Thorne-\.Zytkow Objects

TL;DR

This paper reviews Thorne-Zytkow Objects, rare stellar merger products containing neutron stars, discussing their formation, evolution, stability, observable signatures, and candidates, highlighting their significance in understanding binary star evolution.

Contribution

It provides a comprehensive review of T.Z.O. formation, evolution, stability, and observational searches, summarizing recent progress and theoretical understanding in the field.

Findings

T.Z.O.s can form through specific binary interactions.

Several candidate T.Z.O.s have been identified in observations.

The stability and detectability of T.Z.O.s remain active research areas.

Abstract

Interacting binary star systems play a critical role in many areas of astrophysics. One interesting example of a binary merger product are Thorne-\.Zytkow Objects (T\.ZOs), stars that look like red supergiants but contain neutron stars at their cores. T\.ZOs were theorized nearly five decades ago, and significant work has gone into understanding the physics of their formation, evolution, and stability. Several searches for T\.ZO candidates have also been carried out. Whether or not T\.ZOs could even exist or if they would be stable after formation has also been investigated. Understanding the existence and possible prevalence of T\.ZOs would have important effects on our understanding of binary evolution, stellar mergers, and inform binary population synthesis models. In this chapter, we review the formation channels, evolution and structure, final fates, and observable signatures of T\.ZOs, as well as candidates in the literature, from the inception of T\.ZO theory to recent progress in the field.