A new technique for calculations of binary stellar evolution, with application to magnetic braking

The authors present a new computational technique for the study of the evolution of close binary systems with collapsed primaries and mass-losing secondaries. In this technique, for the secondary a composite polytrope with indices n = 3 and n = 3/2 is used to represent the structure of the radiative core and convective envelope respectively. All other relevant aspects of the binary stellar evolution are calculated in detail. A wide range of possible magnetic braking laws and an extended portion of the binary evolution is investigated. It is found in particular, that if magnetic braking effects cease or are strongly reduced at the time when the radiative core of the secondary vanishes, then the system should temporarily come out of contact; this effect may be relevant to the 2 - 3 hr "gap" in the orbital period distribution among cataclysmic variables. The authors also consider the potential importance of magnetic braking in driving the high mass transfer rates that are evidently necessary to account for the high X-ray luminosities of the bright galactic-bulge X-ray sources.