On the highly reddened members in 6 young galactic star clusters - a multiwavelength study ⋆

The spectral and reddening properties of 211 highly reddened proper motion members with V < 15 mag in 6 young galactic star clusters are investigated using low resolution spectroscopic, broad-band UBV RIJHK and mid-IR data. We report emission features in CaII HK and HI lines for a sample of 29 stars including 11 stars reported for the first time and also provide either a new or more reliable spectral class for a sample of 24 stars. CaII triplet width measurements are used to indicate the presence of an accretion disk for a dozen stars and to hint luminosity for a couple of stars. On the basis of spectral features, near-IR excesses, dereddened color-color diagrams and mid-IR spectral indices we identify a group of 28 pre-main sequence cluster members including 5 highly probable Herbig Ae/Be and 6 classical T Tauri star. A total of 25 non-emission MS stars, amounting to ∼ 10 % early type MS members, appears to show Vega-like characteristics or are precursors to such a phenomenon. The various membership indicators suggest that ∼ 16% of the PM members are non-members. A significant fraction (>70%) of program stars in NGC 1976, NGC 2244, NGC 6530 and NGC 6611 show anomalous reddening with RV = 5.11± 0.11, 3.60± 0.05, 3.87± 0.05 and 3.56±0.02, respectively, indicating the presence of grain size dust larger than that typical to the diffuse medium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 also show normal behavior while the cluster NGC 6823 appears to have a normal reddening. Three highly luminous late type giants, one in NGC 2244 and two in NGC 6530, appears to be member and are in post-hydrogen-core-burning stages suggesting a prolonged duration (∼ 25 Myrs) of star formation.