Obscured clusters. III. Follow-up observations of Mercer 23

Abstract

Context. New infrared surveys have revealed over 1000 new open cluster candidates in the Milky Way, but these candidates need to be confirmed with follow up observations. Of particular interest are young, massive star clusters because they serve as nearby analogues to the distant super star clusters studied as point sources in other galaxies. Aims. We determine the physical parameters and investigate the high-mass stellar content of the infrared star cluster Mercer 23, situated near the Galactic plane (l = 53772, b = +0164). Methods. Our analysis is based on new Baade/PANIC JHKS and ISAAC/VLT imaging of Mercer 23 and ISAAC/VLT moderate resolution (R ≈ 4000) spectroscopy of the brightest cluster members in the H- and K-bands. The cluster age is determined from isochrone main-sequence (MS) and pre-MS fitting. We derive stellar parameters for eight of the stellar members, using a full non-LTE modeling of the obtained spectra. Results. Mercer 23 is a very young cluster, with age of t = 2–4 Myr. The cluster suffers reddening of E(J–KS) = 1.35, AV =7.2 mag. The derived distance is d = 6.5±0.3 kpc. Our spectral modeling allows us to conclude that the three most luminous member are evolved highly massive stars: a WR star, and two mid-O supergiant stars, based on their derived luminosity. Conclusions. Mercer 23 is not a super-massive cluster such as those recently recognized to exist in the Milky Way. However, its mass estimate of 4–6×103 and possibly more, puts it in the class of young Galactic clusters hosting WR stars.