Date of Award
Doctor of Philosophy
Physics and Astronomy
Steven D. Kawaler
Subdwarf B (sdB) stars are horizontal branch stars with thin hydrogen envelopes surrounding a half solar mass helium core. Following their classification in 1966 by Greenstein, little progress was made in understanding these apparently common stars. When pulsators were discovered by Kilkenny et al. (1997), it became plausible that we could explore their interiors through asteroseismology. Our study of four well observed sdB stars, relying on standard stellar models and asteroseismology tools, has produced some new results and pinpointed wide gaps in our understanding of these stars.;We are able to identify specific pulsation modes in the coolest pulsating sdB star, Feige48. We then match these modes to a stellar model. We also confirm the work of Kawaler (1999) and his model match for PG1605. We were able to place a firm lower limit to the evolutionary timescale of Feige48 of 31 million years.;We also developed new tools for use in the asteroseismology of close binaries. Eclipsing binaries can uniquely identify modes of pulsation, provided the pulsation axis is aligned with the rotation axis. If the tidal force of the companion is stronger than the Coriolis force, the pulsation axis may align with the companion. From simulations, we determined that such a condition will present three signatures in the light curves and temporal spectrum, from which it is possible to uniquely identify pulsation modes. From preliminary data on PG1336, we find strong evidence that this pulsating sdB star in a 2.4 hour binary has a pulsation axis that points towards the companion.;Studies of sdB stars represent a rapidly progressing field in stellar astronomy. Through sdB stars, we may hope to constrain nuclear physics by measuring evolution rates driven by helium fusion. Such an understanding will filter down through stellar evolution in an increased understanding of giant branch and white dwarf stars. Subdwarf B stars themselves will place constraints on white dwarf evolution as they will evolve to distinctly lower mass white dwarfs than average.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Michael David Reed
Reed, Michael David, "Studies of subdwarf B stars " (2001). Retrospective Theses and Dissertations. 665.