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Abstract:

Using Eggleton's code, we performed a series of binary evolution calculations in order to investigate the criterion for dynamical instability of mass transfer in binaries. In these calculations; we took the donor's mass on the zero-age main sequence (ZAMS) from 0.8 to 1.9 MD. For each mass, we systematically varied the mass of the core at the beginning of mass transfer and the mass of the companion star. We assumed that mass transfer was completely non-conservative and that all the mass that was lost from the system carried with it the orbital angular momentum of the accreting component. We found that the critical mass ratio, above which mass transfer is dynamically unstable, is from 1.1 to 1.3 in these red-giant binary systems.

Abstract:

Irradiation of the secondaries in close binary systems affects their appearance and can drastically change their internal structure and hence long-term evolution.. In this paper we review the main effects of external irradiation in close binaries with compact primaries, such as the distortion of the shape of the secondary and the conditions for the occurrence of outer critical configurations; and apply these to normal Xray binaries,, including systems such as HZ Her/Her X-1, Vela X-1 and black-hole binaries during outburst (e.g. Nova Sco). Particular emphasis will be given to the role of circulation driven by the external heating and the radiative surface stress: We have developed a three-dimensional fluid dynamics code for modelling these effects: We present initial results of self-consistent calculations for the circulation in irradiated systems and show how the inclusion of these effects is vital to the understanding and interpretation of any system where external irradiation of the secondary is. significant.