My work on cataclysmic variables started with an analysis of a large number of Einstein IPC observations, in search of periodicities in their X-ray emission, and fitting models to the observed spectra. The scientific issue was the origin of X-ray emission in cataclysmic binaries, and in particular, the production site and emission mechanism. The conclusion from the search for periodicities was that the majority of the systems show no periodic modulation of their X-ray flux, which rules out models of a magnetically-controlled accretion flow. The study of X-ray spectra has shown them to be consistent with models of X-ray emission from a boundary layer between the accretion disk and the white dwarf, and has ruled out the corona of the companion star as a likely production site.
AE Aquarii is unique among CVs because of its rapid oscillations, and its ill-understood large flares. After analyzing a large set of UV spectra obtained with the HST we found that the rapid oscillations most likely originate in hot spots at the magnetic poles of the rapidly rotating white dwarf These are the result of heating of the magnetic polar caps by X-rays from the accretion columns. Our HST observations combined with other available data on the system suggest that the white dwarf in AE Aquarii is a magnetic propeller which ejects most of the matter transferred from its companion star, allowing only a small fraction to accrete. This mode of mass transfer, which is though to represent a very brief phase in the evolution of such systems, has been hypothesized for quite some time in the context of low-mass X-ray binaries. However, this is the first cataclysmic variable in which a good case can be made for it. We are now following up on the results of this project by undertaking a detailed modeling of the line-emission mechanisms in the system.
Mass outflows are a ubiquitous property of accretion-powered systems. These can take the form of winds or, in their most dramatic manifestation, powerful relativistic jets in active galaxies. To study the formation of outflows in cataclysmic variables I have carried out UV spectroscopic observations of the prototypical magnetic system, DQ Herculis, with the HST. An important question that these observations are meant to address is whether the accreting white dwarf in DQ Herculis is producing a wind. This is a test of models for the formation of accretion disk winds because in the particular case of DQ Herculis, a strong magnetic field on the white dwarf prevents the formation of an inner disk, and a disk/white dwarf boundary layer which according to some models is the origin of the energetic photons that accelerate the wind. Our detection of a wind in this system supports the recent view that accretion disk winds are accelerated over the entire surface of the disk and not just in the innermost, hottest parts.
I am also interested in the dynamics of eccentric accretion disks in ultrashort period, interacting binaries, These systems have periods of order an hour or less and a full orbital cycle can be easily observed with the Hobby-Eberly telescope. The large collecting area of the telescope allows one to obtain high quality spectra in short exposure times and hence produce maps of the systems and their disks. Dynamical phenomena such as spiral waves can be also searched for with the same technique. I am pursuing these questions as well as others (e.g., modeling of the atmospheres of Helium-rich accretion disks, such as that of AM CVn) in collaboration with Richard Wade. We have secured observing time of the HST for high-speed UV spectroscopy of AM CVn in order to model its atmospheric spectrum and study the dynamics of its accretion disk. we are complementing these data with contemporaneous observations with the Hobby-Eberly Telescope.