Measuring White Dwarf Accretion Rates via their Effective Temperatures

Abstract

Our previous theoretical study of the impact of an accreting envelope on the thermal state of an underlying white dwarf (WD) has yielded equilibrium core temperatures, classical nova ignition masses and thermal luminosities for WDs accreting at time averaged rates of <Mdot> = 10^-11 - 10^-8 Msun/yr. These <Mdot>'s are appropriate to WDs in cataclysmic variables (CVs) of P_orb <~ 7 hr, many of which accrete sporadically as Dwarf Novae. Approximately thirty nonmagnetic Dwarf Novae have been observed in quiescence, when the accretion rate is low enough for spectral detection of the WD photosphere, and a measurement of T_eff. We use our theoretical work to translate the measured T_eff's into local time-averaged accretion rates, confirming the factor of ten drop in <Mdot> predicted for CV's as they transit the period gap. For DN below the period gap, we show that if <Mdot> is that given by gravitational radiation losses alone, then the WD masses are > 0.8 Msun. An alternative conclusion is that the masses are closer to 0.6 Msun and <Mdot> is 3-4 times larger than that expected from gravitational radiation losses. In either case, it is very plausible that a subset of CVs with P_orb < 2 hours will have T_eff's low enough for them to become non-radial pulsators, as discovered by van Zyl and collaborators in GW Lib.