Radiative hydrodynamics simulations of red supergiant stars

II. Simulations of convection on Betelgeuse match interferometric observations

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 1317, 85741 Garching b. München, Germany e-mail: chiavass@mpa-garching.mpg.de
² GRAAL, Université de Montpellier II – IPM, CNRS, Place Eugéne Bataillon, 34095 Montpellier Cedex 05, France
³ Observatoire de Paris, LESIA, UMR 8109, 92190 Meudon, France
⁴ Astrophysics Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France
⁶ Department of Physics and Astronomy, Division of Astronomy and Space Physics, Uppsala University, Box 515, 751  20 Uppsala, Sweden

Received: 18 December 2009
Accepted: 2 March 2010

Abstract

Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection.

Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band to characterize and measure the convection pattern on this star.

Methods. We use a 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations.

Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in both the optical and the H band based on excellent fits to the observed visibility points and closure phases. We determine that the Betelgeuse surface in the H band is covered by small to medium scale (5-15 mas) convection-related surface structures and a large (≈30 mas) convective cell. In this spectral region, H₂O molecules are the main absorbers and contribute to both the small structures and the position of the first null of the visibility curve (i.e., the apparent stellar radius).