Properties of discontinuous and nova-amplified mass transfer in cataclysmic variables

Abstract

We investigate the effects of discontinuous mass loss in recurrent outburst events on the long-term evolution of cataclysmic variables (CVs). Similarly we consider the effects of frictional angular momentum loss (FAML), i.e. interaction of the expanding nova envelope with the secondary. The Bondi-Hoyle accretion model is used to parametrize FAML in terms of the expansion velocity vexp of the nova envelope at the location of the secondary; we find that small vexp causes strong FAML. Numerical calculations of CV evolution over a wide range of parameters demonstrate the equivalence of a discontinuous sequence of nova cycles and the corresponding mean evolution (replacing envelope ejection by a continuous wind), even close to the mass-transfer instability. A formal stability analysis of discontinuous mass transfer confirms this, independent of details of the FAML model. FAML is a consequential angular momentum loss that amplifies the mass-transfer rate driven by systemic angular momentum losses such as magnetic braking. We show that for a given vexp and white dwarf mass the amplification increases with secondary mass and is significant only close to the largest secondary mass consistent with mass-transfer stability. The amplification factor is independent of the envelope mass ejected during the outburst, whereas the mass-transfer amplitude induced by individual nova outbursts is proportional to it. In sequences calculated with nova model parameters taken from Prialnik & Kovetz, FAML amplification is negligible, but the outburst amplitude in systems below the period gap with a white dwarf mass ≃ 0.6 M⊙ is larger than a factor of 10. The mass-transfer rate in such systems is smaller than 10−11 M⊙ yr−1 for ≃ 0.5 Myr (≃ 10 per cent of the nova cycle) after the outburst. This offers an explanation for intrinsically unusually faint CVs below the period gap.