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M. V. McSwain (Georgia State University)
Massive O- and B-type stars evolve significantly faster than cooler spectral types, so their populations include stars at many different evolutionary stages. They provide fascinating laboratories for the study of stellar evolution. In my dissertation, I am investigating two particular categories of evolved stars, Be stars and microquasars.
Be stars are a class of B stars with circumstellar disks that cause Balmer and other line emission. This phenomenon is observed both in pre-main sequence and evolved stars, and the age at which Be stars appear may correspond to a phase of spin up caused by mass transfer in a close binary system or by an internal redistribution of angular momentum. Not all Be stars are observed in binary systems, so it is possible that the spin up phase is common stage in the evolution of single B stars. To test this evolutionary model, I am completing a photometric survey of 61 evolved open clusters to determine their fraction of Be stars relative to B stars as a function of cluster age.
The microquasar LS 5039 represents another interesting phenomenon in massive star evolution. Stellar winds from the O6.5 V star produce a relatively low X-ray luminosity, and we have determined that the companion is probably a neutron star using a wind accretion model. We have used the current orbital elements to study the pre-supernova characteristics of the system. The optical star's N-enriched and C-weak gas probably resulted from accretion of CNO processed gas from its pre-supernova companion.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.