The evolution of massive stars: The Be star and microquasar phenomena

Massive Oand B-type stars evolve significantly faster than stars with cooler spectral types, so their populations include stars at many different evolutionary stages. They provide fascinating laboratories for the study of stellar evolution. In this dissertation, I investigate an unevolved massive star system and two particular categories of evolved stars, Be stars and microquasars. The massive triple system HD 16429 A is largely unevolved, but I present it here as an example of the type of system that will eventually experience a disrupting supernova. I discuss the Doppler tomography technique that I used to isolate the two brightest components, and include my analysis of each star. The stationary component, HD 16429 Aa, is an O9.5 II star. The Ab1 component is a hotter yet less luminous O8 III–IV star, while the unseen Ab2 star is estimated to be a B0 star. Interestingly, HD 16429 A may be associated with the microquasar LS I 61 303. The two systems are only 3 .6 apart in the sky, and they may form a subcluster within the Cas OB6 association. Many massive binary systems, including Be binaries, contain the final product of massive stellar evolution: a neutron star or black hole companion. Mass transfer from the less evolved star onto the compact companion generates X-ray emission, and some of these massive X-ray binaries also have relativistic radio jets that closely resemble small versions of extragalactic quasars. Not only are these microquasars a good test bed for accretion disk and jet models, they also provide fascinating examples of stellar evolution. Based on its large eccentricity and runaway velocity, the microquasar LS 5039 appears to be a recent survivor of the supernova that formed the microquasar. In this dissertation, I derive many of its presupernova characteristics from a spectroscopic study of the system. Be stars are a class of B stars with circumstellar disks that cause Balmer and other line emission. The source of their disks is not well understood, but it is likely that a combination of rapid rotation and nonradial pulsations contribute to their formation. This phenomenon is observed both in pre–main-sequence and evolved B stars, although here I concentrate on main-sequence (MS) and post-MS Be stars. Recent evolutionary models of rapidly rotating massive stars have suggested that the Be phenomenon may be caused by an evolutionary spin-up toward the end of the MS lifetime. Alternatively, binary mass transfer may be responsible for the increase in rotational velocity that induces the Be star disks, although not all Be stars are observed in binary systems. A third possibility is that Be stars may be born as rapid rotators. To investigate these three Be star formation scenarios, I am performing a photometric survey of open clusters. In this work, I present the first results from 20 clusters. I do not find a strong relationship between the percentage of Be stars and the cluster age, but I do find more Be stars turning off the MS rather than close to the zeroage MS. This suggests that either evolutionary spin-up or binary mass transfer is responsible for the phenomenon. I also find a lower percentage of Be stars than other studies have found, although I show that this is probably due to the longterm variability of Be star disks and different sampling methods.