The bifurcation periods in low - mass X - ray binaries : the effect of magnetic braking and mass loss

The bifurcation period in low-mass X-ray binaries is the initial orbital period which separates the formation of converging systems (which evolve with decreasing orbital periods until the donor becomes degenerate) from the diverging systems (which evolve with increasing orbital periods until the donor star loses its envelope and a wide detached binary is formed). We calculate systematically the bifurcation periods of binary systems with a 1.4M⊙ neutron star and a 0.5 − 2M⊙ donor star, taking into account different kinds of magnetic braking and mass loss mechanisms. Our results show that the saturated magnetic braking can considerably decrease the values of bifurcation period compared to the traditional magnetic braking, while the influence of mass loss mechanisms on bifurcation periods is quite weak. We also develop a semi-analytical method to compute the bifurcation period, the result of which agrees well with the numerical method in the leading order. Subject headings: binaries: close – stars: evolution – X-rays: binaries