The Role of Evolutionary Age and Metallicity in the Formation of Classical Be Circumstellar Disks II. Assessing the Evolutionary Nature of Candidate Disk Systems

TL;DR

This study uses polarization imaging to analyze the evolutionary status of Be star disks in different metallicity environments, revealing that disk formation occurs earlier than previously thought and varies with metallicity.

Contribution

It provides the first detailed polarization imaging of Be star clusters in the SMC and LMC, linking disk formation to cluster age and metallicity, and distinguishing between different disk types.

Findings

Classical Be stars are present in clusters aged 5-8 Myr.

Polarimetric signatures decrease with lower metallicity environments.

Approximately 25% of candidate Be stars are not true classical Be stars.

Abstract

(Abridged version) We present the first detailed imaging polarization observations of six SMC and six LMC clusters, known to have large populations of B-type stars which exhibit excess H-alpha emission, to constrain the evolutionary status of these stars and hence better establish links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. The wavelength dependence of our intrinsic polarization data provides a diagnostic of the dominant and any secondary polarigenic agents present, enabling us to discriminate pure gas disk systems, i.e. classical Be stars, from composite gas plus dust disk systems, i.e. Herbig Ae/Be or B[e] stars. Our intrinsic polarization results, along with available near-IR color information, strongly supports the suggestion of Wisniewski et al. that classical Be stars are present in clusters of age 5-8 Myr, and contradict assertions that the Be phenomenon only develops in the second half of a B star's main sequence lifetime, i.e. no earlier than 10 Myr. 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 ~25% are unlikely to arise from true classical Be star-disk systems.