Dense Gas Kinematics and a Narrow Filament in the Orion A OMC1 Region Using NH₃

Abstract

We present combined observations of the NH 3 (J, K) = (1, 1) and (2, 2) inversion transitions toward OMC1 in Orion A obtained by the Karl G. Jansky Very Large Array (VLA) and the 100 m Robert C. Byrd Green Bank Telescope (GBT). With an angular resolution of 6 (0.01 pc), these observations reveal with unprecedented detail the complex filamentary structure extending north of the active Orion BN/KL region in a field covering ∼ 6 × 7 . We find a 0.012 pc wide filament within OMC1, with an aspect ratio of ∼37:1, that was missed in previous studies. Its orientation is directly compared to the relative orientation of the magnetic field from the James Clerk Maxwell Telescope BISTRO survey in Orion A. We find a small deviation of ∼ 11 • between the mean orientation of the filament and the magnetic field, suggesting that they are almost parallel to one another. The filament's column density is estimated to be 2-3 orders of magnitude larger than the filaments studied with Herschel and is possibly self-gravitating given the low values of turbulence found. We further produce maps of the gas kinematics by forward modeling the hyperfine structure of the NH 3 (J, K) = (1, 1) and (2, 2) lines. The resulting distribution of velocity dispersions peaks at ∼ 0.5 km s −1 , close to the subsonic regime of the gas. This value is about 0.2 km s −1 smaller than previously measured in single-dish observations of the same region, suggesting that higher angular and spectral resolution observations will identify even lower velocity dispersions that might reach the subsonic turbulence regime in dense gas filaments.

Physical Conditions and Kinematics of the Filamentary Structure in Orion Molecular Cloud 1

“…These three regions with different velocities converge at the Orion KL region. The velocity difference between the northern and western region was also reported by Wiseman & Ho (1998) and Monsch et al (2018) based on their NH 3 observations. A clearer analysis for the velocity transition among the three regions will be presented in Section 4.2.…”

Relative alignment between dense molecular cores and ambient magnetic field: the synergy of numerical models and observations

“…Such parallel alignment between magnetic field and dense filament has also been reported in the OMC1 region of the Orion A molecular cloud byMonsch et al (2018).MNRAS 000, 1-15(2020)…”

Magnetized filamentary gas flows feeding the young embedded cluster in Serpens South

“…Evidence for such accretion flows have been found in the smooth velocity gradients in N 2 H + 1→0 in the FIL2 region filament 27 (see also Fernandez-Lopez et al 28 ). Field-parallel orientations have been observed recently in a dense filament in Orion's OMC1 region 29 as well as in a distant, hub-filament infrared dark cloud 30 . The combined NIR and FIR polarimetric observations reported here show that filamentary accretion flows affect the local magnetic field orientation and thereby shape the magnetic field structure on filament size scales.…”

“…A V ≤ 2.7 − 3.5 mag to being perpendicular beyond that A V range. Recent submm observations show magnetic field orientations parallel to dense filaments in a few star-forming regions 29,30 , but does not capture a transition.…”