Computer Science
Scientific paper
Nov 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt.........6a&link_type=abstract
Ph.D dissertation, 2005. 202 pages; United States -- Ohio: The Ohio State University; 2005. Publication Number: AAT 3180892. DA
Computer Science
Binary Stars, Cataclysmic Variables, Blue Stragglers
Scientific paper
This work is dedicated to the study of the evolution of CVs and mergers of close binaries. In contrast to previous studies, this work uses modern empirical formulae for angular momentum loss by magnetized stellar winds. Two important differences with previous prescriptions include the saturation of angular momentum loss rate for fast rotators, and the existence of angular momentum loss in low mass, fully convective stars.
Changes in the treatment of angular momentum loss rate have a dramatic effect on the understanding of the evolution of CVs. The timescale for angular momentum loss ( J ) above the fully convective boundary is ~2 orders of magnitude longer than inferred from the older model, and the observed angular momentum loss properties show no evidence for a change in a behavior at the fully convective boundary. This provides evidence against the hypothesis that the period gap is caused by an abrupt change in the angular momentum loss law when secondary becomes fully convective and implies that the timescale for CV evolution is much longer than was thought, comparable to a Hubble time. It is demonstrated that when evolved secondaries are included, a spread in the secondary mass-orbital period plane comparable to that seen in the data is produced for either the saturated prescription for magnetic braking or the unsaturated model commonly used for CVs. It is argued that in order to explain this spread a considerable fraction of all CVs should have evolved stars as the secondaries. The predictions of my models are compared with diagnostics of the mass accretion rate in CVs and it is found that results are intermediate between the saturated and the older braking prescription. Taken together these suggest that either the angular momentum loss rate may be higher in CV secondaries than in single stars of the same rotation period, or that accretion happens in duty cycles. Possible origins of the Period Gap are discussed.
It is demonstrated that main-sequence mergers can account for the observed number of single blue stragglers in M67. Applied to the blue straggler population as a whole, this implies that such mergers are responsible for about one quarter of the population of halo blue metal poor stars, and at least one third of the blue stragglers in open clusters for systems older than 1 Gyr. The observed trends as a function of age are consistent with a saturated angular momentum loss rate for rapidly rotating tidally synchronized systems. The predicted number of blue stragglers from main sequence mergers alone is comparable to the number observed in globular clusters. A population of subturnoff mergers of order 3-4% of the upper main sequence population is also predicted for stars older than 4 Gyr, which is roughly comparable to the small population of highly Li-depleted halo dwarfs.
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