Faint Cataclysmic Variables in Quiescence: Globular Cluster and Field Surveys

Abstract

Current evolutionary models imply that most cataclysmic variables (CVs) have P_orb<2 hr and are dwarf nova (DN) systems that are quiescent most of the time. Observations of nearby quiescent DNs find that the UV spectrum is dominated by the hot white dwarf (WD), indicating that it provides a significant fraction of the optical light in addition to the quiescent disk and main-sequence companion. Hence, identifying a faint, quiescent CV in either the field or a globular cluster (GC) from broadband colors depends on our ability to predict the WD contribution in quiescence. We are undertaking a theoretical study of the compressional heating of WDs, extending down to very low time-averaged accretion rates, <M>~10^-11 M_solar yr^-1, which allows us to self-consistently find the T_eff of the WD. We demonstrate here that most of the compressional heating occurs in the freshly accreted envelope and that the WD core temperature reaches a fixed value on a timescale of less than typical evolutionary times. Since nuclear burning is unstable at these <M>'s, we have incorporated the recurrent heating and cooling of the WD core throughout the classical novae limit cycle in order to find the T_eff-<M> relations. Comparing with observations of field DNs confirms the <M>-P_orb relation of disrupted magnetic braking. We also predict broadband colors of a quiescent CV as a function of <M> and companion mass and show that this leads to the identification of what may be many CVs in deep Hubble Space Telescope images of GCs.