Irradiation and mass transfer in low-mass compact binaries

Abstract

We study the reaction of low-mass stars to anisotropic irradiation and its importance for the long-term evolution of compact binaries binaries. We show by means of a simple homology model that if the energy outflow through the surface layers of a low-mass main sequence star is blocked over a fraction s_eff < 1 of its surface it will inflate only modestly and that the maximum contribution to mass transfer is s_eff times what one obtains in the isotropic case. The duration of this mass transfer phase is roughly -0.1 ln(1-s_eff) times the thermal time scale of the convective envelope. Numerical computations confirm these results. Second, we present a simple analytic one-zone model for computing the blocking effect by irradiation which gives results in acceptable quantitative agreement with detailed numerical computations. Third, we show that unless mass transfer is strongly enhanced by consequential angular momentum losses, cataclysmic variables are stable against mass transfer if the mass of the main sequence donor is less than about 0.7 solar masses. Otherwise systems may be unstable, subject to the efficiency of irradiation. We argue that low-mass X-ray binaries are even less susceptible to this instability. In case of instability, mass transfer must evolve through a limit cycle in which phases of high mass transfer alternate with phases of small (or no) mass transfer.