OMC-1 dust polarisation in ALMA Band 7: Diagnosing grain alignment mechanisms in the vicinity of Orion Source I. (arXiv:2009.14758v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Pattle_K/0/1/0/all/0/1">Kate Pattle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lai_S/0/1/0/all/0/1">Shih-Ping Lai</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wright_M/0/1/0/all/0/1">Melvyn Wright</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coude_S/0/1/0/all/0/1">Simon Coudé</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Plambeck_R/0/1/0/all/0/1">Richard Plambeck</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hoang_T/0/1/0/all/0/1">Thiem Hoang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tang_Y/0/1/0/all/0/1">Ya-Wen Tang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bastien_P/0/1/0/all/0/1">Pierre Bastien</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eswaraiah_C/0/1/0/all/0/1">Chakali Eswaraiah</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Furuya_R/0/1/0/all/0/1">Ray Furuya</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hwang_J/0/1/0/all/0/1">Jihye Hwang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Inutsuka_S/0/1/0/all/0/1">Shu-ichiro Inutsuka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kim_K/0/1/0/all/0/1">Kee-Tae Kim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kirchschlager_F/0/1/0/all/0/1">Florian Kirchschlager</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kwon_W/0/1/0/all/0/1">Woojin Kwon</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_C/0/1/0/all/0/1">Chang Won Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Liu_S/0/1/0/all/0/1">Sheng-Yuan Liu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lyo_A/0/1/0/all/0/1">Aran Lyo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ohashi_N/0/1/0/all/0/1">Nagayoshi Ohashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rawlings_M/0/1/0/all/0/1">Mark Rawlings</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tahani_M/0/1/0/all/0/1">Mehrnoosh Tahani</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tamura_M/0/1/0/all/0/1">Motohide Tamura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Soam_A/0/1/0/all/0/1">Archana Soam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_J/0/1/0/all/0/1">Jia-Wei Wang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ward_Thompson_D/0/1/0/all/0/1">Derek Ward-Thompson</a>
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$_{odot}$ 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 ($>
0.005-0.1,mu$m), 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.
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$_{odot}$ 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 ($>
0.005-0.1,mu$m), 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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