Cataclysmic variable evolution - Clues from the underlying white dwarf

Abstract

The present ensemble of empirically determined (upper limit) cataclysmic variable (CV) effective temperatures reveals a distribution centered near 16 000 K [or a luminosity log (L/L⊙) = -2.5], If core cooling is not accelerated due to erosion of the white dwarf mass by repeated nova explosions, then this distribution implies a mean lower limit total cooling lifetime of 5 × 108 yr for the majority of CV degenerates. If the CV systems were born as detached systems with the timescale to Roche lobe overflow, τpcv ≳ 109 yr, then all such CV degenerates began mass accretion with white dwarf luminosities ≲ 10-3 L⊙ and therefore may have undergone envelope heating, on average, to 10-2 or 10-25 L⊙. If CV systems are born semidetached or if τpcv < 109 yr, then 5 × 108 yr is a typical lower limit CV lifetime. The empirically determined lower limit lifetimes of the CV systems with exposed white dwarfs appear consistent with the McDermott & Taam [ApJ, 342, 1019 (1989) ] timescales for evolution from above the gap to below the gap, down to a limiting orbital period of 80 min. The paucity of hot CV degenerates below the period gap, the absence of cool CV degenerates above the gap (though the latter is possible due to selection) and the general slope of the distribution of observed temperatures relative to the evolutionary tracks of McDermott & Taam (1989) provides some evidence, though far from conclusive evidence, that CV systems evolve across the period gap. The coolest CV degenerates appear to be found among the strongly magnetic AM Her CV's rather than nonmagnetic CVs. At present there is only weak evidence to suggest that magnetic CV degenerates may cool differently than nonmagnetic CV degenerates.