the Ultraviolet and Visual Echelle Spectrograph at VLT, with a signal-to-noise of the order of 100 in the near ultraviolet and 500 in the optical and near infrared wavelength range. Three different oxygen abundance indicators, OH ultraviolet lines around 310.0 nm, the [OI] line at 630.03 nm and the OI lines at 777.1-5 nm were observed in the spectra of 13 metal-poor subgiants with -3.0 <= [Fe/H] <= -1.5. Oxygen abundances were obtained from the analysis of these indicators which was carried out assuming local thermodynamic equilibrium and plane-parallel model atmospheres. Abundances derived from OI were corrected for departures from local thermodynamic equilibrium. Stellar parameters were computed using T-eff-vs.-color calibrations based on the infrared flux method and Balmer line profiles, Hipparcos parallaxes and Fe II lines. [O/Fe] values derived from the forbidden line at 630.03 nm are consistent with an oxygen/iron ratio that varies linearly with [Fe/H] as [O/Fe] = -0.09(+/- 0.08)[Fe/H] + 0.36(+/- 0.15). Values based on the OI triplet are on average 0.19 +/- 0.22 dex(s.d.)higher than the values based on the forbidden line while the agreement between OH ultraviolet lines and the forbidden line is much better with a mean difference of the order of -0.09 +/- 0.25 dex(s.d.). In general, our results follow the same trend as previously published results with the exception of the ones based on OH ultraviolet lines. In that case our results lie below the values which gave rise to the oxygen abundance debate for metal-poor stars.