Near-infrared spectroscopy of candidate red supergiant stars in clusters

TL;DR

This study uses near-infrared spectroscopy combined with photometric data to identify and analyze red supergiant stars in candidate young stellar clusters, revealing new cluster members and emphasizing the importance of spectroscopic follow-up.

Contribution

It introduces a multiwavelength approach to improve the identification of distant young stellar clusters rich in red supergiants, leading to the discovery of new cluster members.

Findings

Six new red supergiant stars identified.

Two red supergiants located at ~3.9 kpc.

Four red supergiants members of a cluster at ~7.0 kpc.

Abstract

Clear identifications of Galactic young stellar clusters farther than a few kpc from the Sun are rare, despite the large number of candidate clusters. We aim to improve the selection of candidate clusters rich in massive stars with a multiwavelength analysis of photometric Galactic data that range from optical to mid-infrared wavelengths. We present a photometric and spectroscopic analysis of five candidate stellar clusters, which were selected as overdensities with bright stars (Ks < 7 mag) in GLIMPSE and 2MASS images. A total of 48 infrared spectra were obtained. The combination of photometry and spectroscopy yielded six new red supergiant stars with masses from 10 Msun to 15 Msun. Two red supergiants are located at Galactic coordinates (l,b)=(16.7deg,-0.63deg) and at a distance of about ~3.9 kpc; four other red supergiants are members of a cluster at Galactic coordinates (l,b)=(49.3deg,+0.72deg) and at a distance of ~7.0 kpc. Spectroscopic analysis of the brightest stars of detected overdensities and studies of interstellar extinction along their line of sights are fundamental to distinguish regions of low extinction from actual stellar clusters. The census of young star clusters containing red supergiants is incomplete; in the existing all-sky near-infrared surveys, they can be identified as overdensities of bright stars with infrared color-magnitude diagrams characterized by gaps.