Investigating episodic mass loss in evolved massive stars

IV. Comprehensive analysis of dusty red supergiants in NGC 6822, IC 10, and WLM

¹ IAASARS, National Observatory of Athens, 15326 Penteli, Greece
² National and Kapodistrian University of Athens, Department of Physics, Panepistimiopolis, 15784 Zografos, 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 Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain

^★ Corresponding author: evachris@noa.gr

Received: 7 February 2025
Accepted: 17 October 2025

Abstract

Mass loss shapes the fate of massive stars; however, the physical mechanism causing it remains uncertain. We present a comprehensive analysis of seven red supergiants, for which we searched evidence of episodic mass loss, in three low-metallicity galaxies: NGC 6822, IC 10, and WLM. Initially, the spectral classification of their optical spectra was refined and compared to previous reported classifications, finding four sources that display spectral variability. We derived the physical properties of five of them using the MARCS atmospheric models corrected for nonlocal thermal equilibrium effects to measure stellar properties from our new near-infrared spectra, such as the effective temperature, surface gravity, metallicity, and microturbulent velocity. Additional empirical and theoretical methods were employed to calculate effective temperatures, finding consistent results. We constructed optical and infrared light curves, discovering two targets in NGC 6822 with photometric variability between 1 and 2.5 mag in amplitude in r and ~0.5 mag in the mid-infrared. Furthermore, we discovered a candidate-dimming event in one of these sources. Periods for three red supergiants were determined using epoch photometry, which were consistent with the empirical estimations from literature period-luminosity relations. Our comprehensive analysis of all the available data for each target provides evidence for episodic mass loss in four red supergiants.