Measuring the alpha-abundance of subsolar-metallicity stars in the Milky Way's central half-parsec: testing globular cluster and dwarf galaxy infall scenarios

TL;DR

This study measures alpha-element and metallicity abundances of two subsolar-metallicity stars in the Milky Way's central region to test their origins, suggesting they likely came from infalling dwarf galaxies or globular clusters.

Contribution

First alpha-element abundance measurements of sub-solar metallicity stars in the Milky Way nuclear star cluster, providing new insights into their origins.

Findings

Stars have intermediate [$ ext{alpha/Fe}$] compared to globular clusters and dwarf galaxies.

Sub-solar metallicity stars differ from the bulk of the nuclear star cluster.

Results support an origin from infalling dwarf galaxies or globular clusters.

Abstract

While the Milky Way Nuclear star cluster 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 $\alpha$-element abundances [$\alpha/Fe$] 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 $\alpha$-element abundance measurements of sub-solar metallicity stars in the Milky Way nuclear star cluster. We measure [$M/H$]=$-0.59\pm 0.11$, [$\alpha/Fe$]=$0.05\pm 0.15$ and [$M/H$]= $-0.81\pm 0.12$, [$\alpha/Fe$]= $0.15\pm 0.16$ for the two stars at the Galactic center; the uncertainties are dominated by systematic uncertainties in the spectral templates. The stars have an [$\alpha/Fe$] in-between the [$\alpha/Fe$] 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 sub-solar metallicity population in the Milky Way nuclear star cluster likely originated from infalling dwarf galaxies or globular clusters and are unlikely to have formed in-situ.