Measuring White Dwarf Accretion Rates via Their Effective Temperatures

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 ⟨ ⟩ = 10 -11 to 10 -8 M ☉ yr -1 . These ⟨ ⟩ values are appropriate to WDs in cataclysmic variables (CVs) of P orb ≲ 7 hr, many of which accrete sporadically as dwarf novae. Approximately 30 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 values into local time-averaged accretion rates, confirming the factor of 10 drop in ⟨ ⟩ predicted for CVs as they transit the period gap. For dwarf novae below the period gap, we show that if ⟨ ⟩ is that given by gravitational radiation losses alone, then the WD masses are greater than 0.8 M ☉ . An alternative conclusion is that the masses are closer to 0.6 M ☉ and ⟨ ⟩ 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 hr will have T eff values low enough for them to become nonradial pulsators, as discovered by van Zyl and collaborators for GW Lib.