Internal Conditions of Accreting White Dwarfs

We explain the physics of compressional heating of the deep interior of an accreting white dwarf (WD) at accretion rates low enough so that the accumulated hydrogen burns unstably and initiates a classical nova (CN). In this limit, the WD core temperature (Tc) reaches an equilibrium value (Tc,eq) after accreting an amount of mass much less than the WD’s mass. Once this equilibrium is reached, the compressional heating from within the envelope exits the surface. This equilibrium yields useful relations between the WD surface temperature, accretion rate and mass that can be employed to measure accretion rates from observed WD effective temperatures, thus testing binary evolution models for cataclysmic variables. 1. Overview and Context Cataclysmic variables (CV) are formed when the WD made during a common envelope event finally comes into contact with its companion as a result of gravitational radiation losses over a few Gyr (see [2]). The WD will have cooled during this time; a 0.20M⊙ He WD would have Tc = 3.3× 10 6 K at 4 Gyr [1], whereas a 0.6M⊙ C/O WD would have Tc = 2.5×10 K in 4 Gyr [4]. These give effective temperatures Teff ≈ 4500 − 5000 K. A subset of the CVs, called Dwarf Novae (DN), contain a WD accreting at low time-averaged rates 〈Ṁ〉 < 10M⊙ yr , where the accretion