Discovery of new Milky Way star cluster candidates in the 2 MASS point source catalog. V. Follow-up observations of the young stellar cluster candidates RCW 87, [BDSB2003] 164 and [DBSB2003] 172

Abstract

Context. Massive clusters are more often found in actively star forming galaxies than in quiescent ones, similar to the Milky Way. Aims. We have carried out an extensive survey of obscured Milky Way clusters to determine whether our Galaxy is still forming such objects. Methods. Near-infrared spectral classification of some probable cluster members was used to derive the distances to the cluster candidates. Isochrone analysis of deep near-infrared color-magnitude diagrams allowed us to obtain age and mass estimates. Results. We report the physical parameters of three cluster candidates: RCW 87 is ∼25 Myr old, located at a distance of D ∼ 7.6kpc in the general direction of the Galactic center. Adding the mass of the suspected cluster members we obtain an estimated total cluster mass of ∼ 10 300 M ⊙ . The mid-infrared photometry indicates that some next-generation star formation is on-going in the associated H II region, probably triggered by supernovae or stellar wind from the older stars in RCW 87. The brightest member of [BDSB2003] 164 is an 05 V type star, based on our spectroscopy. This limits the cluster age to less than a few million years. The estimated total mass is ∼1760 M ⊙ and the distance is D ∼ 3.2kpc. [DBSB2003] 172 lacks central concentration and most likely this is not a cluster but an extended star forming region. Conclusions. The high mass of RCW 87 - if confirmed - puts this object in line with Arches and Quintuplet, among the most young massive clusters in the Galaxy. Further study is necessary to confirm this imoortant result.