Hot and cool: Characterising the companions of red supergiant stars in binary systems

TL;DR

This study investigates hot companions to red supergiant stars in binary systems, compiling observational data and presenting new spectroscopic survey results to better understand their properties and distributions.

Contribution

It provides a comprehensive compilation of known systems with hot companions to RSGs and presents new HST UV spectroscopic data characterizing a subset of these companions.

Findings

Nine systems with well-constrained orbital periods, mostly between 3 and 75 years.

Hot companions show a flat mass-ratio distribution between 0.3 and 1.

88 hot companions identified in the SMC with observational constraints.

Abstract

In this article we study the nature of the recently identified populations of hot companions to red supergiant stars (RSGs). To this end, we compile the literature on the most well characterised systems with the aim of better understanding the hot companions identified with ultra-violet photometry and confirmed with Hubble Space Telescope spectra in the Local Group. We identify 9 systems with current masses greater than around 8 solar masses that have constraints on their orbital periods, which are in the range 3 to 75a. Antares (Alpha Sco) is the obvious outlier in this distribution, having an estimated orbital period of around 2000 a. Mass-ratio (q=M2/MRSG) estimates are available only for 5 of the compiled systems and range between 0.16 < q < 1. Ongoing efforts to identify and characterise hot companions to RSGs in the SMC have revealed 88 hot companions that have observational constraints free of contamination from the RSG component. We present a summary of a recently conducted HST UV spectroscopic survey that aims to characterise a subset of these companions. These companions show a flat-q distribution in the range 0.3 < q < 1.