Measuring the α-abundance of Subsolar-metallicity Stars in the Milky Way's Central Half-parsec: Testing Globular Cluster and Dwarf Galaxy Infall Scenarios

Abstract

While the Milky Way nuclear star cluster (MW NSC) has been studied extensively, how it formed is uncertain. Studies have shown it contains a solar and supersolar metallicity population that may have formed in situ, along with a subsolar-metallicity population that may have formed via mergers of globular clusters and dwarf galaxies. Stellar abundance measurements are critical to differentiate between formation scenarios. We present new measurements of [M/H] and α-element abundances [α/Fe] of two subsolar-metallicity stars in the Galactic center. These observations were taken with the adaptive-optics-assisted high-resolution (R = 24,000) spectrograph NIRSPEC in the K band (1.8-2.6 micron). These are the first α-element abundance measurements of subsolar-metallicity stars in the MW NSC. We measure [M/H] = - 0.59 ± 0.11, [α/Fe] = 0.05 ± 0.15 and [M/H] = - 0.81 ± 0.12, [α/Fe] = 0.15 ± 0.16 for the two stars at the Galactic center; the uncertainties are dominated by systematic uncertainties in the spectral templates. The stars have an [α/Fe] in between the [α/Fe] of globular clusters and dwarf galaxies at similar [M/H] values. Their abundances are very different than the bulk of the stars in the nuclear star cluster. These results indicate that the subsolar-metallicity population in the MW NSC likely originated from infalling dwarf galaxies or globular clusters and are unlikely to have formed in situ.