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 pos = 6.6 ± 4.7 mG, where δB 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 ∼ 1.7 × 10 −7 J m −3 in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼ 10 −7 J m −3 ), and to the energy density in the Orion BN/KL outflow (∼ 10 −7 J m −3 ). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼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.

Abstract. We present the results of high angular resolution millimeter observations of gas and dust toward G31.41+0.31 and G24.78+0.08, two high-mass star forming regions where four rotating massive toroids have been previously detected. The CH 3 CN (12-11) emission of the toroids in G31.41+0.31 and core A1 in G24.78+0.08 has been modeled assuming that it arises from a disk-like structure seen edge-on, with a radial velocity field. For G31.41+0.31 the model properly fits the data for a velocity v rot 1.7 km s −1 at the outer radius R out 13400 AU and an inner radius R inn 1340 AU, while for core A1 in G24.78+0.08 the best fit is obtained for v rot 2.0 km s −1 at R out 7700 AU and R inn 2300 AU. Unlike the rotating disks detected around less luminous stars, these toroids are not undergoing Keplerian rotation. From the modeling itself, however, it is not possible to distinguish between constant rotation or constant angular velocity, since both velocity fields suitably fit the data. The best fit models have been computed adopting a temperature gradient of the type T ∝ R −3/4 , with a temperature at the outer radius T out 100 K for both cores. The M dyn needed for equilibrium derived from the models is much smaller than the mass of the cores, suggesting that such toroids are unstable and undergoing gravitational collapse. The collapse is also supported by the CH 3 13 CN or CH 3 CN line width measured in the cores, which increases toward the center of the toroids. The estimates of v inf anḋ M acc are ∼2 km s −1 and ∼3 × 10 −2 M yr −1 for G31.41+0.31, and ∼1.2 km s −1 and ∼9 × 10 −3 M yr −1 for G24.78+0.08 A1. Such large accretion rates could weaken the effect of stellar winds and radiation pressure and allow further accretion on the star. The values of T rot and N CH 3 CN , derived by means of the RD method, for both G31.41+0.31 and the sum of cores A1 and A2 (core A of Codella et al. 1997, A&A, 325, 282) in G24.78+0.08 are in the range 132-164 K and 2-8 × 10 16 cm −2 . For G31.41+0.31, the most plausible explanation for the apparent toroidal morphology seen in the lower K transitions of CH 3 CN (12-11) is self-absorption, which is caused by the high optical depth and temperature gradient in the core.

We report on the detection of four rotating massive disks in two regions of high-mass star formation. The disks are perpendicular to known bipolar outflows and turn out to be unstable but long-lived. We infer that accretion onto the embedded (proto)stars must proceed through the disks with rates of ∼ yr Ϫ1 .Ϫ2 10 M ,