Polarisation of molecular lines in the circumstellar envelope of the post-asymptotic giant branch star OH 17.7–2.0

Abstract

Context. The role of magnetic field in shaping 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). Aims. Molecular line polarisation of non-maser lines can provide important information about the magnetic field. A comparison between the magnetic 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. Methods. We compared observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) of molecular line polarisation around the post-asymptotic giant branch (post-AGB) or pre-PN star OH 17.7−2.0 with previous observations of polarisation in the 1612 MHz OH maser region. Earlier mid-infrared observations indicate that OH 17.7−2.0 is a young bipolar pre-PN, with both a torus and bipolar outflow cavities embedded in a remnant AGB envelope. Results. We detect CO J = 2 − 1 molecular line polarisation at a level of ∼4% 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⊥ ∼ 1 mG in the CO region, consistent with previous OH Zeeman observations. Conclusions. The consistency of the ALMA CO molecular line polarisation observation with maser observations indicate that both can be used to determine the magnetic field strength and morphology in CSEs. The new observations indicate that the magnetic field has a strong toroidal field component projected on the torus structure and a poloidal field component along the outflow cavity. The existence of a strong, ordered, magnetic-field around OH 17.7−2.0 indicates that the magnetic field is likely involved in the formation of this bipolar pre-PN.