Magnetically-driven explosions of rapidly-rotating white dwarfs following Accretion-Induced Collapse

Abstract

We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD) simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white dwarf. We focus on the dynamical role of MHD processes after the formation of a millisecond-period protoneutron star. We find that including magnetic fields and stresses can lead to a powerful explosion with an energy of a few Bethe, rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of ~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within 500ms after bounce, and the rotational energy extracted from the core is channeled into magnetic energy that generates a strong magnetically-driven wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta, while this wind prevails, precludes it from becoming relativistic. This suggests that a GRB is not expected to emerge from such AICs during the early protoneutron star phase, except in the unlikely event that the massive white dwarf has sufficient mass to lead to black hole formation. In addition, we predict both negligible 56Ni-production (that should result in an optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich nuclei puts strong constraints on the possible rate of such AICs. Moreover, being free from ``fallback,'' such highly-magnetized millisecond-period protoneutron stars may later become magnetars, and the magnetically-driven winds may later transition to Poynting-flux-dominated, relativistic winds, eventually detectable as GRBs at cosmological distances. However, the low expected event rate of AICs will constrain them to be, at best, a small subset of GRB and/or magnetar progenitors.