The cool supergiant population of the massive young star cluster RSGC1

TL;DR

This study uses high-resolution near-IR spectroscopy and maser observations to analyze the population of red supergiants in the young star cluster RSGC1, refining its properties and evolutionary status.

Contribution

It provides the first detailed spectroscopic and maser analysis of RSGC1's RSG population, revises the cluster's age and mass estimates, and links maser activity to the late RSG evolutionary phase.

Findings

Confirmed most candidate RSGs as cluster members

Revised cluster age to 12±2 Myr, older than previous estimates

Identified maser emission as associated with the most luminous RSGs

Abstract

We present new high-resolution near-IR spectroscopy and OH maser observations to investigate the population of cool luminous stars of the young massive Galactic cluster RSGC1. Using the 2.293\micron CO-bandhead feature, we make high-precision radial velocity measurements of 16 of the 17 candidate Red Supergiants (RSGs) identified by Figer et al. We show that F16 and F17 are foreground stars, while we confirm that the rest are indeed physically-associated RSGs. We determine that Star F15, also associated with the cluster, is a Yellow Hypergiant based on its luminosity and spectroscopic similarity to $\rho$ Cas. Using the cluster's radial velocity, we have derived the kinematic distance to the cluster and revisited the stars' temperatures and luminosities. We find a larger spread of luminosities than in the discovery paper, consistent with a cluster age 30% older than previously thought (12$\pm$2Myr), and a total initial mass of $(3\pm1) \times 10^{4}$\msun. The spatial coincidence of the OH maser with F13, combined with similar radial velocities, is compelling evidence that the two are related. Combining our results with recent SiO and H$_2$O maser observations, we find that those stars with maser emission are the most luminous in the cluster. From this we suggest that the maser-active phase is associated with the end of the RSG stage, when the luminosity-mass ratios are at their highest.