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 computed the evolutionary sequences when, at the beginning of mass transfer, 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 incorporate neutron star magnetic field decay. The rotational evolution of a neutron star is presented for the cases when, at the beginning of accretion 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 evaporation of the mass-losing star which reduces the duration of the accretion 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 the rapidly rotating neutron star, and heating of the red dwarf photosphere by hard X- and gamma-rays generated in a neutron star magnetosphere sets in.