Investigating episodic mass loss in evolved massive stars

III. Spectroscopy of dusty massive stars in three northern galaxies

¹ IAASARS, National Observatory of Athens, I. Metaxa & Vas. Pavlou St., 15236 Penteli, Athens, Greece
² Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos 15784, Greece
³ Institute of Astrophysics, FORTH, 71110 Heraklion, Greece
⁴ Instituto de Astrofísica de Canarias, Avenida Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁵ Grantecan S. A., Centro de Astrofísica de La Palma, Cuesta de San José, 38712 Breña Baja, La Palma, Spain
⁶ Royal Observatory of Belgium, Avenue Circulaire, Ringlaan 3, 1180 Brussels, Belgium
⁷ University of Liège, Allée du 6 Août 19c (B5C), 4000 Sart Tilman, Liège, Belgium
⁸ Department of Physics, University of Oxford, Oxford, UK

^★★ Corresponding author.

Received: 20 December 2024
Accepted: 22 April 2025

Abstract

Mass loss in massive stars is crucial to understanding how these stars evolve and explode. Despite increasing evidence of its importance, episodic mass loss remains poorly understood. Here we report the results of an optical spectroscopic survey of evolved massive stars in NGC 6822, IC 10, and IC 1613 conducted as part of the ASSESS project (Episodic Mass Loss in Evolved Massive Stars: Key to Understanding the Explosive Early Universe), which investigated the role of episodic mass loss by targeting stars with infrared excesses indicating a dusty circumstellar environment. We assigned a spectral class to 122 unique sources, the majority of which are dusty. The rate of evolved massive stars was over 60% for the highest-priority targets. We discovered two blue supergiants, one yellow supergiant, and one emission-line object, and confirmed two supernova remnant candidates, a Wolf–Rayet star, and two H II regions. Twenty-eight unique sources were classified as red supergiants (RSGs), 21 of which are new discoveries. In IC 10, we increased the sample of spectroscopically confirmed RSGs from 1 to 17. We used the MARCS models to obtain their surface properties, most importantly the effective temperature, and used spectral energy distribution fitting to obtain the stellar luminosity for 17 of them. The dusty RSGs are cooler, more luminous, more extinguished, and more evolved than the non-dusty ones, in agreement with previous findings. By investigating the optical photometric variability of the RSGs from light curves that cover a period of over a decade, we found that the dusty RSGs are more variable. We further highlight a very extinguished emission-line object, two RSGs that display a significant change in spectral type between two observed epochs, and four dusty K-type RSGs that may have undergone episodic mass loss.