Neutron stars colliding with binary companions: formation of hypervelocity stars, pulsar planets, bumpy superluminous supernovae and Thorne-\.Zytkow objects

TL;DR

This study uses 3D hydrodynamical simulations to explore the diverse outcomes of neutron star collisions with binary companions post-supernova, revealing mechanisms for hypervelocity stars, pulsar planets, and peculiar supernovae.

Contribution

It introduces a semi-analytical formula for drag force in steep density gradients and systematically analyzes collision outcomes across various parameters.

Findings

Neutron star collisions can produce hypervelocity stars exceeding classical limits.

Collisions can lead to formation of Thorne-Zytkow objects or multiple supernova bumps.

Some collisions result in the capture of companion material forming pulsar planets.

Abstract

We perform 3D hydrodynamical simulations of new-born neutron stars (NSs) colliding with main-sequence binary companions after supernova explosions. Based on those hydrodynamical models, we construct a semi-analytical formula that describes the drag force inside stars with steep density gradients. We then compute the outcome of NS--companion collisions over a wide range of parameters using the semi-analytical formula. Depending on the direction and magnitude of the natal kick, we find that the collision may lead to various outcomes. For relatively fast kicks and high impact parameters, the NS may penetrate the companion star envelope without merging. By allowing the NS to plunge through companions, the companion can be accelerated to have runaway velocities up to $\sim10$ per cent above the theoretical upper limit considered in classical binary disruption scenarios. The NS can capture and carry away up to a few per cent of the companion envelope as it escapes, which may form pulsar planets or cause outflows through accretion to heat the ejecta from inside and power the supernova light curve. For lower impact parameters, the neutron star will directly merge with the companion and form a Thorne-\.Zytkow object. In intermediate cases, the NS penetrates the companion envelope several times before merging, possibly causing multiple bumps in the supernova light curve like in SN2015bn and SN2019stc.