A magnetic tertiary in the most massive compact triple-star system

ABSTRACT The system TIC 470710327 is comprised of three main-sequence OB stars, with an inner compact 1.10 d eclipsing binary and a non-eclipsing tertiary on a 52.04 d orbit. With the tertiary mass of 14.5–16 ${\rm M}_{\odot }$ and both components in the inner eclipsing binary with individual masses of 6–7 and 5.5–6.3 ${\rm M}_{\odot }$, it is currently the most massive compact system known. The formation scenario of such a compact triple is uncertain. It has been suggested that ‘2 + 2’ quadruple dynamics can lead to a stellar merger in the initially more massive binary and finally result in a highly magnetized tertiary. Our study confirms the presence of a kG-order magnetic field in the tertiary and the slow rotation typical for massive magnetic stars. We conclude that finding massive merger candidates by studies of dynamics in compact, multiple-star systems is an efficient way to understand the evolution of massive stellar multiplicity and the generation of magnetic fields.