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J.P. Wisniewski (USRA/NASA GSFC)
While rapid rotation is likely the dominant mechanism which influences the development of classical Be circumstellar disks, recent observational and theoretical work suggest that evolutionary age and/or metallicity may also influence the onset of the Be phenomenon. We use a simple 2-color diagram photometric technique to identify the candidate Be population in 16 LMC, SMC, and Galactic clusters having a wide range of ages and metallicities. We detect an enhancement in the fractional early-type candidate Be star population relative to the fractional later-type candidate population in clusters whose early-type stars are near the end of their main sequence lifetimes, suggesting the Be phenomenon is enhanced with evolutionary age. Furthermore, in contrast to the suggestion of Fabregat & Torrejon (2000) that the Be phenomenon should begin at least 10 Myr after the zero-age-main-sequence, we detect a substantial number of candidate Be stars in clusters as young as 5 Myr. Follow-up photo-polarimetric observations of these young candidates reveal many are true classical Be stars, indicating that a significant number of zero-age-main-sequence stars must be rotating close to their critical breakup velocities. The improved statistics offered by our study also reveal clear evidence of an enhancement of the Be phenomenon in low metallicity environments.
It is commonly assumed in the literature that all B-type objects detected as excess H\alpha emitters via 2-color diagrams are ``Be stars''. We explore the nature of many of these candidate Be stars with additional photo-polarimetric observations, and find that ~25% of these objects exhibit properties which aren't consistent with those expected from classical Be stars. We also find that the prevalence of polarization Balmer jumps in Be stars located in low metallicity environments is lower than that typically observed for Galactic Be stars. One interpretation of this result is that disk systems in low metallicity environments have fundamentally different properties, i.e. smaller disks and/or lower disk temperatures, than their Galactic counterparts.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.