Discovery and spectroscopic study of the massive Galactic cluster Mercer 81

TL;DR

This study confirms Mercer 81 as a very massive young Galactic cluster hosting Wolf-Rayet stars and blue supergiants, identified through infrared photometry and spectroscopy, and located at the far end of the Galactic bar.

Contribution

First spectroscopic confirmation of Mercer 81 as a massive young cluster with Wolf-Rayet stars, expanding knowledge of Galactic cluster populations.

Findings

Mercer 81 hosts Nitrogen-rich Wolf-Rayet stars (WN) and blue supergiants.

The cluster is located at the far end of the Galactic bar.

Brightest stars are foreground stars, not cluster members.

Abstract

During the last decade, hundreds of young massive cluster candidates have been detected in the disk of the Milky Way. We investigate one of these candidates, Mercer 81, which was discovered through a systematic search for stellar overdensities, with follow-up NICMOS/HST infrared narrow-band photometry to find emission-line stars and confirm it as a massive cluster. Surprisingly, the brightest stars turned out to be a chance alignment of foreground stars, while a real massive cluster was found among some fainter stars in the field. From a first spectroscopic study of four emission-line stars (ISAAC/VLT), it follows that Mercer 81 is a very massive young cluster, placed at the far end of the Galactic bar. Additionally, in this work we present some unpublished spectra from a follow-up observation program which confirm that the cluster hosts several Nitrogen-rich Wolf-Rayet stars (WN) and blue supergiants.