Cold C + O white dwarfs and neutron stars

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) ONeMg white dwarfs, we show CO white dwarfs to be the most natural candidates to the progenitors of such neutron stars. Cool CO 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 CO mixtures (although the range leading to colsapse depends upon it). The results of dynamical calculations of such collapses are briefly summarized. © 1988.