White Dwarfs in Cataclysmic Variables: Probes of Evolution and Accretion Physics

Villanova University, Villanova PA

Investigators

Abstract

A cataclysmic variable is a compact binary (with a period less than twelve hours) in which the primary (a white dwarf) accretes matter and angular momentum from the secondary star (a main sequence star) filling its Roche-lobe. In non-magnetic systems, the matter is transferred, at continuous or sporadic rates, by means of an accretion disk around the white dwarf. Ongoing accretion at a low rate (quiescence) is interrupted every few weeks to months by intense accretion (outburst) of days to weeks, and every few thousand years by a thermonuclear explosion (a classical nova event). Thus the white dwarfs in these systems are probes of cataclysmic evolution and accretion physics because they bear the thermal, chemical and rotational imprint of their long-term accretion and explosive thermonuclear history. Here, Professor Sion, accompanied by collaborators and students, will examine white dwarfs in cataclysmic binaries and make measurements of their surface temperatures, rotation rates and chemistry of their accreted atmospheres, using multi-component synthetic spectral fitting. This will allow them to probe the age, evolutionary history, time-averaged accretion rate, white dwarf mass and core temperature, and the mass transfer driving mechanisms in these systems. In particular, measurements of photospheric chemical abundances will test accretion and diffusion theories, while overabundances of thermonuclear-processed elements could indicate pre-historical nova explosions or processed core material from an originally more massive secondary. The distribution of white dwarf rotation rates above and below the period gap, which will be explored here, is key to understanding the physics of angular momentum transfer during accretion. Through this project they will also enlarge the sample of white dwarfs with known surface temperatures across all cataclysmic variable subtypes. They will carry out evolutionary accretion simulations with time-variable accretion and thus provide a new and independent means of determining white dwarf masses, constraining white dwarf rotation velocities, and determining the thermal impact that prolonged accretion heating has on the white dwarfs. An understanding of the consequences of accretion in cataclysmic variables is the first step in a global understanding of accretion-related phenomena throughout the universe (e.g. around neutron stars and black holes) which cannot be easily observed. Also the ejected material into the interstellar medium from novae explosions, and hence the composition of the outer envelopes of white dwarfs in cataclysmic variables, tells us about which elements (metals) are expected to be recycled in the interstellar medium for the next generation of stars. In addition, some cataclysmic variable systems are possible progenitors for Type Ia supernovae and changes in our understanding of these accreting white dwarfs translates into change in the possible size of the population from which Type Ia supernovae arise. Throughout this project, undergraduate students will play key roles in the analysis and synthetic spectral modeling of the data. They will also participate in the dissemination of the results via presentation of poster papers at professional meetings and co-authoring refereed publications.