The close circumstellar environment of Betelgeuse

III. SPHERE/ZIMPOL imaging polarimetry in the visible^⋆

¹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
e-mail: pkervell@das.uchile.cl
² LESIA (UMR 8109), Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: pierre.kervella@obspm.fr
³ Laboratoire Lagrange (UMR 7293), UNSA, CNRS, Obs. de la Côte d’Azur, Bd de l’Observatoire, 06304 Nice Cedex 4, France
⁴ Institut de Radio-Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’ Hères, France
⁵ National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁷ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁸ Obs. de Lyon, CRAL, ENS Lyon, CNRS, Univ. Lyon 1, UMR 5574, 9 avenue Charles André, 69230 Saint-Genis Laval, France
⁹ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile

Received: 7 August 2015
Accepted: 29 October 2015

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 Hα 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.