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

Abstract

Aims. Polarization 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 Band 5 science verification observations of the red supergiant VY CMa to study the polarization of SiO thermal/masers lines and dust continuum at ∼ 1.7 mm wavelength. We analyse both linear and circular polarization and derive the magnetic field strength and structure, assuming the polarization 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 polarization. Results. We detect, for the first time, significant polarization (∼ 3%) of the circumstellar dust emission at millimeter wavelengths. The polarization is uniform with an electric vector position angle of ∼ 8• . Varying levels of linear polarization are detected for the J = 4−3 28 SiO v = 0, 1, 2, and 29 SiO v = 0, 1 lines, with the strongest polarization fraction of ∼ 30% found for the 29 SiO v = 1 maser. The linear polarization vectors rotate with velocity, consistent with earlier observations. We also find significant (up to ∼ 1%) circular polarization 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 polarization. This implies that the dust is embedded in a magnetic field > 13 mG. The maser line polarization 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 polarization is due to Zeeman splitting, it indicates a magnetic field strength of ∼ 1 − 3 Gauss, consistent with previous maser observations.

Characterizing maser polarization: effects of saturation, anisotropic pumping, and hyperfine structure

“…Very-long-baseline interferometry (VLBI) observations have shown that SiO masers orient themselves in a ring-like structure around the central stellar object. The polarization of these SiO masers, irrespective of their angular momentum transition, show well-ordered polarization vectors with respect to this structure (Kemball & Diamond 1997;Cotton et al 2004;Plambeck et al 2003;Vlemmings et al 2011bVlemmings et al , 2017. This is taken to be an indicator of an ordered magnetic field.…”

The Unexpected Spectrum of the Innermost Ejecta of the Red Hypergiant VY CMa∗

“…′′ 5 from the star with the brightest (Clump C) having a total mass of at least 2.5 × 10 −3 M ⊙ (O'Gorman et al 2015). More recent ALMA measurements of Clump C yield a dust mass alone of > 1.2 × 10 −3 M ⊙ , implying a total mass of ∼ 0.1 M ⊙ (Vlemmings et al 2017). These measurements imply short term, high mass loss in separate events over several hundred years.…”

“…The presence of magnetic fields is supported by the Zeeman splitting and polarization of the masers in its circumstellar ejecta (Vlemmings et al 2002(Vlemmings et al , 2003(Vlemmings et al , 2005Shinnaga et al 2017). Recent ALMA continuum observations (Vlemmings et al 2017) reveal polarized dust emission from magnetically aligned gains within 0. ′′ 5 of the star.…”

PORTAL: Three-dimensional polarized (sub)millimeter line radiative transfer

“…It is difficult to directly observe magnetic fields; one typically has to use the polarization properties of the observed light (e.g., Han 2017). For (sub)millimeter interferometers, such as ALMA, magnetic field detection is done mostly through dust (e.g., Hull et al 2017) and line polarization observations (e.g., Vlemmings et al 2017). However, it has recently become increasingly clear that dust polarization does not always faithfully trace the magnetic field morphology, but instead it can be affected by processes such as self-scattering (Kataoka et al 2015(Kataoka et al , 2017.…”