Photospheric and dust structures of Betelgeuse during its Great Dimming

Betelgeuse is a red supergiant surrounded by a dust envelope characteristic to its evolutionary stage. From November 2019 to March 2020, it underwent the so-called “Great Dimming”. Images in total intensity obtained with SPHERE/VLT (Montargès et al. 2021) show significant darkening of the southern half of the stellar disk. Meanwhile, our differential speckle polarimetry observations, carried out with 2.5-m telescope of Caucasian Mountain Observatory of SAI MSU (Safonov et al. 2020), indicate the presence of an envelope surrounding the star at a distance of about 1 R⨀ from the photosphere, which was reported previously by Haubois et al. 2023.
Two main mechanisms have been discussed in literature to explain the dimming: the formation of a dust cloud along the line of sight or local cooling of the stellar photosphere. To investigate a scenario in which both mechanisms act simultaneously, we employ radiative transfer modeling. It is performed using the RADMC3D package (Dullemond et al. 2012), which we modified to account for an inhomogeneous photosphere. Our model includes a cold spot, a dust cloud on the line of sight and a dust 
envelope and allows us to reproduce resolved observations in both total and polarized intensity. The modeling demonstrates the possibility of using a single dust composition for both the cloud and the envelope, namely corundum with grain sizes from 0.005 to 0.13 μm, in agreement with Gail et al. 2020. We find that the cloud and the envelope had masses of  2.5x10^-9 M⨀ and 8x10^-9 M⨀, respectively. The cold spot had a temperature of 3150 K and covered 80% of the visible photosphere.