Magnetically aligned dust and SiO maser polarisation in the envelope of the red supergiant VY Canis Majoris
Journal article, 2017

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 similar to 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 (similar to 3%) of the circumstellar dust emission at millimeter wavelengths. The polarisation is uniform with an electric vector position angle of similar to 8 degrees. Varying levels of linear polarisation are detected for the J = 4-3 (SiO)-Si-28 nu = 0; 1; 2; and (SiO)-Si-29 nu = 0; 1 lines, with the strongest polarisation fraction of similar to 30% found for the (SiO)-Si-29 nu = 1 maser. The linear polarisation vectors rotate with velocity, consistent with earlier observations. We also find significant (up to similar 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 similar to 1-3 Gauss, consistent with previous maser observations.

Late-Type Stars

Field Direction

stars: individual: VY CMa


Evolved Stars

Radiofrequency Lines


Circumstellar Envelopes

3-Dimensional Morphology

Molecular Clouds


stars: mass-loss

stars: magnetic field

Grain Alignment


Radiative Torques


Wouter Vlemmings

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Theo Khouri

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Ivan Marti-Vidal

Chalmers, Earth and Space Sciences, Onsala Space Observatory

Daniel Tafoya Martinez

Chalmers, Earth and Space Sciences, Onsala Space Observatory

A. Baudry

Laboratoire d'Astrophysique de Bordeaux

S. Etoka

University of Hamburg

E. M. Humphreys

European Southern Observatory (ESO)

T. J. Jones

University of Minnesota

A. Kemball

University of Illinois

Eamon O Gorman

A. F. Perez-Sanchez

European Southern Observatory Santiago

A. M. S. Richards

University of Manchester

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 603 A92- A92

Subject Categories

Astronomy, Astrophysics and Cosmology



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7/4/2018 6