The close circumstellar environment of Betelgeuse - III. SPHERE/ZIMPOL visible polarimetry of the inner envelope and photosphere

TL;DR

This study uses visible polarimetry with SPHERE/ZIMPOL to resolve Betelgeuse's inner envelope and photosphere, revealing asymmetries, dust, and gaseous structures that inform understanding of mass loss mechanisms in red supergiants.

Contribution

First high-resolution polarimetric imaging of Betelgeuse's inner envelope in visible light, revealing asymmetric dust and gas structures near the photosphere.

Findings

Asymmetric gaseous envelope detected within 2-3 stellar radii.

Significant Halpha emission within 3 stellar radii.

Asymmetric dust shell at similar radius, influencing wind acceleration.

Abstract

The physical mechanism through which the outgoing material of massive red supergiants is accelerated above the escape velocity is unclear. Thanks to the transparency of its circumstellar envelope, the nearby red supergiant Betelgeuse gives the opportunity to probe the innermost layers of the envelope of a typical red supergiant down to the photosphere, i.e. where the acceleration of the wind is expected to occur. We took advantage of the SPHERE/ZIMPOL adaptive optics imaging polarimeter to resolve the visible photosphere and close envelope of Betelgeuse. We detect an asymmetric gaseous envelope inside a radius of 2 to 3 times the near-infrared photospheric radius of the star (R*), and a significant Halpha emission mostly contained within 3 R*. From the polarimetric signal, we also identify the signature of dust scattering in an asymmetric and incomplete dust shell located at a similar radius. The presence of dust so close to the star may have a significant impact on the wind acceleration through radiative pressure on the grains. The 3 R* radius emerges as a major interface between the hot gaseous and dusty envelopes. The detected asymmetries strengthen previous indications that the mass loss of Betelgeuse is likely tied to the vigorous convective motions in its atmosphere.