The close circumstellar environment of Betelgeuse

V. Rotation velocity and molecular envelope properties from ALMA

¹ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
² LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: pierre.kervella@obspm.fr
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D B2401, 3001 Leuven, Belgium
⁴ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁵ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309, USA
⁶ Dublin Institute for Advanced Studies, Dublin 2, Ireland
⁷ Universidad Católica del Norte, Instituto de Astronomía, Avenida Angamos 0610, Antofagasta, Chile

Received: 11 August 2017
Accepted: 9 November 2017

Abstract

We observed Betelgeuse using ALMA’s extended configuration in band 7 (f ≈ 340 GHz, λ ≈ 0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the ²⁸SiO(ν= 2, J = 8−7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of ν_eqsini = 5.47 ± 0.25 km s^-1 at the equivalent continuum angular radius R_star = 29.50 ± 0.14 mas. This corresponds to an angular rotation velocity of ω sini = (5.6 ± 1.3) × 10^-9 rad s^-1. The position angle of its north pole is PA = 48.0 ± 3.5°. The rotation period of Betelgeuse is estimated to P/ sini = 36 ± 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ≈ 10 au (45 mas or 1.5 × the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong “rogue” convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants.