CATACLYSMIC VARIABLE PRIMARY EFFECTIVE TEMPERATURES: CONSTRAINTS ON BINARY ANGULAR MOMENTUM LOSS

Abstract

We review the most decisive currently available measurements of the surface\neffective temperatures, Teff, of white dwarf (WD) primaries in cataclysmic\nvariables (CVs) during accretion quiescence, and use these as a diagnostic for\ntheir time averaged accretion rate, <Mdot>. Using time-dependent calculations\nof the WD envelope, we investigate the sensitivity of the quiescent Teff to\nlong term variations in the accretion rate. We find that the quiescent Teff\nprovides one of the best available tests of predictions for the angular\nmomentum loss and resultant mass transfer rates which govern the evolution of\nCVs. While gravitational radiation is sufficient to explain the <Mdot> of\nstrongly magnetic CVs at all Porb, faster angular momentum loss is required by\nthe temperatures of dwarf nova primaries (non-magnetic systems). This provides\nevidence that a normal stellar magnetic field structure near the secondary is\nessential for the enhanced braking mechanism to work, supporting the well-known\nstellar wind braking hypothesis. The contrast in <Mdot> is most prominent for\norbital periods Porb > 3 hours, above the period gap, but a modest enhancement\nis also present at shorter Porb. The averaging time which <Mdot> reflects is as\nmuch as 10^5 years for low-<Mdot> systems and as little as 10^3 years for\nhigh-<Mdot> systems. We discuss the security of conclusions drawn about the CV\npopulation in light of these time scales and our necessarily incomplete sample\nof systems. Measurements for non-magnetic systems above the period gap fall\nbelow predictions from traditional stellar wind braking prescriptions, but\nabove more recent predictions with somewhat weaker angular momentum loss. We\nalso discuss the apparently high Teff's found in the VY Scl stars. (abridged)\n