Can Thorne-$\dot{\textrm{Z}}$ytkow Objects source GW190814-type events?

TL;DR

This paper investigates whether Thorne-Zytkow objects can explain GW190814-like events, analyzing their formation, rate, and potential to produce low-mass black hole mergers consistent with observations.

Contribution

It introduces a new scenario where Thorne-Zytkow objects in hierarchical triples can produce GW190814-like events, highlighting the importance of field populations over dense clusters.

Findings

T$ ext{ extperiodcentered}$Z$ ext{ extperiodcentered}$s in dense clusters cannot explain the GW190814 rate.

Hierarchical triples in the field can produce comparable merger rates.

Future observations may detect mergers involving low mass gap black holes.

Abstract

The LIGO-Virgo collaboration reported in their third run the coalescence event GW190814 involving a 2.6 $M_{\odot}$ object with a 23 $M_{\odot}$ black hole. In this letter we study the conditions under which Thorne-$\dot{\textrm{Z}}$ytkow objects (T$\dot{\textrm{Z}}$Os) can be connected to that type of events. We evaluate first the rate of appearance of T$\dot{\textrm{Z}}$Os in the local Universe. Under the assumption that T$\dot{\textrm{Z}}$Os eventually become low mass gap black holes we evaluate how those black holes end up in binaries with other stellar mass black holes and compare to the reported rate for GW190814-type of events (1-23 $\textrm{Gpc}^{-3} \textrm{yr}^{-1}$). We find that T$\dot{\textrm{Z}}$Os in dense stellar clusters can not explain the LIGO-Virgo rate without a T$\dot{\textrm{Z}}$Os population in the field providing a dominant contribution. We also find that T$\dot{\textrm{Z}}$Os formed within hierarchical triple systems in the field with the third more distant star being the progenitor of a stellar mass black hole, may be able to give a rate comparable to that of GW190814-type events. In that case, future observations should discover mergers between stellar mass and low mass gap black holes, with the lower mass spanning the entire low mass gap range.