Multi-dimensional simulations of the accretion-induced collapse of white dwarfs to neutron stars

We have performed 2.5D radiation-hydrodynamics simulations of the accretion-induced collapse (AIC) of white dwarfs, starting from 2D rotational equilibrium configurations of a 1.92-M_⊙ model. Electron capture leads to the collapse to nuclear densities of the core within a few tens of milliseconds. The shock generated at bounce moves slowly, but steadily, outwards. Within 50-100 ms, 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 ∼ 40° opening angle about the poles, with a mass loss rate of 5 × 10^-3M _⊙yr^-1. 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 ≥600 ms after bounce, the explosion energy has reached 3 × 10⁴⁹ erg and the total ejecta mass has reached a few times 0.001 M_⊙. We estimate the asymptotic explosion energies to be slightly lower than 10⁵⁰ 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 summarize the numerous effects of the fast rotation of the progenitor: The neutron star is aspherical; the "ν_μ" and ν̄_e neutrino luminosities are reduced compared to the ν_e neutrino luminosity; the deleptonized region has a "butterfly" shape; the neutrino flux and electron fraction depend strongly upon latitude (à la von Zeipel); and a quasi-Keplerian 0.5-M_⊙ accretion disk is formed.