Dr. Szkody and her collaborators will conduct three projects on interacting binary stars. In the first, they will measure changes in the orbital period of the only known eclipsing binary of the AM CVn type; this will yield the rate of orbital energy lost through the emission of gravitational radiation. The second project will measure the temperatures and other properties of pulsating white dwarf stars to determine the location of the onset of instability. Finally, they will investigate certain other systems to measure changes in angular momentum with mass accretion, which may shed light on the formation of Type Ia supernovae.
These projects are expected to have an impact in several areas of binary evolution. For example, understanding the gravitational wave radiation from binaries will help distinguish signals of individual objects from cosmological signals in future gravitational wave experiments such as LISA (Laser Gravitational Wave Antenna). This team will also provide research and analysis opportunities for undergraduate students and high school teachers.
Cataclysmic variables are close binary systems in which mass is transferred from a cool star to a hot, high gravity white dwarf. The main goal of our three distinct projects was to understand the implications of the mass transfer and accretion on the long term evolution of systems of this type. These projects have broad applications in understanding the stability and evolution of binaries, and also help in modeling the production of a type of explosive supernova. We describe our projects below. 1] AMCVns are an extremely rare type of cataclysmic variable with the shortest orbital periods of any known class of binaries, ranging from periods near an hour to extreme cases at 5 minutes. Our first project involved the indirect detection of gravitational waves, emanating from the eclipsing compact interacting AMCVn binary, SDSS0926+3624. With the help of our collaborators, we have accomplished this goal of measuring the rate of decay of its binary orbital period with time to be (35.4 +/- 5.8)e-14 s/s. The attached image shows our latest O-C diagram with a timebase of observations from 2006 to 2013. We expect to add another season of observations by Spring of 2015 prior to final publication of our results. 2] We have acquired time-series photometry on the cataclysmic variable V455 And (HS2331+3905) since its discovery, thereby establishing a timebase of 11 years from 2003 to 2014. Using photometric observations from October 2007 to January 2014, we constrain the rate of change of its spin period with time to be (-9.2 +/- 6.0)e-15 s/s employing the O-C method, and (-7.2 +/- 6.0)e-15 s/s with a direct non-linear least squares fit. The large uncertainties imply that there is no significant detection of a changing spin period for the duration 2007--2014. V455 And underwent a large amplitude dwarf nova outburst, an episode of enhanced accretion, beginning on the 4th of September 2007. The data obtained prior to the outburst have relatively larger uncertainties, but reflect a period 3.5 +/- 1.2 microseconds longer than the best-fit post-outburst spin period. The angular momentum gained by the white dwarf from matter accreted during outburst as well as the white dwarf's slight subsequent shrinking should both cause the star to spin slightly faster after the outburst, theoretically estimated to be about 5 microseconds. This expectation is at par with the empirical constraint of 3.5 +/- 1.2 microseconds, thus providing preliminary evidence that the spin period is affected by an outburst at the level of a few microseconds. 3] The third aspect involved determining the location and width of the pulsational instability strip for accreting white dwarfs. We were involved in an observational search for pulsations in new systems to add more points to the scarcely populated instability strip, and to improve the statistics. Our work on the 16 known pulsating white dwarfs that are undergoing accretion have allowed us to delineate a much broader instability strip than the one for isolated non-interacting white dwarfs. Our second approach was to track how the pulsations ceased during episodes of enhanced accretion when the white dwarf was heated beyond the hot edge of the instability strip, and how pulsations resumed upon subsequent cooling to quiescence in a few years. We followed several systems such as GW Librae, SDSS0745+4538, SDSS0804+5103, as well as V455 And, throughout the period of this grant. We find that these systems can start and stop pulsations with no indication of large accretion events; all systems stop pulsating for a while after an outburst, but some return to pre-outburst instability in a few years while others remain hotter and show different instability patterns for timescales up to 7 years. We are hopeful these observational results will stimulate further theoretical studies of the instability process under conditions of faster rotation, and enhanced metals compared to single star evolution. This research involved training of 8 undergraduate students in observational and data analysis techniques (6 of which are currently in or will attend graduate school), graduate students from the University of Washington, University of Texas, the University of Warwick (United Kingdom) and the Victoria University of Wellington (New Zealand) and a postdoc from the University of Washington. The PI was also involved in a Journal Workshop for young scientists at the IAU in Beijing which promoted optimum scientific writing of journal papers in developing countries.
