¹ Indian Institute of Astrophysics, Bangalore 560034, India
² CREST, Siddalaghatta Road, Hosakote, Bangalore

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and evolutionary state. 207 open star clusters were observed using slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20% clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0–10 and 20–30 Myr age bin (∼40% each). Most of the emission stars in our survey belong to Classical Be class (∼92%) while a few are Herbig Be stars (∼6%) and Herbig Ae stars (∼2%). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and NGC 1624 (all 4 Myr old) while NGC 6756 (125–150 Myr) is the oldest cluster to have Classical Be stars. The Classical Be stars are located all along the MS in the optical CMDs of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. The distribution of Classical Be stars as a function of spectral type shows peaks at B1–B2 and B6–B7 spectral types. The Be star fraction (N(Be)/N(B+Be)) is found to be less than 10% for most of the clusters and NGC 2345 is found to have the largest fraction (∼26%). Our results indicate there could be two mechanisms responsible for the Classical Be phenomenon. Some are born Classical Be stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to Classical Be stars, witin the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.

Accepted by MNRAS
Preprints available on the web at
http://www3.interscience.wiley.com/journal/120776211/abstract.