A dusty veil shading Betelgeuse during its Great Dimming

Abstract

Red supergiants represent the most common final stage of the evolution of stars with initial masses between 8 and 30-35 times the mass of the Sun 1 . During this phase of lifetime lasting ≈ 10 5 yrs 1 , they experience substantial mass loss of unknown mechanism 2 . This mass loss can affect their evolutionary path, collapse, future supernova light curve 3 , and ultimate fate as a neutron star or a black hole 4 . From November 2019 to March 2020, the second closest red supergiant (RSG, 222 +48 −34 pc 5, 6 ) Betelgeuse experienced a historic dimming of its visible brightness, witnessed worldwide. Usually between 0.1 and 1.0 mag, it went down to 1.614 ± 0.008 mag around 7-13 February 2020 7 . Here we report high angular resolution observations showing that the southern hemisphere of the star was ten times darker than usual in the visible. Observations and modeling support the scenario of a dust clump recently formed in the vicinity of the star due to a local temperature decrease in a cool patch appearing on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. This event suggests that an inhomogeneous component of red supergiant mass loss 8 is linked to a very contrasted and rapidly changing photosphere.

Did a close tidal encounter cause the Great Dimming of Betelgeuse?

“…the spectral dependence of the observed darkening. One of the possible explanations that accommodates these observations is the presence of a large cloud that obscures Betelgeuse from our perspective on the Earth (Levesque & Massey 2020;Dupree et al 2020;Montargès et al 2021). Such a cloud could have been expelled by Betelgeuse by means of large convective outflows -but, in fact, might also have been triggered by tidal overflow in a close encounter with an unseen companion.…”

“…We finally note that, even though a close tidal encounter does not appear viable to explain the Great Dimming by means of gravity darkening alone, it remains possible, of course, that such an interaction may either have caused or contributed to the launching of a stellar outburst or wind, which has been invoked to explain the obscuring of Betel- geuse by a dust cloud (Levesque & Massey 2020;Dupree et al 2020;Montargès et al 2021).…”

“…The "Great Dimming" of Betelgeuse, observed from late 2019 into early 2020, reduced its visual brightness by about one apparent magnitude, corresponding to a reduction in intensity by about 60%. A number of different effects have been suggested that might have caused this dimming, including changes in the star's photosphere (Dharmawardena et al 2020), a critical transition in the pulsation dynamics (George et al 2020), strong outflows, resulting from a conjunction of shock waves and convective motion and leading to an increase in molecular opacity (Kravchenko et al 2021), the appearance of a large dark spot (Alexeeva et al 2021), as well as obscuring by a dust cloud expelled during a recent mass-loss episode (Levesque & Massey 2020;Dupree et al 2020;Montargès et al 2021).…”

Locating Red Supergiants in the Galaxy NGC 6822

“…An alternate explanation to this cooler population of RSGs is that higher luminosity RSGs are known to be both more variable and also have higher mass-loss rates than lower luminosity RSGs (Meynet et al 2015;Humphreys et al 2020). It is possible that these high luminosity RSGs have recently gone through eruptive mass-loss events similar to the recent outburst by Betelgeuse (Levesque & Massey 2020;Montargès et al 2021) and thus appear cooler since much of their light has been blocked by dust. However, as discussed briefly by Massey et al (2021a), when a similar group of cooler RSGs was discovered in the SMC, spectroscopy confirmed them to be cool M-type RSGs as opposed to highly reddened and dusty K-type stars.…”

Multiple components in the molecular outflow of the red supergiant NML Cyg

“…Large convective cells on the surfaces of RSGs are another possible underlying mechanism for multiple discrete mass-loss ejection events (Josselin & Plez 2007), but the variations in these cells are predicted to occur over short timescales of days and weeks, with survival periods of the convective cells believed to last over timescales of only a few years (López Ariste et al 2018). Additional links between large convective cells on the surface of RSGs to mass-loss ejection events were drawn in the case of the recent 'Great Dimming' event of Betelgeuse (Montargès et al 2021), which occured over a period of about 150 days. In Humphreys et al (2021), the 'Great Dimming' event of Betelgeuse was then compared to possible episodic outflows in different directions lasting a few decades around VY CMa, determined from the presence of large dust clumps around VY CMa and coincident temporal variability in the light curve of the star, though the authors point out a clear difference in the timescales and energies involved.…”