DISCRETE MASS-LOSS IN A LOW-MASS BINARY WITH A RAPIDLY SPINNING NEUTRON-STAR

We present the results of evolutionary computations for a low-mass binary (LMB) with a 1.4 M. neutron star (NS) as the compact companion and a 1 M. red dwarf as the donor star. At the moment when the Roche lobe is filled by the red dwarf, the orbital period of the system is 9.(h)4. The illumination of the donor star by energetic quanta and particles from the NS is taken into account. We focus on the regime of ''noncatastrophic'' heating, which occurs at the beginning of the mass-transfer phase when illumination by the pulsar activates mass loss by the donor star rather than vaporizing its outer layers. We also include the effect of sweeping the overflowing plasma out of the system by the magnetodipole radiation of the pulsar. We find that the evolution of such a system may result in the formation of an LMB with a rapidly rotating NS and a 0.2 M. red dwarf companion and having an orbital period similar to(6-7) hr. We also investigate the regime of episodic mass loss (at a super-Eddington rate) by the donor star, commencing at a late stage of evolution, before the donor star reaches the minimum mass similar to 0.2 M. and shrinks within its Roche lobe.