Role of Evolutionary Age and Metallicity in the FOrmation of Classical BE CIrcumstellar Disks II. Assessing the True Nature of Candidate Disk Systems

摘要

Photometric 2-color diagram (2-CD) surveys of young cluster populations have been used to identify populations of B-type stars exhibiting excess Ha emission. The prevalence of these excess emitters, assumed to be 'Be stars'. has led to the establishment of links between the onset of disk formation in classical Be stars and cluster age and or metallicity. We have obtained imaging polarization observations of six SMC and six LMC clusters whose candidate Be populations had been previously identified via 2-CDs. The interstellar polarization (ISP) associated with these data has been identified to facilitate an examination of the circumstellar environments of these candidate Be stars via their intrinsic polarization signatures, hence determine the true nature of these objects. These data imply that a significant number of B-type stars must emerge onto the zero-age-main-sequence rotating at near-critical rotation rates, although we can not rule out the possibility that these data instead reveal the presence of a sub-group of the Be phenomenon characterized by sub-critically rotating objects. Comparing the polarimetric properties of our dataset to a similar survey of Galactic classical Be stars, we find that the prevalence of polarimetric Balmer jump signatures decreases with metallicity. We speculate that these results might indicate that either it is more difficult to form large disk systems in low metallicity environments, or that the average disk temperature is higher in these low metallicity environments. We have characterized the polarimetric signatures of all candidate Be stars in our data sample and find approx. 25 are unlikely to arise from true classical Be star-disk systems. This detection of such a substantial number 'contaminants' suggests one should proceed with caution when attempting to determine the role of evolutionary age and or metallicity in the Be phenomenon purely via 2-CD results.