Binary evolution and formation of binary millisecond pulsars.

The high incidence of binaries among millisecond pulsars in combination with rapid spin and relatively weak magnetic fields strongly suggest a large fraction of them (if not all) are old neutron stars that have been 'recycled' by accretion of mass and angular momentum from a companion star (Bhattacharya and van den Heuvel, 1991, Phys. Rep., 203, 1). We investigate the formation of binary millisecond pulsars via simulations of the evolution of ZAMS-stars in binary systems. Formation processes of neutron stars in interacting binaries, and the subsequent evolution of such systems, are discussed. The effects of various mass-transfer mechanisms in a binary system, different mass-loss rates from stellar winds (Vanbeveren and De Greve, 1979, Astron. Astrophys., 77, 295) and asymmetry in supernova explosions (e.g. Bailes, 1989, Astrophys. J., 342, 917); Hills, 1983, Astrophys. J., 267, 322) are discussed. We regard only pulsars formed in normal type II supernova explosions, but do not exclude the formation of neutron stars via coalescence of white dwarfs. We suggest that, depending on the conditions that lead to evolution via spiral-in and a common envelope (CE), binary millisecond pulsars are likely to originate from binary systems containing a primary main-sequence star with initial mass in the range approximately 10-15 stellar mass and a secondary main-sequence star with initial mass less than 1.5 stellar mass. Further, we present evidence (depending on the physical assumptions used during the CE evolution) for a correlation betweeen the orbital period and the recoil velocity of evolved binary systems containing a millisecond radio pulsar and a white dwarf companion.