OMC-1 dust polarisation in ALMA Band 7: Diagnosing grain alignment mechanisms in the vicinity of Orion Source I

TL;DR

This study uses ALMA polarisation data to investigate dust grain alignment mechanisms near Orion Source I, revealing evidence for radiative precession effects and magnetic field structures in the Orion molecular cloud.

Contribution

It provides the first evidence of k-RAT alignment outside protostellar discs, demonstrating the influence of intense radiation fields on dust grain alignment mechanisms.

Findings

Radiation from Source I can cause dust grains to precess around the radiation anisotropy vector.

Magnetic field geometry is radial around the BN/KL explosion center.

The magnetic field in the Main Ridge remains aligned with larger-scale magnetic structures.

Abstract

We present ALMA Band 7 polarisation observations of the OMC-1 region of the Orion molecular cloud. We find that the polarisation pattern observed in the region is likely to have been significantly altered by the radiation field of the $>10^{4}$ L$_{\odot}$ high-mass protostar Orion Source I. In the protostar's optically thick disc, polarisation is likely to arise from dust self-scattering. In material to the south of Source I - previously identified as a region of 'anomalous' polarisation emission - we observe a polarisation geometry concentric around Source I. We demonstrate that Source I's extreme luminosity may be sufficient to make the radiative precession timescale shorter than the Larmor timescale for moderately large grains ($> 0.005-0.1\,\mu$m), causing them to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This requires relatively unobscured emission from Source I, supporting the hypothesis that emission in this region arises from the cavity wall of the Source I outflow. This is one of the first times that evidence for k-RAT alignment has been found outside of a protostellar disc or AGB star envelope. Alternatively, the grains may remain aligned by B-RATs and trace gas infall onto the Main Ridge. Elsewhere, we largely find the magnetic field geometry to be radial around the BN/KL explosion centre, consistent with previous observations. However, in the Main Ridge, the magnetic field geometry appears to remain consistent with the larger-scale magnetic field, perhaps indicative of the ability of the dense Ridge to resist disruption by the BN/KL explosion.