ALMA observations of anisotropic dust mass loss in the inner circumstellar environment of the red supergiant VY Canis Majoris^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: eamon.ogorman@chalmers.se
² Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
³ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
⁴ CNRS, LAB, UMR 5804, 33270 Floirac, France
⁵ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁶ School of Physics, Trinity College Dublin, 2 Dublin, Ireland
⁷ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁸ LESIA, Observatoire de Paris, CNRS, UPMC, Université Paris-Diderot, PSL, 5 place Jules Janssen, 92195 Meudon, France
⁹ UMI-FCA, CNRS/INSU,(UMI 3386), France
¹⁰ Dept. de Astronomía, Universidad de Chile, Casilla 26- D Santiago, Chile
¹¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands

Received: 2 October 2014
Accepted: 14 November 2014

Abstract

The processes leading to dust formation and the subsequent role it plays in driving mass loss in cool evolved stars is an area of intense study. Here we present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, thus allowing us to trace dust on spatial scales down to 11 R_⋆ (71 AU). Two prominent dust components are detected and resolved. The brightest dust component, C, is located 334 mas (61 R_⋆) southeast of the star and has a dust mass of at least 2.5 × 10^-4 M_⊙. It has a dust emissivity spectral index of β = −0.1 at its peak, implying that it is optically thick at these frequencies with a cool core of T_d ≲ 100 K. Interestingly, not a single molecule in the ALMA data has emission close to the peak of this massive dust clump. The other main dust component, VY, is located at the position of the star and contains a total dust mass of 4.0 × 10^-5 M_⊙. It also contains a weaker dust feature extending over 60 R_⋆ to the north with the total component having a typical dust emissivity spectral index of β = 0.7. We find that at least 17% of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of 5 × 10^-6 M_⊙yr^-1. The anisotropic morphology of the dust indicates a continuous, directed mass loss over a few decades, suggesting that this mass loss cannot be driven by large convection cells alone.