The Star Population of Young Open Clusters: A Photometric and Spectroscopic Study

Abstract

I have determined physical parameters (interstellar reddening, distance, and age) for three young open clusters: h Persei, x Persei, and NGC 1893. I have studied their massive star populations and their formation epochs. I present photometric data for 350 stars in the double open cluster h and x Persei. For both clusters, the distance moduli and reddening are determined to be the same: V0 M p V and E(b y) p . However, the ages for 11.6 0.2 0.42 0.2 the clusters are different. Most stars in h Persei follow the isochrone, although some of the massive stars seem log t p 7.3 younger. By contrast, the stars in x Persei better fit isochrones for (A. Marco & G. Bernabeu 2001, A&A, log t p 7.10–7.15 372, 477). I give photometric data for 114 stars in NGC 1893. The distance modulus is found to be V0 . This M p 13.9 0.2 V distance is much larger than the traditional value for Aur OB2. I have also obtained spectroscopy for photometrically peculiar stars in NGC 1893. Among them, I have found six emissionline stars, identified as pre–main-sequence (PMS) stars with spectral types between B0 and late F. For two of these stars, the derived spectral types indicate that they are very massive Herbig Be stars. However, one other B-type emission-line star looks like a typical Be star. The presence of massive PMS stars in NGC 1893 suggests that many other fainter PMS stars could be present. A spectroscopic study of objects not falling on the cluster main sequence found several objects whose spectral types and colors suggested that they could be absorption-line PMS stars. While many early and intermediate B-type stars are settled onto the main sequence, the detection of early-B Herbig Be stars indicates that star formation has been taking place for some time in NGC 1893. This work has produced a list of new b standard stars uvby for the study of moderately reddened clusters and has shown that the combination of Stromgren photometry and low-resolution spectroscopy is a powerful tool for the detection of a PMS population in young open clusters (A. Marco, G. Bernabeu, & I. Negueruela 2001, AJ, 121, 2075).