Oxygen Abundances in Metal-poor Stars (-2.2<[Fe/H]<-1.2) from Infrared OH Lines

Abstract

Infrared OH lines at 1.55-1.56 μm in the H band were obtained with the Phoenix high-resolution spectrograph at the 2.1 m telescope of the Kitt Peak National Observatory for a sample of 14 metal-poor stars. Detailed analyses of the sample stars have been carried out, deriving stellar parameters based on two methods: (a) spectroscopic parameters and (b) infrared flux method (IRFM) effective temperatures, trigonometric gravities, and metallicities from Fe II lines. The Fe I lines present in the H-band region observed were well fitted by the stellar parameters within Δ[Fe/H]<=0.15 dex. The oxygen abundances were derived from fits of spectrum synthesis calculations to the infrared OH lines. CO lines in the H and K bands were obtained for a subsample in order to determine their carbon abundances. Adopting the spectroscopic parameters, a mean oxygen-to-iron ratio of [O/Fe]~0.52 is obtained, whereas using the IRFM temperatures, Hipparcos gravities, and [Fe II/H], [O/Fe]~0.25 is found. A mean of the two methods gives a final value of [O/Fe]~0.4 for the metallicity range -2.2<[Fe/H]<-1.2 of the sample metal-poor stars.