Near-infrared spectra of Galactic stellar clusters detected on Spitzer/GLIMPSE images

TL;DR

This study uses near-infrared spectroscopy to analyze three Galactic stellar clusters, including a newly discovered one, determining their stellar composition, masses, ages, and distances, thus enhancing understanding of massive star clusters in the inner Galaxy.

Contribution

First near-infrared spectroscopic analysis of three Galactic clusters, including a new discovery, with detailed stellar classification and cluster property estimations.

Findings

Identification of massive stars in the clusters

Estimation of cluster masses, ages, and distances

Discovery of a new stellar cluster, the Quartet

Abstract

We present near-infrared spectroscopic observations of massive stars in three stellar clusters located in the direction of the inner Galaxy. One of them, the Quartet, is a new discovery while the other two were previously reported as candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we firmly establish the nature of the brightest stars in these clusters, yielding new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an early WC star in GLIMPSE20. We combine this information with the available photometric measurements from 2MASS, to estimate cluster masses, ages, and distances. The presence of several massive stars places the Quartet and GLIMPSE20 among the small sample of known Galactic stellar clusters with masses of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about 3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant stars identified in GLIMPSE13 indicates that it is another massive (~ 6500 Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of about 3 kpc.