Investigating episodic mass loss in evolved massive stars

II. Physical properties of red supergiants at subsolar metallicity^★

¹ IAASARS, National Observatory of Athens, Metaxa & Vas. Pavlou St., 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 University of Liège, Allée du 6 Août 19c (B5C), 4000 Sart Tilman, Liège, Belgium
⁵ Royal Observatory of Belgium, Avenue Circulaire/Ringlaan 3, 1180 Brussels, Belgium
⁶ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA
⁷ Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA

Received: 14 February 2024
Accepted: 2 April 2024

Abstract

Mass loss during the red supergiant (RSG) phase plays a crucial role in the evolution of an intermediate-mass star; however, the underlying mechanism remains unknown. We aim to increase the sample of well-characterized RSGs at subsolar metallicity by deriving the physical properties of 127 RSGs in nine nearby southern galaxies. For each RSG, we provide spectral types and used MARCS atmospheric models to measure stellar properties from their optical spectra, such as the effective temperature, extinction, and radial velocity. By fitting the spectral energy distribution, we obtained the stellar luminosity and radius for 92 RSGs, finding that ~50% of them have log(L/L_⊙) ≥ 5.0 and six RSGs have R ≳ 1400 R_⊙. We also find a correlation between the stellar luminosity and mid-IR excess of 33 dusty variable sources. Three of these dusty RSGs have luminosities exceeding the revised Humphreys-Davidson limit. We then derived a metallicity-dependent J − K_s color versus temperature relation from synthetic photometry and two new empirical J − K_s color versus temperature relations calibrated on literature TiO and J-band temperatures. To scale our derived cool TiO temperatures to values that are in agreement with the evolutionary tracks, we derived two linear scaling relations calibrated on J-band and i-band temperatures. We find that the TiO temperatures are more discrepant as a function of the mass-loss rate, and discuss future prospects of the TiO bands as a mass-loss probe. Finally, we speculate that three hot dusty RSGs may have experienced a recent mass ejection (12% of the K-type sample) and classify them as candidate Levesque-Massey variables.