JCMT POL-2 observations of magnetic fields potentially shaped by outflows in the pre-planetary nebulae CRL 618 and OH231.8+4.2

TL;DR

This study presents the first JCMT POL-2 observations of magnetic fields in pre-planetary nebulae, revealing ordered fields influenced by outflows and providing insights into magnetic field geometry and dust properties in these evolved stellar objects.

Contribution

It provides the first polarization measurements of magnetic fields in PPNe, showing their alignment with outflows and suggesting the influence of outflow-driven dust cavity walls.

Findings

Magnetic fields are ordered and aligned with outflows in PPNe.

Polarization fractions are similar despite different dust chemistries.

Magnetic field geometry complexity influences polarization measurements.

Abstract

We present the first observations of magnetic fields in pre-planetary nebulae (PPNe) made with the POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We observed the PPNe CRL 618 and OH231.8+4.2 in 850 $\mu$m polarized light. In both cases, we observe ordered magnetic fields that appear to arise from dusty circumstellar material that has been swept up by the passage of outflows driven by the central post-Asymptotic Giant Branch (post-AGB) star. CRL 618 shows a magnetic field aligned with one of the most extreme position angles of the outflowing bullets ejected from the central source. We hypothesize that polarized emission in CRL 618 may preferentially arise from material in the walls of the dust cavity opened by the ejected bullets. Conversely, OH231.8+4.2 shows a magnetic field that is aligned approximately perpendicular to the outflow direction, which may preferentially arise from an infrared-bright dense clump embedded near the base of the outflow. Despite CRL 618 being carbon-rich and OH231.8+4.2 being oxygen-rich, there is no significant difference in the polarization fractions of the two sources. This suggests that at linear resolutions $\sim 10^{4}$ au, the complexity of the magnetic field geometry on scales smaller than the beam, rather than grain composition, sets the measured polarization fraction of these sources.