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

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 the 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 per cent 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 per cent each). Most of the emission stars in our survey belong to Classical Be class (∼92 per cent) while a few are Herbig Be stars (∼6 per cent) and Herbig Ae stars (∼2 per cent). The youngest clusters to have Classical Be stars are IC 1590, NGC 637 and 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 main sequence (MS) in the optical colour–magnitude diagrams (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 per cent for most of the clusters and NGC 2345 is found to have the largest fraction (∼26 per cent). 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, within the MS lifetime, as indicated by the enhancement in the fraction of clusters with Classical Be stars in the 20–30 Myr age bin.

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

“…Figure 12 presents the percentages per spectral subclass, referred to the total sample of Be stars, but separately for SMC and Galaxy. The Galactic data are from , McSwain & Gies (2005), and Mathew et al (2008). The distributions for the two galaxies are similar but the incompleteness of the SMC data becomes apparent beyond B2 (cf.…”

“…6 .0 -6 .3 6 .3 -6 .6 6 .6 -6 .9 6 .9 -7 .2 7 .2 -7 .5 7 .5 -7 . Mathew et al (2008), there seems to be a decrease first in the number of open clusters with Be stars towards 30-40 Myr (log (age) = 7.5-7.6). Thereafter, there is an increase during the evolution, and another decrease after 50-60 Myr (log (age) = 7.7-7.8).…”

Search for Southern Galactic Be Star Candidates

“…More recent surveys of Be stars in galactic open clusters and OB associations concluded that stars in these environments also show spectroscopic and photometric irregular variations (McSwain & Gies 2005;Mathew, Subramaniam, & Bhatt 2008). These photometric variations have been observed in three time-scales that are often superimposed: (1) The short-term variations with time-scales from hours to days and amplitudes up to 0.1 mag, are probably caused by nonradial pulsations or rotation (Percy et al 2002;Percy, Harlow, & Wu 2004).…”

Dust formation of Be stars with large infrared excess

“…Furthermore, Currie et al (2008) excess, implying that these CBe stars are just in the beginning of the dust formation. Recently, Mathew et al (2008) claimed to have identified a CBe star in IC 1590 which is only ∼ 4 Myr old, marking the nature of CBe stars even more puzzling ever. …”