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

Abstract

We present ALMA Band 7 polarisation observations of the centre of the OMC-1 region of the Orion molecular cloud. We find that the polarisation pattern observed in the region is significantly altered by the radiation field of the > 10 4 L high-mass protostar Orion Source I. In the optically thick disc of Source I, polarisation is likely to arise from dust self-scattering, while 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 in this region the extreme luminosity of Source I can shorten the radiative precession timescale to be less than the Larmor timescale for moderately large grains (> 0.005 − 0.1 µm), causing these grains to be aligned by Radiative Alignment Torques (RATs) to precess around the radiation anisotropy vector (k-RATs) rather than the magnetic field direction (B-RATs). This is the first time that k-RAT alignment has been observed outside of a protostellar disc or AGB star envelope. Elsewhere in OMC-1, we find that grains remain aligned perpendicular to the magnetic field direction, and that the magnetic field geometry is consistent with that inferred in the region from lower-resolution single-dish observations. The persistence of this geometry over orders of magnitude in size-scale and density suggests that the magnetic field is dynamically important and plays a significant role in mediating star formation in OMC-1.

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