Neutron Star-Main-Sequence Star Collision: Formation and Long-Term Evolution of Thorne-Żytkow Objects

Abstract

Thorne-$\dot{\mathrm{Z}}$ytkow objects (T$\dot{\mathrm{Z}}$Os) are stars containing a neutron-star (NS) or black-hole (BH) core embedded in a stellar envelope and powered primarily by mass accretion onto the core. Classical evolution models of T$\dot{\mathrm{Z}}$Os begin with an assumed equilibrium configuration and predict chemical anomalies in the stellar envelope resulting from nuclear burning in the vicinity of the NS core. Here we connect dynamical formation to long-term evolution by combining a three-dimensional simulation of a post-supernova collision between a $1.4M_\odot$ NS and a $10M_\odot$ main-sequence star with a subsequent stellar evolution calculation using MESA. The collision successfully produces a bound, quasi-spherical T$\dot{\mathrm{Z}}$O with the NS settled at the center of the stellar envelope. In contrast to the classical picture, the collision heats a substantial fraction of the star and leads to widespread nuclear burning during the subsequent thermal-relaxation phase. As the envelope becomes convective, material processed by the carbon-nitrogen cycle is transported to the surface, leaving a potentially observable chemical imprint of formation that persists after the star attains the standard accretion-supported T$\dot{\mathrm{Z}}$O structure. At the end of our simulation, the stellar luminosity greatly exceeds Eddington in the outer envelope, suggesting that significant mass ejection will occur, leaving behind an isolated NS or BH.