A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster

Abstract

We report metallicities for three ∼Gyr-old stars in the Milky Way nuclear star cluster (NSC) using high-resolution near-infrared spectroscopy. We derive effective temperatures from a calibration with Sc line strength, which yields results in good agreement with other methods, and metallicities from spectral fits to Fe i lines. Our derived metallicities range from −1.2 < [Fe/H] < + 0.5, a span of 1.7 dex. In addition we use isochrone projection to obtain masses of 1.6–4.3 M ⊙, and ages assuming single-star evolution. The oldest of these stars is 1.5 Gyr while the youngest and most metal-rich is only 100 Myr. The wide range in metallicity poses interesting questions concerning the chemical evolution and enrichment of the NSC and adds to the evidence for the presence of a young, metal-rich population in the NSC. We suggest that the candidate intermediate-age, metal-poor ([Fe/H] = −1.2) star may be best explained as a blue straggler from an underlying old population.