- The merger of white dwarf-neutron star binaries: Simulations in full general relativity (arXiv)
Author : Vasilios Paschalidis, Yuk Tung Liu, Zacharias Etienne, Stuart L. Shapiro
Abstract: We present fully general relativistic (GR) simulations of the inspiral and merger white dwarf-neutron star (WDNS) binary. 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 (Mmax) that can support a cool, degenerate EOS against gravitational collapse. The time and length scales span several orders of magnitude, making fully general relativistic hydrodynamic (GRHD) simulations computationally prohibitive. For this reason, we model the WD as a “pseudo-white dwarf” (pWD) as in our WDNS head-on collision binary study [PRD83:064002,2011]. Our GRHD simulations of a pWDNS system with a WD of 0.98 solar masses and a NS of 1.4 solar masses show that the merger remnant is a rotating Thorne-Zytkow (TZlO)-type object surrounded by a massive disk. The final total rest mass exceeds Mmax, but the remainder does not immediately collapse. To estimate whether the object will eventually collapse after cooling, we introduce radiative thermal cooling. We first apply the cooling algorithm to TZlOs formed in WDNS head-on collisions and show that these objects collapse to form black holes on the cooling time-scale, as expected. However, when we cool the spin TZlO formed in the merger of a circular orbit WDNS binary, the remnant does not collapse, demonstrating that the differential spin support is sufficient to prevent collapse. As the final total mass exceeds Mmax, magnetic fields and/or viscosity may redistribute angular momentum and eventually lead to delayed collapse into a BH. We conclude that merging realistic massive WDNS binaries will likely lead to the formation of rotating TZlOs that undergo delayed collapse.
2. The ELM Research. II. Twelve White Dwarf Merger Binaries (arXiv)
Author : Mukremin Kilic, Warren R. Brown, Carlos Allende Prieto, MA Agueros, Craig Heinke, SJ Kenyon
Abstract: We describe new radial velocity and X-ray observations of extremely low-mass white dwarfs (ELM WDs, ~0.2 Msol) in the Sloan Digital Sky Survey Data Release 4 and the MMT Hypervelocity Star survey. We detect four new short-period binaries, including two merging systems. These observations bring the total number of short-period binaries detected in our survey to 20. No main-sequence or neutron star companions are visible in the available optical, radio, and X-ray photometry data. Thus, the companions are most likely WDs. Twelve of these systems will merge within a Hubble time due to gravitational wave radiation. We’ve now tripled the number of known merging WD systems. We discuss the characteristics of this merger sample and possible links to subluminous supernovae, extreme helium stars, AM CVn systems, and other merger products. We provide new observational tests of WD mass-period distribution and cooling models for ELM WDs. We also find evidence for a new formation channel for individual low-mass WDs through binary mergers of two lower-mass objects.