Cool White Dwarfs in Cataclysmic Variables: Hubble Space Telescope Results on EG Cancri and HV Virginis

Abstract

Our Hubble Space Telescope (HST) ultraviolet spectra of the low mass transfer rate cataclysmic variables EG Cnc and HV Vir at 4 yr and 8 yr past their last superoutburst have shown that the white dwarfs in these systems are among the coolest yet observed in disk-accreting close binaries. The UV-optical fluxes of EG Cnc are consistent with a 11,700-13,000 K white dwarf at a distance of 350-475 pc, while those of HV Vir fit with a 12,500-14,000 K white dwarf at 400-550 pc, with logg=8.0+/-0.5 for both stars. In each case, the metal abundance is near 0.3 solar, while the rotation rates are considerably less than breakup. If these systems formed above the period gap and evolved to their short periods in an evolution time of several Gyr, then their white dwarfs are hotter than field white dwarfs of comparable age and hotter than those in close binary systems of similar orbital period that contain magnetic white dwarfs. If these systems have indeed passed the period minimum in their evolution, the implication is that they are significantly heated by the long-term effects of mass transfer from their close companions. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555 and with the Apache Point Observatory (APO) 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium (ARC).