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

Abstract

We present the first detailed imaging polarization observations of six SMC and six LMC clusters, known to have large populations of B-type stars that exhibit excess H emission from 2-CD photometric studies, 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. We parameterize and remove the interstellar polarization (ISP) associated with each line of sight, thereby isolating the presence of any intrinsic polarization. We use the wavelength dependence of this intrinsic polarization 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, support the suggestion of Wisniewski et al. that classical Be stars are present in clusters of age 5Y8 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. The prevalence of polarimetric Balmer jump signatures decreases with metallicity; we speculate that either it is more difficult to form large disk systems in low-metallicity environments or that average disk temperatures are higher in low-metallicity environments. The polarimetric signatures of $25% of our sample appear unlikely to arise from true classical Be star disk systems, suggesting 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.

A slitless spectroscopic survey for Hαemission-line objects in SMC clusters

“…Thereafter, there is an increase during the evolution, and another decrease after 50-60 Myr (log (age) = 7.7-7.8). Figures 13 and 14 indicate that some Be stars could be born as Be stars (Wisniewski et al 2007b), while others only happen to have Be characteristics during the evolution as mentioned by Fabregat & Torrejón (2000). The first decrease, if real (the differences between the studies being compared are large), could be caused by Be stars reaching the terminal-age main-sequence or by an evolutionary change in the angular velocities so that not every initial Be star can sustain a high enough surface rotation rate to remain a Be star throughout its entire main-sequence life (Martayan et al 2007b).…”

“…Of the 7 remaining emission-line stars, 4 (WBBe5, WBBe10, WBBe17, WBBe20) are in common to both studies. One star (WBBe7), for which Wisniewski et al (2007b) did not publish polarimetry, is found not to have emission. The two others (WBBe6 and WBBe21) are faint and have a too low S/N to provide a reliable conclusion about the presence of emission in their spectra.…”

“…Owing to the large spread in age of the stars in this cluster, some of them could be pre-mainsequence stars. Where possible, cross-references to the studies by Hennekemper et al (2008), Wisniewski & Bjorkman (2006), Wisniewski et al (2007b), andHunter et al (2008) and to the Simbad database are, therefore, also included.…”

DISK-LOSS AND DISK-RENEWAL PHASES IN CLASSICAL Be STARS. I. ANALYSIS OF LONG-TERM SPECTROPOLARIMETRIC DATA

“…and larger statistical ensembles (Coyne & Kruszewski 1969;McLean & Brown 1978;Poeckert et al 1979;Wisniewski et al 2007b). Electron scattering modified by pre-or postscattering absorption by hydrogen atoms in the disk produces the characteristic wavelength-dependent polarization signature of Be stars (Wood et al 1996a(Wood et al , 1996b.…”

“…It inherently assumes the existence of uniform interstellar medium conditions in the vicinity of the target and also assumes that the field stars themselves are devoid of any intrinsic polarization component. While the technique can be highly successful when used to analyze the ISP properties of open clusters, where one can assume all members are located at the same distance and there are sufficient numbers of stars to lessen the effects of small intrinsic polarization components in the field stars by brute force averaging of a large number of stars (see, e.g., Wisniewski et al 2007b), the application of the technique to isolated stars often yields imprecise ISP estimates (see McLean & Clarke 1979;Bjorkman et al 1998;and Wisniewski et al 2006).…”