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We simulate head-on collisions from rest at large separation of binary whitedwarf -- neutron stars (WDNSs) in full general relativity. Our study serves asa prelude to our analysis of the circular binary WDNS problem. We focus oncompact binaries whose total mass exceeds the maximum mass that a colddegenerate star can support, and our goal is to determine the fate of suchsystems. A fully general relativistic hydrodynamic computation of a realisticWDNS head-on collision is prohibitive due to the large range of dynamical timescales and length scales involved. For this reason, we construct an equation ofstate (EOS) which captures the main physical features of NSs while, at the sametime, scales down the size of WDs. We call these scaled-down WD models"pseudo-WDs (pWDs)". Using pWDs, we can study these systems via a sequence ofsimulations where the size of the pWD gradually increases toward the realisticcase. We perform two sets of simulations; One set studies the effects of the NSmass on the final outcome, when the pWD is kept fixed. The other set studiesthe effect of the pWD compaction on the final outcome, when the pWD mass andthe NS are kept fixed. All simulations show that 14%-18% of the initial totalrest mass escapes to infinity. All remnant masses still exceed the maximum restmass that our cold EOS can support (1.92 solar masses), but no case leads toprompt collapse to a black hole. This outcome arises because the finalconfigurations are hot. All cases settle into spherical, quasiequilibriumconfigurations consisting of a cold NS core surrounded by a hot mantle,resembling Thorne-Zytkow objects. Extrapolating our results to realistic WDcompactions, we predict that the likely outcome of a head-on collision of arealistic, massive WDNS system will be the formation of a quasiequilibriumThorne-Zytkow-like object.

Head-on collisions of binary white dwarf--neutron stars: Simulations in full general relativity