Molecular line polarisation from the circumstellar envelopes of Asymptotic Giant Branch stars

Abstract

Polarisation observations of masers in the circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars have revealed strong magnetic fields. However, masers probe only specific lines-of-sight through the CSE. Non-masing molecular line polarisation observation can more directly reveal the large scale magnetic field morphology and hence probe the effect of the magnetic field on AGB mass-loss and the shaping of the AGB wind. Observations and models of CSE molecular line polarisation can now be used to describe the magnetic field morphology and estimate its strength throughout the entire CSE. We use observations taken with ALMA of molecular line polarisation in the envelope of two AGB stars (CW~Leo and R~Leo). We model the observations using the multi-dimensional polarised radiative transfer tool PORTAL. We find linearly polarised emission, with maximum fractional polarisation on the order of a few percent, in several molecular lines towards both stars. We can explain the observed differences in polarisation structure between the different molecular lines by alignment of the molecules by a combination of the Goldreich-Kylafis effect and radiative alignment effects. We specifically show that the polarisation of CO traces the morphology of the magnetic field. Competition between the alignment mechanisms allows us to describe the behaviour of the magnetic field strength with radius throughout the circumstellar envelope of CW~Leo. The magnetic field strength derived using this method is inconsistent with the magnetic field strength derived using a structure function analysis of the CO polarisation and the strength previously derived using CN Zeeman observations. In contrast with CW~Leo, the magnetic field in the outer envelope of R~Leo appears to be advected outwards by the stellar wind. {abridged abstract}

arXiv comment23 pages, 21 figures, accepted for publication in A&A

References in corpus (47)

• Gaia Data Release 2. Summary of the contents and survey properties
• An atomic and molecular database for analysis of submillimetre line observations
• 2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules
• Dispersion of Magnetic Fields in Molecular Clouds. I
• LIME - a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths
• Dispersion of Magnetic Fields in Molecular Clouds. II
• A magnetically collimated jet from an evolved star
• Molecular abundances in the inner layers of IRC +10216
• Circular Polarization of Water Masers in the Circumstellar Envelopes of Late Type Stars
• The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
• The magnetic field around late-type stars revealed by the circumstellar H2O masers
• The circumstellar envelope of IRC+10216 from milli-arcsecond to arcmin scales
• Distance estimates for AGB stars from parallax measurements
• Full polarization study of SiO masers at 86 GHz
• The alignment of molecular cloud magnetic fields with the spiral arms in M33
• Dispersion of Magnetic Fields in Molecular Clouds. IV - Analysis of Interferometry Data
• ALMA Science Verification Data: Millimeter Continuum Polarimetry of the Bright Radio Quasar 3C 286
• Line Polarization of Molecular Lines at Radio Frequencies: The case of DR21(OH)
• An independent distance estimate to CW Leo
• Search for surface magnetic fields in Mira stars. First detection in chi Cyg
• IRC +10 216 in 3-D: morphology of a TP-AGB star envelope
• Magnetic field structure in a high-mass outflow/disk system
• Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae
• Magnetic Fields in Evolved Stars: Imaging the Polarized Emission of High-Frequency SiO Masers
• Magnetically aligned dust and SiO maser polarization in the envelope of the red supergiant VY CMa
• A synchrotron jet from a post-AGB star
• Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust
• Near-infrared spectro-interferometry of Mira variables and comparisons to 1D dynamic model atmospheres and 3D convection simulations
• Photodissociation of CO in the outflow of evolved stars
• Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at $159.0-368.5$ GHz
• Magnetic fields around evolved stars: further observations of H${}_2$O maser polarization
• Polarization of thermal molecular lines in the envelope of IK Tau
• Non-Zeeman Circular Polarization of Molecular Rotational Spectral Lines
• Mapping the Linearly Polarized Spectral Line Emission around the Evolved Star IRC+10216
• ALMA reveals the magnetic field evolution in the high-mass star forming complex G9.62+0.19
• The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars
• Magnetic Field Properties in High Mass Star Formation from Large to Small Scales - A Statistical Analysis from Polarization Data
• Magnetic field in IRC+10216 and other C-Rich Evolved Stars
• Low Level Carbon Monoxide Line Polarization in two Protoplanetary Disks: HD 142527 and IM Lup
• PORTAL--three-dimensional polarized (sub)millimeter line radiative transfer
• Characterizing the accuracy of ALMA linear-polarization mosaics
• Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars
• Characterizing maser polarization: effects of saturation, anisotropic pumping and hyperfine structure
• Mapping circumstellar magnetic fields of late-type evolved stars with the Goldreich-Kylafis effect: CARMA observations at $\lambda 1.3$ mm of R Crt and R Leo
• Non-Zeeman Circular Polarization of Molecular Spectral Lines in the ISM
• The generation and transformation of polarisation signals in molecular lines through collective anisotropic resonant scattering
• Polarisation of molecular lines in the circumstellar envelope of the post-Asymptotic Giant Branch star OH 17.7-2.0