Oxygen abundances in low- and high-alpha field halo stars and the
  discovery of two field stars born in globular clusters
  Oxygen abundances of 67 dwarf stars in the metallicity range -1.6<[Fe/H]<-0.4
are derived from a non-LTE analysis of the 777 nm O I triplet lines. These
stars have precise atmospheric parameters measured by Nissen and Schuster, who
find that they separate into three groups based on their kinematics and
alpha-element (Mg, Si, Ca, Ti) abundances: thick-disk, high-alpha halo, and
low-alpha halo. We find the oxygen abundance trends of thick-disk and
high-alpha halo stars very similar. The low-alpha stars show a larger
star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower
oxygen abundances compared to the other two groups. Thus, we find the behavior
of oxygen abundances in these groups of stars similar to that of the alpha
elements. We use previously published oxygen abundance data of disk and very
metal-poor halo stars to present an overall view (-2.3<[Fe/H]<+0.3) of oxygen
abundance trends of stars in the solar neighborhood. Two field halo dwarf stars
stand out in their O and Na abundances. Both G53-41 and G150-40 have very low
oxygen and very high sodium abundances, which are key signatures of the
abundance anomalies observed in globular cluster (GC) stars. Therefore, they
are likely field halo stars born in GCs. If true, we estimate that at least
3+/-2% of the local field metal-poor star population was born in GCs.
