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

摘要

Context. New infrared surveys have revealed over 1000 newopen cluster candidates in the Milky Way, but these candidates need tobe confirmed with follow up observations. Of particular interest areyoung, massive star clusters because they serve as nearby analogues tothe distant super star clusters studied as point sources in othergalaxies. Aims. We determine the physical parameters and investigate thehigh-mass stellar content of the infrared star cluster Mercer23,situated near the Galactic plane (,). Methods. Our analysis is based on new Baade/PANIC and ISAAC/VLT imaging of Mercer23 and ISAAC/VLT moderate resolution ()spectroscopy of the brightest cluster members in the H- and K-bands.The cluster age is determined from isochrone main-sequence (MS) andpre-MS fitting. We derive stellar parameters for eight of the stellarmembers, using a full non-LTE modeling of the obtained spectra. Results. Mercer23 is a very young cluster, with age of t=2-4Myr. The cluster suffers reddening of E(J-)=1.35, AV=7.2mag. The derived distance is kpc.Our spectral modeling allows us to conclude that the three mostluminous member are evolved highly massive stars: a WR star, and twomid-O supergiant stars, based on their derived luminosity. Conclusions. Mercer23 is not a super-massive cluster suchas those recently recognized to exist in the Milky Way. However, itsmass estimate of 4-and possibly more, puts it in the class of young Galactic clusters hosting WR stars. Key words: Galaxy: open clusters and associations: general - stars:Wolf-Rayet - open clusters and associations: individual: [MCM2005b] 23