Magnetically aligned dust and SiO maser polarisation in the envelope of the red supergiant VY Canis Majoris

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: wouter.vlemmings@chalmers.se
² Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55455, USA
⁶ Department of Astronomy and National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA
⁷ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁸ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK

Received: 6 March 2017
Accepted: 11 April 2017

Abstract

Aims. Polarisation observations of circumstellar dust and molecular (thermal and maser) lines provide unique information about dust properties and magnetic fields in circumstellar envelopes of evolved stars.

Methods. We use Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 science verification observations of the red supergiant VY CMa to study the polarisation of SiO thermal/maser lines and dust continuum at ~1.7 mm wavelength. We analyse both linear and circular polarisation and derive the magnetic field strength and structure, assuming the polarisation of the lines originates from the Zeeman effect, and that of the dust originates from aligned dust grains. We also discuss other effects that could give rise to the observed polarisation.

Results. We detect, for the first time, significant polarisation (~3%) of the circumstellar dust emission at millimeter wavelengths. The polarisation is uniform with an electric vector position angle of ~8°. Varying levels of linear polarisation are detected for the J = 4 − 3²⁸SiO v = 0, 1, 2, and ²⁹SiO v = 0, 1 lines, with the strongest polarisation fraction of ~30% found for the ²⁹SiO v = 1 maser. The linear polarisation vectors rotate with velocity, consistent with earlier observations. We also find significant (up to ~1%) circular polarisation in several lines, consistent with previous measurements. We conclude that the detection is robust against calibration and regular instrumental errors, although we cannot yet fully rule out non-standard instrumental effects.

Conclusions. Emission from magnetically aligned grains is the most likely origin of the observed continuum polarisation. This implies that the dust is embedded in a magnetic field >13 mG. The maser line polarisation traces the magnetic field structure. The magnetic field in the gas and dust is consistent with an approximately toroidal field configuration, but only higher angular resolution observations will be able to reveal more detailed field structure. If the circular polarisation is due to Zeeman splitting, it indicates a magnetic field strength of ~1–3 Gauss, consistent with previous maser observations.