Planet Hunters TESS IV: A massive, compact hierarchical triple star system TIC 470710327

TL;DR

This paper reports the discovery and detailed analysis of a rare massive hierarchical triple star system with a compact eclipsing binary and a more massive tertiary, providing insights into massive star formation and evolution.

Contribution

It presents the first detailed dynamical and spectroscopic analysis of TIC 470710327, revealing its unique configuration and potential evolutionary outcomes.

Findings

The tertiary star is more massive than the inner binary.

The system is likely to undergo multiple mass transfer phases.

It may evolve into a double neutron star or an exotic Thorne-Zytkow object.

Abstract

We report the discovery and analysis of a massive, compact, hierarchical triple system (TIC 470710327) initially identified by citizen scientists in data obtained by NASA's Transiting Exoplanet Survey Satellite (TESS). Spectroscopic follow-up observations obtained with the HERMES spectrograph, combined with eclipse timing variations (ETVs), confirm that the system is comprised of three OB stars, with a compact 1.10 d eclipsing binary and a non-eclipsing tertiary on a 52.04 d orbit. Dynamical modelling of the system (from radial velocity and ETVs) reveal a rare configuration wherein the tertiary star (O9.5-B0.5V; 14-17 M$_{\odot}$) is more massive than the combined mass of the inner binary (10.9-13.2 M$_{\odot}$). Given the high mass of the tertiary, we predict that this system will undergo multiple phases of mass transfer in the future, and likely end up as a double neutron star gravitational wave progenitor or an exotic Thorne-Zytkow object. Further observational characterisation of this system promises constraints on both formation scenarios of massive stars as well as their exotic evolutionary end-products.