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

We present alpha element to iron abundance ratios, [$α$/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 [$α$/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 [$α$/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].

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