Irradiation and mass transfer in low-mass compact binaries
H. Ritter 1,
Z.-Y. Zhang 2,3 and
U. Kolb 4
Received 20 December 1999 / Accepted 18 May 2000
This paper studies the reaction of low-mass stars to anisotropic irradiation and its implications for the long-term evolution of compact binaries (cataclysmic variables and low-mass X-ray binaries).
First, we show by means of simple homology considerations that if the energy outflow through the surface layers of a low-mass main sequence star is blocked over a fraction of its surface (e.g. as a consequence of anisotropic irradiation) it will inflate only modestly, by a factor . The maximum contribution to mass transfer of the thermal relaxation of the donor star is times what one obtains for isotropic () irradiation. The duration of this irradiation-enhanced mass transfer is of the order of times the thermal time scale of the convective envelope. Numerical computations involving full 1D stellar models 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 in a detailed stability analysis that if mass transfer is not strongly enhanced by consequential angular momentum losses, cataclysmic variables are stable against irradiation-induced runaway mass transfer if the mass of the main sequence donor is . If systems may be unstable, subject to the efficiency of irradiation. Low-mass X-ray binaries, despite providing much higher irradiating fluxes, are even less susceptible to this instability.
If a binary is unstable, mass transfer must evolve through a limit cycle in which phases of irradiation-induced high mass transfer alternate with phases of small (or no) mass transfer. At the peak rate mass transfer proceeds on times the thermal time scale rate of the convective envelope. A necessary condition for the cycles to be maintained is that this time scale has to be much shorter () than the time scale on which mass transfer is driven.
Key words: accretion, accretion disks stars: binaries: close stars: evolution stars: interiors stars: novae, cataclysmic variables X-rays: stars
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© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000