The Evolutionary Status of Be Stars: Results from a Photometric Study of Southern Open Clusters

By M. McSwain, D. Gies. 2005

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Be stars are a class of rapidly rotating B stars with circumstellar disks that cause Balmer and other line emission. There are three possible reasons for the rapid rotation of Be stars: they may have been born as rapid rotators, spun up by binary mass transfer, or spun up during the main-sequence (MS) evolution of B stars. To test the various formation scenarios, we have conducted a photometric survey of 55 open clusters in the southern sky. Of these, five clusters are probably not physically associated groups and our results for two other clusters are not reliable, but we identify 52 definite Be stars and an additional 129 Be candidates in the remaining clusters. We use our results to examine the age and evolutionary dependence of the Be phenomenon. We find an overall increase in the fraction of Be stars with age until 100 Myr, and Be stars are most common among the brightest, most massive B-type stars above the zero-age main sequence (ZAMS). We show that a spin-up phase at the terminal-age main sequence (TAMS) cannot produce the observed distribution of Be stars, but up to 73% of the Be stars detected may have been spun-up by binary mass transfer. Most of the remaining Be stars were likely rapid rotators at birth. Previous studies have suggested that low metallicity and high cluster density may also favor Be star formation. Our results indicate a possible increase in the fraction of Be stars with increasing cluster distance from the Galactic center (in environments of decreasing metallicity). However, the trend is not significant and could be ruled out due to the intrinsic scatter in our data. We also find no relationship between the fraction of Be stars and cluster density.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/d9698bef28b5b4d315b01089942534c22fae0640

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In Astronomy and Astrophysics

1. S. Ekström
2. G. Meynet
3. C. Martayan
4. T. Rivinius
5. Andre Maeder
6. A. Carciofi
7. R. Hirschi
8. F. Barblan

Citations in the corpus

• Evolution towards the critical limit and the origin of Be stars (2007, background): “Forinstance McSwain & Gies (2005b) found that Be stars were generally found among the brighter B-stars in their survey of southern sky Galactic clusters.”
• THE EFFECT OF STARBURST METALLICITY ON BRIGHT X-RAY BINARY FORMATION PATHWAYS (2010, methodology): “Regardless of the accuracy of this association, we note that McSwain & Gies (2005) found the observed number of Be-HMXBs to be greatest between 25-100 Myr after star formation, relegating the vast majority of BeHMXB activity until after the end of the simulations and the questions considered here.”
• Classical Be stars (2013, methodology): “In a later study, Martayan et al. (2010a) found that among early-type stars (B0–B3) the frequency of Be stars is 3 to 5 times higher in the SMC than in the MW as reported by McSwain and Gies (2005) or Mathew et al. (2008).”
• A STELLAR ROTATION CENSUS OF B STARS: FROM ZAMS TO TAMS (2010, background): “McSwain & Gies (2005) found that the Be fraction increases as high mass B stars evolve.”