  Recently published episodic spectral (Levesque and Massey, 2020) and high
angular resolution (Montarges et al, 2020) observations of Betelgeuse suggest
that the deep minimum of 2019-2020 was caused by an enhanced dust abundance in
the stellar atmosphere. Detailed monitoring of such events may prove useful for
constructing consistent physical models of mass loss by evolved stars. For such
observations it is fundamentally important to employ methods resolving an
inhomogeneous stellar atmosphere. We present the differential speckle
polarimetric observations of Betelgeuse at 2.5-m telescope of Caucasian
Mountain Observatory of SAI MSU covering the period of 2019-2020 minimum. The
observations were secured on 17 dates at wavelengths 465, 550, 625 and 880 nm.
The circumstellar reflection nebula with the angular size of
$\approx0.1^{\prime\prime}$ was detected for all the dates and at all
wavelengths. The morphology of the nebula changed significantly over the
observational period. Net polarized brightness of the envelope remained
constant until February 2020, while the stellar $V$ band flux decreased 2.5
times. Starting from mid-February 2020, polarized flux of the envelope rose 2.1
times, at the same time the star returned to the pre-minimum state of October
2019. Basing on these data and our low resolution spectrum obtained on
2020-04-06 we confirm a conclusion that the minimum is caused by the formation
of a dust cloud located on the line of sight. A quantitative characterisation
of this cloud will be possible when the data on its thermal radiation are
employed.
