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Astron. Astrophys. 360, 969-990 (2000)

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Irradiation and mass transfer in low-mass compact binaries

H. Ritter 1, Z.-Y. Zhang 2,3 and U. Kolb 4

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85740 Garching, Germany
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, P.R. China
3 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
4 Department of Physics and Astronomy, The Open University, Milton Keynes MK7 6AA, UK

Received 20 December 1999 / Accepted 18 May 2000

Abstract

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 [FORMULA] of its surface (e.g. as a consequence of anisotropic irradiation) it will inflate only modestly, by a factor [FORMULA]. The maximum contribution to mass transfer of the thermal relaxation of the donor star is [FORMULA] times what one obtains for isotropic ([FORMULA]) irradiation. The duration of this irradiation-enhanced mass transfer is of the order of [FORMULA] 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 [FORMULA]. If [FORMULA] 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 [FORMULA] 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 ([FORMULA]) 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

Send offprint requests to: hsr@mpa-garching.mpg.de

This article contains no SIMBAD objects.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
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