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.
Period of Support:3 yrs, 9/2008 - 9/2011. During the 3 years of this NSF grant, we carried out the following investigations which yielded the science described below. (1) We completed the first modeling analysis of the archival far ultraviolet spectra of the hot components in the AM CVn helium cataclysmics, using model helium accretion disks and model helium-rich atmospheres. The AM CVn systems have the shortest orbital periods (5 minutes to one hour!), and are the most compact of any known interacting double stars. Their spectra are dominated by helium spectral features arising from the accretion disk, a swirling disk of helium-rich gas surrounding the helium-rich white dwarf star (WD) which is accreting matter from the disk. The disk formed via mass transfer from a degenerate or semidegenerate helium donor star filling its Roche lobe. Objects like the AM CVn systems may be a significant channel for the production of Type Ia supernovae (the cosmological white dwarf supernovae that have been used to show that the expansion rate of the universe is accelerating. and can contribute up top 25% of the galactic Type Ia supernova rate (Nelemans et al 2001). Their mass transfer is driven by General Relativity, and the AM CVn binary systems are predicted to be a significant source of the low frequency gravitational radiation background (Warner 1995). We fixed the distance of each system at its parallax-derived value and adopted appropriate values of orbital inclination and white dwarf mass. We find that the accretion-heated "DO/DB" WDs are contributing significantly to the FUV flux in four of the systems (ES Ceti, CR Boo, V803 Cen, HP Lib, GP Com). In two of the systems, GP Com and ES Ceti, the WD dominates the FUV/NUV flux. We present model-derived accretion rates which agree with the low end of the range of accretion rates derived earlier from black body fits over the entire spectral energy distribution. We find that the WD in ES Ceti is very likely not a direct impact accretor but has a small disk. The WD in ES Ceti has Teff = 40,000±10,000K. This is far cooler than the previous estimate of Espaillat et al.(2005). We find that the WD in GP Com has Teff = 14,800 ± 500K, which is hotter than the previously estimated temperature of 11,000K. We present a comparison between our empirical results and current theoretical predictions for these systems. (2) We modeled the far ultraviolet (FUV) Hubble Space Telescope (HST), Far Ultraviolet Spectroscopic Explorer spectra (FUSE), and International Ultraviolet Explorer (IUE) spectra of cataclysmic variables of 76 accreting binaries: dwarf novae (DNe), nova-like variable stars (NL), and recurrent novae/symbiotics in quiescence/low optical brightness states, outburst/high optical brightness states and transitional states to obtain the surface temperatures (Teff), rotation rates (V sin i), surface gravity (log g),chemical abundances, possible additional components of FUV flux (accretion belts, rings, spots, or boundary layer) thus enlarging the sample of CV WDs. We also determined rates of mass accretion for selected samples of nova-like variables having IUE archival spectra and distances uniformly determined using an infrared method by Knigge (2006). Based upon two independent modeling approaches, we find no significant difference between the accretion rates of SW Sextantis systems and non-SW Sextantis nova-like systems insofar as optically thick disk models are appropriate. We find little evidence to suggest that the SW Sex stars have higher accretion rates than other nova-like CVs above the period gap within the same range of orbital periods. (3) We carried out 1D evolutionary model simulations with time-variable accretion to study the thermal response of the WD in VW Hyi, HT Cas, V455 And, GW Lib and other DN, to accretion during DN cycles and in comparing our derived accreting WD properties with theoretical predictions. We analyzed the X-ray emitting boundary layer (BL), the region between the inner disk edge and stellar surface, of the massive, very hot WD in RU Peg finding a 116s delay of the X-ray variations with respect to the UV variations from which we calculated the BL a viscosity parameter (0.003) and radius (1.4 Rwd). The BL viscosity is much smaller than the disk viscosity. The publications list for NSF RUI grant AST08-07892 consists of 29 refereed publications in the top five highest impact major journals, which have 18 different Villanova undergraduates as co-authors or lead authors, 24 conference papers and AAS poster paper abstracts In an educational sense, the broadest impact of this project has been the inspiration and training of undergraduate students via their key role in the analysis and synthetic spectral modeling of the data, presentation of poster papers at professional meetings and co-authoring refereed publications.
