Multi-Dimensional Simulations of the Accretion-Induced Collapse of White Dwarfs to Neutron Stars

Abstract

We present 2.5D radiation-hydrodynamics simulations of the accretion-induced collapse (AIC) of white dwarfs, starting from 2D rotational equilibrium configurations of a 1.46-Msun and a 1.92-Msun model. Electron capture leads to the collapse to nuclear densities of these cores within a few tens of milliseconds. The shock generated at bounce moves slowly, but steadily, outwards. Within 50-100ms, the stalled shock breaks out of the white dwarf along the poles. The blast is followed by a neutrino-driven wind that develops within the white dwarf, in a cone of ~40deg opening angle about the poles, with a mass loss rate of 5-8 x 10^{-3} Msun/yr. The ejecta have an entropy on the order of 20-50 k_B/baryon, and an electron fraction distribution that is bimodal. By the end of the simulations, at >600ms after bounce, the explosion energy has reached 3-4 x 10^{49}erg and the total ejecta mass has reached a few times 0.001Msun. We estimate the asymptotic explosion energies to be lower than 10^{50}erg, significantly lower than those inferred for standard core collapse. The AIC of white dwarfs thus represents one instance where a neutrino mechanism leads undoubtedly to a successful, albeit weak, explosion. We document in detail the numerous effects of the fast rotation of the progenitors: The neutron stars are aspherical; the ``nu_mu'' and anti-nu_e neutrino luminosities are reduced compared to the nu_e neutrino luminosity; the deleptonized region has a butterfly shape; the neutrino flux and electron fraction depend strongly upon latitude (a la von Zeipel); and a quasi-Keplerian 0.1-0.5-Msun accretion disk is formed.