The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks. I. New Candidate Be Stars in the LMC, SMC, and Milky Way

We present B, V, R, and H alpha photometry of 8 clusters in the SmallMagellanic Cloud, 5 in the Large Magellanic Cloud, and 3 Galactic clusters, anduse 2 color diagrams (2-CDs) to identify candidate Be star populations in theseclusters. We find evidence that the Be phenomenon is enhanced in lowmetallicity environments, based on the observed fractional early-type candidateBe star content of clusters of age 10-25 Myr. Numerous candidate Be stars ofspectral types B0 to B5 were identified in clusters of age 5-8 Myr, challengingthe suggestion of Fabregat & Torrejon (2000) that classical Be stars shouldonly be found in clusters at least 10 Myr old. These results suggest that asignificant number of B-type stars must emerge onto the zero-age-main-sequenceas rapid rotators. We also detect an enhancement in the fractional content ofearly-type candidate Be stars in clusters of age 10-25 Myr, suggesting that theBe phenomenon does become more prevalent with evolutionary age. We brieflydiscuss the mechanisms which might contribute to such an evolutionary effect. Adiscussion of the limitations of utilizing the 2-CD technique to investigatethe role evolutionary age and/or metallicity play in the development of the Bephenomenon is offered, and we provide evidence that other B-type objects ofvery different nature, such as candidate Herbig Ae/Be stars may contaminate theclaimed detections of ``Be stars'' via 2-CDs.