SCYON Abstract

Received on July 1 2008

Be phenomenon in open clusters: Results from a survey of emission-line stars in young open clusters

Authors	Blesson Mathew, Annapurni Subramaniam, and Bhuwan Chandra Bhatt
Affiliation	Indian Institute of Astrophysics, Bangalore 560034, India
Accepted by	Monthly Notices of the Royal Astronomical Society
Contact	blesson@iiap.res.in
URL
Links	Berkeley 62 / Berkeley 63 / Berkeley 86 / Berkeley 87 / Berkeley 90 / Bochum 2 / Bochum 6 / Collinder 96 / IC 1590 / IC 4996 / King 10 / King 21 / NGC 146 / NGC 436 / NGC 457 / NGC 581 / NGC 637 / NGC 654 / NGC 659 / NGC 663 / NGC 869 / NGC 884 / NGC 957 / NGC 1220 / NGC 1624 / NGC 1893 / NGC 2345 / NGC 2414 / NGC 2421 / NGC 6649 / NGC 6756 / NGC 6823 / NGC 6834 / NGC 6910 / NGC 7039 / NGC 7128 / NGC 7235 / NGC 7261 / NGC 7380 / NGC 7419 / NGC 7510 / Roslund 4

Abstract

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.