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