The close circumstellar environment of Betelgeuse

IV. VLTI/PIONIER interferometric monitoring of the photosphere^⋆

¹ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’Hères, France
e-mail: miguel.montarges@obspm.fr
² LESIA, Observatoire de Paris, PSL Research University, CNRS UMR 8109, Sorbonne Universités, UPMC, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
³ Unidad Mixta Internacional Franco-Chilena de Astronomía (UMI 3386), CNRS/INSU, France
⁴ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁵ Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
⁶ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), UMR 5274, 38041 Grenoble Cedex 9, France
⁷ Université de Toulouse, UPS-OMP, IRAP, 31028 Tarbes, France
⁸ CNRS, IRAP, 57 avenue d’Azereix, BP 826, 65008 Tarbes Cedex, France
⁹ Université de Toulouse, UPS-OMP, Institut de Recherche en Astrophysique et Planétologie, 31028 Toulouse, France
¹⁰ CNRS, UMR 5277, Institut de Recherche en Astrophysique et Planétologie, 14 avenue Édouard Belin, 31400 Toulouse, France
¹¹ National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹² European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile

Received: 21 July 2015
Accepted: 7 February 2016

Abstract

Context. The mass-loss mechanism of cool massive evolved stars is poorly understood. The proximity of Betelgeuse makes it an appealing target to study its atmosphere, map the shape of its envelope, and follow the structure of its wind from the photosphere out to the interstellar medium.

Aims. A link is suspected between the powerful convective motions in Betelgeuse and its mass loss. We aim to constrain the spatial structure and temporal evolution of the convective pattern on the photosphere and to search for evidence of this link.

Methods. We report new interferometric observations in the infrared H-band using the VLTI/PIONIER instrument. We monitored the photosphere of Betelgeuse between 2012 January and 2014 November to look for evolutions that may trigger the outflow.

Results. Our interferometric observations at low spatial frequencies are compatible with the presence of a hot spot on the photosphere that has a characteristic width of one stellar radius. It appears to be superposed on the smaller scale convective pattern. In the higher spatial frequency domain, we observe a significant difference between the observations and the predictions of 3D hydrodynamical simulations.

Conclusions. We bring new evidence for the presence of a convective pattern in the photosphere of red supergiants. The inferred hot spot is probably the top of a giant convection cell although an asymmetric extension of the star cannot be excluded by these interferometric observations alone. The properties of the observed surface features show a stronger contrast and inhomogeneity as predicted by 3D radiative hydrodynamical simulations. We propose that the large observed feature is modifying the signature of the convective pattern at the surface of the star in a way that simulations cannot reproduce.