Episodic mass loss in the very luminous red supergiant [W60] B90 in the Large Magellanic Cloud

¹ IAASARS, National Observatory of Athens, Metaxa & Vas. Pavlou St., 15236 Penteli, Athens, Greece
² Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, Zografos 15784, Greece
³ Cerro Tololo Inter-American Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
⁴ Las Campanas Observatory, Carnegie Observatories, Colina El Pino, Casilla 601, La Serena, Chile
⁵ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

^★ Corresponding author; gonzalomu16@gmail.com

Received: 16 May 2024
Accepted: 7 August 2024

Abstract

Context. Despite mounting evidence that extreme red supergiants (RSGs) undergo episodic mass-loss events, their role in RSG evolution remains uncertain. Critical questions remain unanswered, such as whether or not these events can strip the star, and their timescale and frequency.

Aims. This study delves into [W60] B90, one of the most luminous and extreme RSGs in the Large Magellanic Cloud (LMC), with our aim being to search for evidence of episodic mass loss. Our discovery of a bar-like nebular structure at 1 pc, which is reminiscent of the bar around Betelgeuse, raised the question of whether [W60] B90 also has a bow shock, motivating the present study.

Methods. We collected and analyzed proper motion data from Gaia, as well as new multi-epoch spectroscopic and imaging data, and archival time-series photometry in the optical and mid-infrared (MIR). We used MARCS models to derive the physical properties of the star from the spectra.

Results. We find [W60] B90 to be a walkaway star, with a supersonic peculiar velocity in the direction of the bar. We detect shocked emission between the bar and the star, based on the [S II]/Hα > 0.4 criterion, providing strong evidence for a bow shock. The 30 yr optical light curve reveals semi-regular variability, showing three similar dimming events with ΔV ~ 1 mag, a recurrence of ~12 yr, and a rise time of 400 days. We find the MIR light curve to vary by 0.51 mag and 0.37 mag in the WISE1 and WISE2 bands, respectively, and by 0.42 mag and 0.25 mag during the last dimming event. During this event, optical spectroscopy reveals spectral variability (M3 I to M4 I), a correlation between the T_eff and the brightness, increased extinction, and, after the minimum, spectral features incompatible with the models. We also find a difference of >300 K between the T_eff measured from the TiO bands in the optical and the atomic lines from our J-band spectroscopy.

Conclusions. [W60] B90 is a more massive analog of Betelgeuse in the LMC and therefore the first single extragalactic RSG with a suspected bow shock. Its high luminosity of log(L/L_⊙) = 5.32 dex, mass-loss rate, and MIR variability compared to other RSGs in the LMC indicate that it is in an unstable evolutionary state, undergoing episodes of mass loss. Investigating other luminous and extreme RSGs in low-metallicity environments using both archival photometry and spectroscopy is crucial to understanding the mechanism driving episodic mass loss in extreme RSGs in light of the Humphreys-Davidson limit and the “RSG problem”.