Merger of binary white dwarf-neutron stars: Simulations in full general relativity

We perform fully general relativistic simulations to address the inspiral and merger of binary white dwarf-neutron stars. The initial binary is in a circular orbit at the Roche critical separation. The goal is to determine the ultimate fate of such systems. We focus on binaries whose total mass exceeds the maximum mass (M-max) a cold, degenerate equation of state can support against gravitational collapse. The time and length scales span many orders of magnitude, making fully general relativistic hydrodynamic simulations computationally prohibitive. For this reason, we model the white dwarf as a pseudo-white dwarf as in our binary white dwarf-neutron star (WDNS) head-on collisions study [V. Paschalidis, Z. Etienne, Y. T. Liu, and S. L. Shapiro, Phys. Rev. D 83, 064002 (2011)]. Our general relativistic hydrodynamic simulations of a pseudo-WDNS (pWDNS) system with a 0.98M(circle dot) white dwarf and a 1.4M(circle dot) neutron star show that the merger remnant is a spinning Thorne-Zytkow-like object (TZlO) surrounded by a massive disk. The final total rest mass exceeds M-max, but the remnant does not collapse promptly. To assess whether the object will ultimately collapse after cooling, we introduce radiative thermal cooling. We first apply our cooling algorithm to TZlOs formed in pWDNS head-on collisions, and show that these objects collapse and form black holes on the cooling time scale, as expected. However, when we cool the spinning TZlO formed in the merger of a circular-orbit pWDNS binary, the remnant does not collapse, demonstrating that differential rotational support is sufficient to prevent collapse. Given that the final total mass exceeds Mmax for our cold equation of state, magnetic fields and/or viscosity may redistribute angular momentum, ultimately leading to delayed collapse to a black hole. We infer that the merger of realistic massive WDNS binaries likely will lead to the formation of spinning TZlOs that undergo delayed collapse.