Polarisation of molecular lines in the circumstellar envelope of the post-Asymptotic Giant Branch star OH 17.7-2.0

TL;DR

This study uses ALMA observations of molecular line polarisation to compare magnetic field structures in the circumstellar envelope of a pre-PNe star, confirming the magnetic field's role in shaping bipolar planetary nebulae.

Contribution

It demonstrates that CO molecular line polarisation can reliably trace magnetic fields in CSEs, aligning with maser observations and revealing the magnetic field's influence on nebula morphology.

Findings

CO line polarisation shows an ordered magnetic field structure.

Magnetic field strength in the CO region is about 1 mG.

CO polarisation results are consistent with maser-based magnetic field measurements.

Abstract

(abridged) The role of magnetic field in the shaping of Planetary Nebulae (PNe), either directly or indirectly after being enhanced by binary interaction, has long been a topic of debate. Large scale magnetic fields around pre-PNe have been inferred from polarisation observations of masers. However, because masers probe very specific regions, it is still unclear if the maser results are representative of the intrinsic magnetic field in the circumstellar envelope (CSE). Molecular line polarisation can provide important information about the magnetic field. A comparison between the field morphology determined from maser observations and that observed in the more diffuse CO gas, can reveal if the two tracers probe the same magnetic field. We compare observations taken with ALMA of molecular line polarisation around the post-Asymptotic Giant Branch)/pre-PNe star OH~17.7-2.0 with previous observations of polarisation in the 1612~MHz OH maser region. We detect CO~$J=2-1$ molecular line polarisation at a level of $\sim4\%$ that displays an ordered linear polarisation structure. We find that, correcting for Faraday rotation of the OH~maser linear polarisation vectors, the OH and CO linearly polarised emission trace the same large scale magnetic field. A structure function analysis of the CO linear polarisation reveals a plane-of-the-sky magnetic field strength of $B_\perp\sim1$~mG in the CO region, consistent with previous OH Zeeman observations. The consistency of the ALMA CO molecular line polarisation with maser observations indicate that both can be used to determine the magnetic field in CSEs. The existence of a strong, ordered, magnetic-field around OH 17.7-2.0 indicates that magnetic fields are likely involved in the formation of this bipolar pre-PNe.