FORMATION OF LOW-MASS BINARIES WITH MILLISECOND PULSARS

An evolutionary scheme is considered which results in the formation o a ow-mass binary (with orbital period 3-6 h) consisting of a red dwarf of mass < 0.3 M. and a rapidly rotating neutron star with spin-period approximately few ms and surface magnetic field < 10(9) G. We have co mputed the evolutionary sequences when, at the beginning of mass trans fer, the donor star has a mass of 1 and 0.5 M., and the orbital period of the system is 9.4 and 4.5 h respectively. Our computations incorpo rate neutron star magnetic field decay. The rotational evolution of a neutron star is presented for the cases when, at the beginning of accr etion on to the neutron star, the magnetic field is 2 x 10(8), 5 x 10( 8) or 10(9) G, and has a characteristic decay time of more than 10(9) yr. One of the main ingredients of our scheme is the stage of evaporat ion of the mass-losing star which reduces the duration of the accretio n stage to a few times 10(7) yr. The stage of evaporation begins when the infalling plasma is pushed out by the magnetodipole radiation of t he rapidly rotating neutron star, and heating of the red dwarf photosp here by hard X- and gamma-rays generated in a neutron star magnetosphe re sets in.