On the highly reddened members in six young galactic star clusters - A multiwavelength study

The spectral and reddening properties of 211 highly reddened proper-motion members with V < 15 mag in six young galactic star clusters are investigated using low-resolution spectroscopic, broad-band UBVRIJHK and mid-infrared (IR) data. We report emission features in Ca II HK and H I 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. Can triplet width measurements are used to indicate the presence of an accretion disc for a dozen stars and to indicate luminosity for a couple of stars. On the basis of spectral features, near-IR excesses, dereddened colour-colour diagrams and mid-IR spectral indices we identify a group of 28 pre-main-sequence cluster members including five highly probable Herbig Ae/Be and six classical T Tauri stars. A total of 25 non-emission main-sequence (MS) stars, amounting to ∼ 10 per cent early-type MS members, appears to show Vega-like characteristics or are precursors to such a phenomenon. The various membership indicators suggest that ∼ 16 per cent of the propermotion members are non-members. A significant fraction (>70 per cent) of programme stars in NGC 1976, NGC 2244, NGC 6530 and NGC 6611 show anomalous reddening with R_v = 4.78 ± 0.10, 3.54 ± 0.04, 3.87 ± 0.05 and 3.56 ± 0.02, respectively, indicating the presence of grain size dust larger than that typical of the diffuse medium. A small number of stars in NGC 1976, NGC 2244 and NGC 6611 also show normal behaviour while the cluster NGC 6823 appears to have normal reddening. Three highly luminous late-type giants, one in NGC 2244 and two in NGC 6530, appear to be members and are in post-hydrogen-coreburning stages, suggesting a prolonged duration (∼25 Myr) of star formation.