Cold C + O white dwarfs and neutron stars

Abstract

Neutron stars in low mess binary systems (QPOs and other galactic bulge X-ray sources, millisecond pulsars, wide binary pulsars, globular cluster X-ray sources) pose a problem as to their origin. Since neither collapse of the evolved core of massive stars (the systems would not survive the attending explotions) nor capture of a previously formed neutron star (star densities too low and/or relative velocities too large in the corresponding regions) can explain those systems, direct collapse of a white dwarf into a neutron star has to be invoked. Rather than postulating evolutionary scenarios involving hypothetical (and in any case very rare) ONeMg white dwarfs, we show CO white dwarfs to be the most natural candidates to the progenitors of such neutron stars. Cool CO white dwarfs totally or partially solid can collapse upon mess accretion for a sizeable range of the relevant parameters (initial messes and temperatures, accretion rates). This conclusion is shown to be independent from the hypothesis made as to the chemical Composition of solid CO mixtures (although the range leading to colsapse depends upon it). The results of dynamical calculations of such collapses are briefly summarized.