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

TL;DR

The study investigates head-on collisions between neutron stars and white dwarfs using a simplified dynamical model that treats the WD as a fluid and the NS as a rigid body, with an effective drag coefficient $C_n$ guiding deceleration. By examining He-, CO-, and ONe-WD types, the work identifies parameter regimes in which the collision heats WD material to ignition temperatures ($T_{ignite}$) while preserving degeneracy, potentially triggering thermonuclear explosions and sub-Chandrasekhar Type Ia supernovae, as well as regimes where the NS remains embedded, forming a TZlO, especially for low-mass CO- and ONe-WDs with large $C_n$. He-WD cases show ignition possible at low $C_n$, but higher $C_n$ tends to lift degeneracy without a global explosion, while ONe-WD outcomes depend strongly on mass, with massive WDs capable of oxygen-deflagration-driven explosions. Overall, NS–WD head-on collisions emerge as an extremely rare but potentially consequential channel for exotic SNe and TZlO formation, offering a framework to predict observable signatures and motivate more detailed GR/MHD simulations to capture angular momentum, magnetic fields, and full disruption dynamics.

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.

Figures (4)

• Figure 1: Evolution of the relative distance between the pseudo–WD and the NS after the collision, for a WD mass of $1.0~M_{\odot}$. The solid lines represent the relative distance for seven different values of $C_{\rm n}$ , while the dashed line denotes the sum of the WD and NS radii. Due to the energy loss during the collision, all results consistently show that the NS ultimately sinks toward the center of the WD.
• Figure 2: The relative displacement of NS with the center of WD for different parameters. For the purpose of visualization, a constant offset was applied to the relative distance such that the initial separation is normalized to zero. The light gray dashed line denotes the coincidence of the NS and WD centers of mass, while the black dashed line indicates the condition where the separation between the two centers of mass exceeds the sum of their radii, corresponding to the NS emerging from the WD.
• Figure 3: Taking the collision between the NS and WD as the initial moment, we measure the maximum separation between them during their relative motion over the subsequent 10 seconds. The red line indicates that the NS was just able to pierce through the WD.
• Figure 4: The temperature profile of the reaction region. The sharp drop in the curve corresponds to the NS traversing the WD. The black dashed line denotes a temperature of $1\times 10^{9} \rm K$, which is the ignition temperature of carbonChen2014, while the pink dashed line represents $3\times 10^{9} \rm K$, corresponding to the critical burning temperature of oxygen Wu2018. The red dashed line denotes the temperature at which the thermal pressure equals the electron degeneracy pressure.