MASSIVE QUIESCENT CORES IN ORION: DYNAMICAL STATE REVEALED BY HIGH-RESOLUTION AMMONIA MAPS

Abstract

We present combined VLA and Green Bank Telescope images of \\ammonia\\\ninversion transitions (1,1) and (2,2) toward OMC2 and OMC3. We focus on the\nrelatively quiescent Orion cores, which are away from the Trapezium cluster and\nhave no sign of massive protostars nor evolved star formation, such as IRAS\nsource, water maser, and methanol maser. The 5\\arcsec\\ angular resolution and\n$0.6 \\rm{}km\\,s^{-1}$ velocity resolution of these data enable us to study the\nthermal and dynamic state of these cores at $\\sim{}0.02 \\rm{}pc$ scales,\ncomparable to or smaller than those of the current dust continuum surveys. We\nmeasure temperatures for a total of 30 cores, with average masses of $11\\,\\Ms$,\nradii of $0.039 \\rm{}pc$, virial mass ratio $\\bar{R_{vir}}$ = 3.9, and critical\nmass ratio $\\bar{R_{C}}$ = 1.5. Twelve sources contain \\textit{Spitzer}\nprotostars. The thus defined starless and protostellar subsamples have similar\ntemperature, line width, but different masses, with an average of $7.3\\,\\Ms$\nfor the former and $16\\,\\Ms$ for the latter. Compared to others Gould Belt\ndense cores, mores Orion cores have a high gravitational-to kinetic energy\nratio and more cores have a larger thant unity critical mass ratio. Orion dense\ncores have velocity dispersion similar to those of cores in low-mass\nstar-forming regions but larger masses for fiven size. Some cores appear to\nhave truly supercritical gravitational-to-kinetic energy ratios, even when\nconsidering significant observational uncertainties: thermal and non-thermal\ngas mothins alone cannot prevent collapse.\n