Multidimensional Simulations of the Accretion‐induced Collapse of White Dwarfs to Neutron Stars

Abstract

We present 2.5-dimensional radiation-hydrodynamics simulations of the accretion-induced collapse (AIC) of white dwarfs, starting from two-dimensional rotational equilibrium configurations, thereby accounting consistently for the effects of rotation prior to and after core collapse. We focus our study on a 1.46 and a 1.92 M☉ a model. Electron capture leads to the collapse to nuclear densities of these cores a few tens of milliseconds after the start of the simulations. The shock generated at bounce moves slowly, but steadily, outward. 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 (5-8) × 10-3 M☉ s-1. The ejecta have an entropy on the order of (20-50)kB baryon-1 and an electron fraction that is bimodal. By the end of the simulations, at ≳600 ms after bounce, the explosion energy has reached (3-4) × 1049 ergs and the mass has reached a few times 10-3 M☉. We estimate the asymptotic explosion energies to be lower than 1050 ergs, 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 ``νμ'' and e neutrino luminosities are reduced compared to the νe neutrino luminosity; the deleptonized region has a butterfly shape; the neutrino flux and electron fraction depends strongly upon latitude (a la von Zeipel); and a quasi-Keplerian 0.1-0.5 M☉ accretion disk is formed.