Black hole formation via hypercritical accretion during common-envelope evolution

Neutron stars inspiralling into a stellar envelope can accrete at rates vastly exceeding the Eddington limit if the flow develops pressures high enough to allow neutrinos to radiate the released gravitational energy. It has been suggested that this hypercritical mode of accretion leads inevitably to the formation of stellar mass black holes, with implications both for some gamma ray burst models and for the predicted event rate in future Laser Interferometer Gravitational-Wave Observatory (LIGO) observations. We study the hydrodynamics of this flow at large radii (R ≫ R_ns), and show that for low Mach number flows, in two dimensions, modest density gradients in the stellar envelope suffice to produce a hot, advection-dominated accretion disk around the accreting object. The fate of the neutron star depends critically on the highly uncertain nature and strength of outflows generated by such a disk. We argue that strong outflows are likely, in which case insufficient accretion occurs to force collapse to a black hole before the envelope has been ejected.