Super-Solar Metallicity Stars in the Galactic Center Nuclear Star
  Cluster: Unusual Sc, V, and Y Abundances
  We present adaptive-optics assisted near-infrared high-spectral resolution
observations of late-type giants in the nuclear star cluster of the Milky Way.
The metallicity and elemental abundance measurements of these stars offer us an
opportunity to understand the formation and evolution of the nuclear star
cluster. In addition, their proximity to the supermassive black hole ($\sim
0.5$ pc) offers a unique probe of the star formation and chemical enrichment in
this extreme environment. We observed two stars identified by medium
spectral-resolution observations as potentially having very high metallicities.
We use spectral-template fitting with the PHOENIX grid and Bayesian inference
to simultaneously constrain the overall metallicity, [M/H], alpha-element
abundance [$\alpha$/Fe], effective temperature, and surface gravity of these
stars. We find that one of the stars has very high metallicity ([M/H] $> 0.6$)
and the other is slightly above solar metallicity. Both Galactic center stars
have lines from scandium (Sc), vanadium (V), and yttrium (Y) that are much
stronger than allowed by the PHOENIX grid. We find, using the spectral
synthesis code Spectroscopy Made Easy, that [Sc/Fe] may be an order of
magnitude above solar. For comparison, we also observed an empirical calibrator
in NGC6791, the highest metallicity cluster known ([M/H] $\sim 0.4$). Most
lines are well matched between the calibrator and the Galactic center stars,
except for Sc, V, and Y, which confirms that their abundances must be
anomalously high in these stars. These unusual abundances, which may be a
unique signature of nuclear star clusters, offer an opportunity to test models
of chemical enrichment in this region.
