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Normal star evolution

We consider stars with a constant (solar) initial chemical composition. Before the Roche lobe overflow, the components evolve as single stars (see for details LPP96). Analytical approximation describing evolutionary tracks are used. For massive stars, we use two types of evolutionary tracks: those calculated by the Geneva group (Schaller et al., 1992) with high stellar wind mass-loss (up to 90 per cent of the initial mass), and those calculated earlier by different groups assuming low stellar wind mass-loss (up to 30 per cent of the initial mass).

When a star fills its Roche lobe, the mass transfer time-scales are treated differently depending on the star's mass and time it fills the Roche lobe. For the most close binaries we aslo calculate Roche lobe overfilling due to angular momentum loss by magnetic stellar wind or gravitational radiation.

If the mass transfer onto a normal star occurs on a time-scale ten time shorter than the thermal Kelvin-Helmholz time for this star or a compact star is engulfed by a giant companion (e.g. as a result of the kick), the common envelope (CE) stage of the binary evolution is set in (Paczynski 1976). The CE stage is treated conventionally by by introducing a parameter that measures what fraction of the system's orbital energy goes, between the beginning and the end of the spiralling-in process, into the binding energy (gravitational minus thermal) of the ejected common envelope. Spiral-in during the CE stage can result in the binary coalescence. In the case of coalescence of the normal star with a compact object (NS or BH) a Thorne-Zytkow object is formed.

Sergei B. Popov
Fri Jun 21 19:17:07 MSD 1996