Detailed abundance analysis of a metal-poor giant in the Galactic Center

TL;DR

This study presents the first high-resolution abundance analysis of a metal-poor, alpha-enhanced red giant near the Galactic Center, revealing insights into the cluster's old stellar population and its relation to the bulge.

Contribution

It provides the first detailed chemical abundance analysis of a metal-poor giant in the Galactic Center using high-resolution spectroscopy.

Findings

Star has [Fe/H]~-1.0 and [alpha/Fe]~+0.4, similar to bulge stars.

Star is likely part of the old (~10 Gyr) population.

This is the most metal-poor red giant confirmed near the Galactic Center.

Abstract

We report the first results from our program to examine the metallicity distribution of the Milky Way nuclear star cluster connected to SgrA*, with the goal of inferring the star formation and enrichment history of this system, as well as its connection and relationship with the central 100 pc of the bulge/bar system. We present the first high resolution (R~24,000), detailed abundance analysis of a K=10.2 metal-poor, alpha-enhanced red giant projected at 1.5 pc from the Galactic Center, using NIRSPEC on Keck II. A careful analysis of the dynamics and color of the star locates it at about 26 pc line-of-sight distance in front of the nuclear cluster. It probably belongs to one of the nuclear components (cluster or disk), not to the bar-bulge or classical disk. A detailed spectroscopic synthesis, using a new linelist in the K band, finds [Fe/H]~-1.0 and [alpha/Fe]~+0.4, consistent with stars of similar metallicity in the bulge. As known giants with comparable [Fe/H] and alpha enhancement are old, we conclude that this star is most likely to be a representative of the ~10 Gyr old population. This is also the most metal poor confirmed red giant yet discovered in vicinity of the nuclear cluster of the Galactic Center. We consider recent reports in the literature of a surprisingly large number of metal poor giants in the Galactic Center, but the reported gravities of log g ~4 for these stars calls into question their reported metallicities.