Binary evolution is performed by the rapid binary-star evolution (BSE)
Circularization of eccentric orbits and synchronization of stellar rotation
with the orbital motion owing to tidal interaction is modelled in detail.
Angular momentum loss mechanisms, such as gravitational radiation and magnetic
braking, are also modelled.
Wind accretion, where the secondary may accrete some of the material lost from
the primary in a wind, is allowed with the necessary adjustments made to the
orbital parameters in the event of any mass variations.
Mass transfer also occurs if either star fills its Roche lobe and may proceed
on a nuclear, thermal or dynamical time-scale.
In the latter regime, the radius of the primary increases in response to
mass-loss at a faster rate than the Roche-lobe of the star.
Stars with deep surface convection zones and degenerate stars are unstable to
such dynamical time-scale mass loss unless the mass ratio of the system is
less than some critical value.
The outcome is a common-envelope event if the primary is a giant star.
This results in merging or formation of a close binary, or a direct merging
if the primary is a white dwarf or low-mass main-sequence star.
On the other hand, mass transfer on a nuclear or thermal time-scale is
assumed to be a steady process.
Prescriptions to determine the type and rate of mass transfer, the response of
the secondary to accretion and the outcome of any merger events are in place in
BSE and the details can be found in
the BSE paper:
Hurley, Tout & Pols (2002).