White Dwarfs in Cataclysmic Variables

Prior to the last 15 years, the literature on cataclysmic variables (CVs) contained little on the properties of the underlying white dwarf accreter other than estimates of their masses. They were regarded simply as nondescript potential wells for studies of accretion disk structure and stability. Estimates of their masses and associated core compositions were discussed, but only in the context of thermonuclear runaway theory. With the advent of space spectroscopy, especially HST, IUE, EUVE, and HUT, direct spectroscopic observations of exposed white dwarfs in CVs were carried out during dwarf nova quiescence or low brightness states of nova-like variables and magnetic CVs when accretion rates were very low. This review covers new insights and physical properties of white dwarfs in nonmagnetic and magnetic CVs, including surface temperatures, heating and cooling measurements, rotational velocities, CV white dwarf masses (including Einstein redshift masses), photospheric chemical abundances (including composition relics of ancient novae), accretion belts, the physics of accretion heating, and long-term CV evolution. For an ensemble of 37 CV degenerates with secure temperatures, the average T_eff=20,800 K, for nonmagnetic CV degenerates T_eff=24,100 K, and for magnetic CV degenerates T_eff=16,400 K. The lowest T_eff values are associated with magnetic CVs and with systems whose orbital periods are less than 2 hr. Thermal e-folding times of the white dwarf envelope in response to accretion heating are in the range 6-600 days, photospheric chemical abundances range from moderately subsolar to greatly above solar, and rotational velocities lie within the range 50 km s^-1<Vsini<1200 km s^-1. If pre-CV white dwarfs had time to cool to 10^-3L_solar prior to the onset of Roche lobe overflow, then their average lower limit lifetime is 2.5x10^8 yr, and they have been accretion heated to an average luminosity log(L/L_solar)=1.90 or, on average, by ~11,000 K since the end of their pre-CV evolution phase.