Time-Domain Studies of Hot Subdwarf Stars

Hot subdwarf B (sdB) stars represent one of the least-understood stages of stellar evolution, yet they dominate surveys of faint blue objects and are found in almost all Galactic stellar populations. Understanding these stars better would illuminate the enigmatic second parameter problem in globular cluster evolution, improve synthesis modeling of the UV upturn in giant elliptical galaxies, and constrain the physical properties of the dense plasmas present in these compact objects. Presumably, hot subdwarfs were once red giant branch stars stripped of their outer hydrogen layers, possibly due to interactions with binary companions. Searching for sdB binaries, determining their orbital parameters, and measuring the masses of their hot subdwarf components are some of the best ways to test binary formation models. Since pulsations offer the chance to measure sdB properties through a combination of spectral analysis and stellar seismology, we conducted a Southern-hemisphere survey with the PROMPT telescopes to extend the number of pulsators available for such studies. CS 1246, one of the pulsators discovered in our survey, shows a single, large-amplitude radial mode permitting the application of techniques rarely applied to other stars. Using time-resolved spectroscopy from the SOAR telescope, we employ the Baade-Wesselink method to derive a mass, radius, and distance for the star; this represents the first application of the method to a hot subdwarf. Pulse timing measurements from PROMPT over two years imply the presence of a previously-unknown, low-mass companion orbiting CS 1246 with a period of two weeks. We verify this hypothesis with radial velocity measurements of the orbital reflex motion, confirming for the first time that the rapid pulsations of sdB stars can be sufficiently accurate clocks for the detection of companions using the O-C diagram. Future applications of these techniques to other pulsating hot subdwarfs will help address the mysteries surrounding their formation histories.Doctor of Philosoph