Red supergiant stars in binary systems

II. Confirmation of B-type companions of red supergiants in the Small Magellanic Cloud using Hubble ultraviolet spectroscopy

¹ Centro de Astrobiologia (CAB), CSIC-INTA, Carretera de Ajalvir km4, 28850 Torrejón de Ardoz, Madrid, Spain
² Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ Departamento de Física Aplicada, Universidad de Alicante, E-03690 San Vicente del Raspeig, Alicante, Spain
⁷ Universidad Europea de Canarias, La Orotava, Tenerife, Spain

^⋆ Corresponding author: lrpatrick@cab.inta-csic.es

Received: 24 December 2024
Accepted: 29 April 2025

Abstract

Red supergiant (RSG) stars represent the final evolutionary phase of the majority of massive stars and hold a unique role in testing the physics of stellar models. Eighty-eight RSGs in the Small Magellanic Cloud (SMC) were recently found to have an ultraviolet (UV) excess, which was attributed to a B-type companion. We present follow-up Hubble Space Telescope (HST) UV (1700–3000 Å) spectroscopy for 16 of these stars to investigate the nature of the UV excess and confirm the presence of a hot companion. In all cases, we are able to confirm that the companion is a main-sequence B-type star based on the near-UV continuum. We determine effective temperatures, radii, and luminosities from fitting the UV continuum with TLUSTY models and find stellar parameters in the expected range for SMC B-type stars. We display these results on a Hertzsprung–Russell diagram and assess the previously determined stellar parameters using UV photometry alone. From this comparison, we conclude that UV photometric surveys are vital to identify such companions and that UV spectroscopy is similarly vital to characterise the hot companions. From a comparison with IUE spectra of 32 Cyg, a well-known RSG binary system in the Galaxy where the companion is embedded in the wind of the RSG, four targets display evidence of also being embedded in such a wind. The ages of seven targets, determined via the stellar parameters of the hot companions, are found to be in tension with the ages determined for the RSG. An explanation for the discrepancy could be unaccounted for binary mass-transfer or red straggler stars.