Study of emission line stars in young open clusters

Abstract

Emission-line stars in young open clusters are identired 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 identired 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 rest time. From optical/near-IR photometry and spectroscopy, we suggest that Bochum 6(1), IC 1590(1) and NGC 6823(1) are Herbig Be while IC 1590(2) and NGC 7380(4) are Herbig Ae candidates. The Classical Be stars are located all along the main sequence (MS) in the optical colour magnitude diagram (CMD) of clusters of all ages, which indicates that the Be phenomenon is unlikely only due to core contraction near the turn-o_. A spectroscopic study of 152 Classical Be stars in 42 young open clusters was performed using medium resolution spectra in 3700􀀀9000 _A range, to understand the Be phenomenon. We found Lymanr uorescence as the mechanism for the production of Oi 8446 _A line in 24% of the surveyed stars, while collisional excitation is the likely excitation mechanism for Oi 8446 _A and 7772_A lines found in 47% of the stars, suggesting a denser disk. The Balmer decrement ((I( H_)/I(H_)) is found to have a bimodal distribution which is correlated with the nature of H_ pro_le. Candidates with higher Balmer decrement (76%) were found to have more number of spectral lines, higher H_ equivalent width and (H􀀀K)0 values, suggesting an optically and geometrically thick disk. Massive Be stars of spectral type B0􀀀B4 are found to have enhanced H_ emission at the end of their main sequence lifetime, as inferred from the enhancements observed at 12.5 and 25 Myr. The angular momentum evolution of stars as a function of age and spectral type also suggest a bimodal origin of Be stars. Stars in the B0􀀀B2 spectral bin are found be spun up towards the end of their MS life time, suggesting that early type stars evolve to become Be stars. Similar variation in properties were not found for stars in the later spectral types (B4􀀀A0), suggesting that the Be phenomenon dirers in early type and late type stars.