The Head-on Collision of a Neutron Star with a White Dwarf

TL;DR

This paper investigates the physical processes and outcomes of head-on collisions between neutron stars and white dwarfs, exploring conditions for thermonuclear explosions and formation of Thorne-Zytkow-like objects.

Contribution

It provides a detailed analysis of collision outcomes for different WD types, identifying conditions for supernovae and exotic objects, which was not comprehensively studied before.

Findings

NS-WD collisions can trigger thermonuclear explosions under certain conditions.

Low-mass CO and ONe WDs can form Thorne-Zytkow-like objects if specific parameters are met.

Conditions for supernovae and TZlO formation depend on WD type and collision parameters.

Abstract

We have computed the physical processes involved in a head-on collision between a neutron star (NS) and a white dwarf (WD). The outcomes of such collisions vary depending on the mass and type of the WD. We have separately examined the dynamical processes for collisions between NSs and helium WDs (He-WDs), carbon-oxygen WDs (CO-WDs), and oxygen-neon WDs (ONe-WDs). We aim to investigate whether the collision can trigger a thermonuclear explosion of the WD, and if not, whether the NS can remain bound within the WD to form a Thorne-Zytkow-like object (TZlO). For a thermonuclear explosion to occur, at least two conditions must be satisfied: (i) the collision-induced temperature must reach the ignition threshold of the relevant nuclear reactions, and (ii) the burning material must remain in a degenerate state. For different types of WDs, there exist parameter ranges where both conditions are fulfilled, implying that NS-WD collisions can indeed induce thermonuclear explosions, leading to sub-Chandrasekhar Type Ia supernovae or other exotic optical transients powered by thermonuclear explosion. On the other hand, the formation of a TZlO requires that the WD material exerts sufficient drag on the NS to prevent its escape, while the interaction must not trigger a thermonuclear explosion of the WD. Our results indicate that such conditions can be realized in the case of low-mass CO-WDs and low-mass ONe-WDs, provided that the viscous coefficient is sufficiently large.