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

TL;DR

This study measures metallicities of three Gyr-old stars in the Milky Way's nuclear star cluster, revealing a wide metallicity range and suggesting complex chemical evolution and multiple stellar populations.

Contribution

First high-resolution near-infrared spectroscopic metallicity measurements for Gyr-old stars in the NSC, showing a broad metallicity span and implications for its star formation history.

Findings

Metallicities range from -1.2 to +0.5 dex.

Stars have masses between 1.6 and 4.3 solar masses.

Age estimates vary from 100 Myr to 1.5 Gyr.

Abstract

We report metallicities for three $\sim$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 to 4.3 M$_\odot$, 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.