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arXiv:1707.05269 [astro-ph.GA]AbstractReferencesReviewsResources

The JCMT BISTRO Survey: The magnetic field strength in the Orion A filament

Kate Pattle, Derek Ward-Thompson, David Berry, Jennifer Hatchell, Huei-Ru Chen, Andy Pon, Patrick M. Koch, Woojin Kwon, Jongsoo Kim, Pierre Bastien, Jungyeon Cho, Simon Coudé, James Di Francesco, Gary Fuller, Ray S. Furuya, Sarah F. Graves, Doug Johnstone, Jason Kirk, Jungmi Kwon, Chang Won Lee, Brenda C. Matthews, Joseph C. Mottram, Harriet Parsons, Sarah Sadavoy, Hiroko Shinnaga, Archana Soam, Tetsuo Hasegawa, Shih-Ping Lai, Keping Qiu, Per Friberg

Published 2017-07-17Version 1

We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of $B_{\rm pos}=6.6\pm4.7$ mG, where $\delta B_{\rm pos}=4.7$ mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of $\sim1.7\times 10^{-7}$ Jm$^{-3}$ in OMC 1, comparable both to the gravitational potential energy density of OMC 1 ($\sim 10^{-7}$ Jm$^{-3}$), and to the energy density in the Orion BN/KL outflow ($\sim 10^{-7}$ Jm$^{-3}$). We find that neither the Alfv\'{e}n velocity in OMC 1 nor the velocity of the super-Alfv\'{e}nic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the $\sim$500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.

Comments: 23 pages, 14 figures, 2 tables. Accepted for publication in ApJ
Categories: astro-ph.GA
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