Nuclear-dominated accretion flows in two dimensions – II. Ejecta dynamics and nucleosynthesis for CO and ONe white dwarfs

Abstract

We study mass ejection from accretion disks formed in the merger of a white dwarf with a neutron star or black hole. These disks are mostly radiatively-inefficient and support nuclear fusion reactions, with ensuing outflows and electromagnetic transients. Here we perform time-dependent, axisymmetric hydrodynamic simulations of these disks including a physical equation of state, viscous angular momentum transport, a coupled 19-isotope nuclear network, and self-gravity. We find no detonations in any of the configurations studied. Our global models extend from the central object to radii much larger than the disk. We evolve these global models for several orbits, as well as alternate versions with an excised inner boundary to much longer times. We obtain robust outflows, with a broad velocity distribution in the range 10 2 − 10 4 km s −1 . The outflow composition is mostly that of the initial white dwarf, with burning products mixed in at the 10 − 30% level by mass, including up to ∼ 10 −2 M of 56 Ni. These heavier elements (plus 4 He) are ejected within 40 • of the rotation axis, and should have higher average velocities than the lighter elements that make up the white dwarf. These results are in broad agreement with previous one-and two-dimensional studies, and point to these systems as progenitors of rapidly-rising (∼ few day) transients. If accretion onto the central BH/NS powers a relativistic jet, these events could be accompanied by high energy transients with peak luminosities ∼ 10 47 − 10 50 erg s −1 and peak durations of up to several minutes, possibly accounting for events like CDF-S XT2.

SN 2019ehk: A Double-peaked Ca-rich Transient with Luminous X-Ray Emission and Shock-ionized Spectral Features

“…transition between the outer and inner regions due to the compositional structure on the stripped WD surface; and (iv) the observed low CSM velocities derived from early-time Hα and He II lines. It should also be noted that similar observables could be obtained from the disruption of HybCO (hybrid He/ C/O) WD by a NS (e.g., see Fernández et al 2019), although the rates associated with such binary systems are not consistent with CaSTs.…”

Magnetars from Neutron Star–White Dwarf Mergers: Application to Fast Radio Bursts

“…The explosive outcomes of NSWD mergers have been explored in the literature (Metzger 2012;Margalit & Metzger 2016, 2017Zenati et al 2019a,b;Fernández et al 2019), but the final remnants of these events have been little investigated (see Paschalidis et al 2011a,b;Margalit & Metzger 2016). Generally, there are two evolutionary pathways for NSWD binaries, which depend on the critical mass ratio q crit = M WD,crit /M NS , where M WD,crit is the critical WD mass and M NS is the NS mass.…”

Faint rapid red transients from neutron star–CO white dwarf mergers

“…Mergers of neutron stars (NSs) or black holes (BH) with white dwarfs (WDs) have been explored through several hydrodynamical and nuclear-hydrodynamical simulations, e.g. Fernández & Metzger (2013a); Bobrick et al (2017); Zenati et al (2019); Fernández et al (2019). All CO WD-NS mergers are expected to lead to unstable mass transfer (Bobrick et al 2017) where the WD is tidally disrupted on dynamical timescales and forms an extended debris disk around the NS (Papaloizou & Pringle 1984;Fryer et al 1999).…”