Oxygen abundances in low- and high-alpha field halo stars and the discovery of two field stars born in globular clusters

TL;DR

This study analyzes oxygen abundances in 67 dwarf stars across different galactic components, revealing similarities and differences in their chemical evolution, and identifies two stars likely born in globular clusters, estimating their contribution to the field star population.

Contribution

It provides detailed oxygen abundance trends in halo and disk stars and discovers two field stars with globular cluster origins, estimating their prevalence in the local population.

Findings

Oxygen abundance trends are similar in thick-disk and high-alpha halo stars.

Low-alpha halo stars show more scatter and lower oxygen compared to other groups.

Two field stars exhibit globular cluster chemical signatures, suggesting at least 3% of local metal-poor stars originated in GCs.

Abstract

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.