Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography

Abstract

Context. Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms, such as episodic mass loss and the presence of dark spots in the photosphere, were invoked. Aims. We aim to relate the atmospheric dynamics of Betelgeuse to its photometric variability, with the main focus on the dimming event. Methods. We used the tomographic method which allowed us to probe different depths in the stellar atmosphere and to recover the corresponding disk-averaged velocity field. The method was applied to a series of high-resolution HERMES observations of Betelgeuse. Variations in the velocity field were then compared with photometric and spectroscopic variations. Results. The tomographic method reveals that the succession of two shocks along our line-of-sight (in February 2018 and January 2019), the second one amplifying the effect of the first one, combined with underlying convection and/or outward motion present at this phase of the 400 d pulsation cycle, produced a rapid expansion of a portion of the atmosphere of Betelgeuse and an outflow between October 2019 and February 2020. This resulted in a sudden increase in molecular opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the observed unusual decrease of the star’s brightness.

A dusty veil shading Betelgeuse during its Great Dimming

“…However, later sub-millimeter observations and TiO photometry 12 have favored a singular or multiple dark and cool photospheric spots to explain the event 13 while preserving compatability with optical spectrophotometry, arguing in favor of a molecular opacity increase. A similar conclusion is reached by tomography from high resolution spectroscopy, which suggests that the propagation of two shock waves in the upper atmosphere, aided by underlying convection or outward gas motion altered the molecular opacity in the line of sight 14 . This scenario would be compatible with both anterior spectropolarimetric imaging 15 , and 3D simulations of stellar convection in evolved stars carried out with the CO5BOLD code [16][17][18][19] .…”

An Exceptional Dimming Event for a Massive, Cool Supergiant in M51

“…UV observations of chromospheric variability also indicate the formation of a dense, outflowing structure-possibly enhanced by a timing coincidence with outward motion of the pulsational cycle-just before the event (Dupree et al 2020). The most likely explanation for the dimming, then, was an increase in the opacity of this ouflowing material related to the formation of molecules (see, e.g., evidence from TiO-band monitoring and tomography in Harper et al 2020;Kravchenko et al 2021, anddiscussion in Davies &Plez 2021) and possibly also large-grain dust (e.g., Levesque & Massey 2020). As shown in Figure 4, the evolution of Betelgeuse in V and I from the American Association of Variable Star Observers 8 (AAVSO) as it recovers from near the photometric minimum of the 2019-2020 dimming is remarkably similar to the "red"class variables, though smaller in amplitude.…”

Evidence for dynamical changes in Betelgeuse using multi-wavelength data

“…Three main hypotheses have been suggested as reasons for the dimming, namely changes in pulsation dynamics, star spots and a dust cloud. Combinations of these hypotheses have also been suggested as reasons for the dimming 4,5 . We will explore each of these explanations below, before explaining how the nonlinear dynamics of the light curve can contribute towards understanding the dimming.…”

“…In addition, high angular resolution images showed a significant dimming in the southern hemisphere of Betelgeuse during the dimming event 7 . Observations from multiple wavelengths suggest that an outflow from the star itself, enhanced by pulsations, condensed around the southern hemisphere of the star 4,5 . It has also been suggested that the dimming could be explained by an ejection from the star that cools below 3000 K to form molecules, before condensing to dust 8 .…”