Chemistry and Kinematics of Red Supergiant Stars in the Young Massive Cluster NGC 2100

TL;DR

This study analyzes red supergiant stars in NGC 2100 using near-IR spectroscopy to determine metallicity, kinematics, and dynamical properties, providing new insights into the cluster's characteristics and stellar evolution.

Contribution

It presents the first detailed spectroscopic analysis of RSGs in NGC 2100, including metallicity, radial velocities, and dynamical mass estimates, using the J-band technique.

Findings

Average metallicity of [Z] = -0.43 dex for NGC 2100.

No significant difference in RSG positions in the Hertzsprung--Russell diagram compared to Galactic clusters.

Upper limit of 3.9 km/s for the cluster's velocity dispersion.

Abstract

We have obtained K-band Multi-Object Spectrograph (KMOS) near-IR spectroscopy for 14 red supergiant stars (RSGs) in the young massive star cluster NGC 2100 in the Large Magellanic Cloud (LMC). Stellar parameters including metallicity are estimated using the J-band analysis technique, which has been rigorously tested in the Local Universe. We find an average metallicity for NGC 2100 of [Z]=$-$0.43$\pm$0.10 dex, in good agreement with estimates from the literature for the LMC. Comparing our results in NGC 2100 with those for a Galactic cluster (at Solar-like metallicity) with a similar mass and age we find no significant difference in the location of RSGs in the Hertzsprung--Russell diagram. We combine the observed KMOS spectra to form a simulated integrated-light cluster spectrum and show that, by analysing this spectrum as a single RSG, the results are consistent with the average properties of the cluster. Radial velocities are estimated for the targets and the dynamical properties are estimated for the first time within this cluster. The data are consistent with a flat velocity dispersion profile, and with an upper limit of 3.9 km/s, at the 95% confidence level, for the velocity dispersion of the cluster. However, the intrinsic velocity dispersion is unresolved and could, therefore, be significantly smaller than the upper limit reported here. An upper limit on the dynamical mass of the cluster is derived as $M_{dyn}$ $\le$ $15.2\times10^{4}M_{\odot}$ assuming virial equilibrium.