[$\alpha$/Fe] Abundances of Four Outer M 31 Halo Stars

TL;DR

This study measures alpha element to iron ratios in four outer halo stars of M 31, marking the first such measurements beyond the Milky Way, revealing rapid chemical enrichment similar to our galaxy.

Contribution

First measurement of [$ ext{alpha}$/Fe] in M 31 halo stars, extending chemical abundance studies beyond the Milky Way.

Findings

[$ ext{alpha}$/Fe] ratios are elevated, indicating rapid enrichment.

Outer halo stars have similar [$ ext{alpha}$/Fe] to Milky Way halo stars.

Sample spans metallicities from -2.2 to -1.4 dex.

Abstract

We present alpha element to iron abundance ratios, [$\alpha$/Fe], for four stars in the outer stellar halo of the Andromeda Galaxy (M 31). The stars were identified as high-likelihood field halo stars by Gilbert et al. (2012) and lie at projected distances between 70 and 140 kpc from M 31's center. These are the first alpha abundances measured for a halo star in a galaxy beyond the Milky Way. The stars range in metallicity between [Fe/H]= -2.2 and [Fe/H]= -1.4. The sample's average [$\alpha$/Fe] ratio is +0.20+/-0.20. The best-fit average value is elevated above solar which is consistent with rapid chemical enrichment from Type II supernovae. The mean [$\alpha$/Fe] ratio of our M31 outer halo sample agrees (within the uncertainties) with that of Milky Way inner/outer halo stars that have a comparable range of [Fe/H].