Formation of Millisecond Pulsars with Heavy White Dwarf Companions: Extreme Mass Transfer on Subthermal Timescales.

We have performed detailed numerical calculations of the nonconservative evolution of close X-ray binary systems with intermediate-mass (2.0-6.0 M middle dot in circle) donor stars and a 1.3 M middle dot in circle accreting neutron star. We calculated the thermal response of the donor star to mass loss in order to determine its stability and follow the evolution of the mass transfer. Under the assumption of the "isotropic reemission model," we demonstrate that in many cases it is possible for the binary to prevent a spiral-in and survive a highly super-Eddington mass transfer phase (1<<M&d2;&solm0;M&d2;Edd<105) on a subthermal timescale if the convective envelope of the donor star is not too deep. These systems thus provide a new formation channel for binary millisecond pulsars with heavy CO white dwarfs and relatively short orbital periods (3-50 days). However, we conclude that to produce a binary pulsar with a O-Ne-Mg white dwarf or Porb approximately 1 day (e.g., PSR B0655+64) the above scenario does not work, and a spiral-in phase is still considered the most plausible scenario for the formation of such a system.