The inner dust shell of Betelgeuse seen with high-angular-resolution polarimetry

Abstract

Context. The characteristics of the innermost layer of dust winds from red supergiants have not been identified. In 2019–2020, Betelgeuse exhibited an important dimming event that has been partially attributed to dust formation, highlighting the importance of understanding dust properties in the first stellar radii from the photosphere. Aims. We aim to detect and characterize the inner dust environment of Betelgeuse at high spatial resolution. Methods. We obtained SPHERE/ZIMPOL and SPHERE/IRDIS linear polarimetric observations from January 2019, before the dimming event, and compared them to a grid of synthetic radiative transfer models. Results. We detect a structure that is relatively centro-symmetric with a 60 mas diameter (1.3–1.4 stellar diameter). We computed synthetic images using radiative transfer modeling assuming a spherical dust shell composed of MgSiO3 grains. We find that most of the data are best reproduced with a dust shell whose outer radius is approximately 10 AU (i.e., ~2 stellar radii) and a maximum grain size in the 0.4–0.6 µm range. These results are close to the ones we obtained from 2013 NACO/SAMPOL data, indicating that the shell radius and grain size can show some stability for at least 6 yr despite morphological changes of the dust shell. The residuals after the subtraction of the best-fitting centro-symmetric model suggest complex asymmetric density structures and photospheric effects.