Neutron star formation by collapse of white dwarfs

Mass-accreting carbon-oxygen white dwarfs become thermally and dynamically unstable when they reach high enough central densities. Carbon ignition at the star's center likely propagates subsonically and, in the case of an initially solid core, leads to collapse if the rate of increase of the core's mass is sufficiently fast. Recent results indicate, however, that solidification of the core induces carbon-oxygen separation. The central regions are then made of pure oxygen while carbon is rejected to lower-density layers. Carbon ignition happens only after neutronization of the central (oxygen) regions. Collapse to a neutron star is then independent from the rate of mass increase and the only possible restrictions are set by the behaviour of the outer, accreted layers. X-ray sources, pulsars and Type I supernovae are likely outcomes of this process.