FORMATION OF BINARY MILLISECOND PULSARS BY ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS UNDER WIND-DRIVEN EVOLUTION

Abstract

Accretion-induced collapse of massive white dwarfs (WDs) has been proposed to be an important channel to form binary millisecond pulsars (MSPs). Recent investigations on thermal timescale mass transfer in WD binaries demonstrate that the resultant MSPs are likely to have relatively wide orbit periods ($\gtrsim 10$ days). Here we calculate the evolution of WD binaries taking into account the excited wind from the companion star induced by X-ray irradiation of the accreting WD, which may drive rapid mass transfer even when the companion star is less massive than the WD. This scenario can naturally explain the formation of the strong-field neutron star in the low-mass X-ray binary 4U 1822$-$37. After AIC the mass transfer resumes when the companion star refills its Roche lobe, and the neutron star is recycled due to mass accretion. A large fraction of the binaries will evolve to become binary MSPs with a He WD companion, with the orbital periods distributed between $\gtrsim 0.1$ day and $\lesssim 30$ days, while some of them may follow the cataclysmic variable-like evolution towards very short orbits. If we instead assume that the newborn neutron star appears as an MSP and part of its rotational energy is used to ablate its companion star, the binaries may also evolve to be the redback-like systems.