Massive quiescent cores in Orion: dynamical state revealed by high-resolution ammonia maps.
LI D., KAUFFMANN J., ZHANG Q. and CHEN W.
Abstract (from CDS):
We present combined Very Large Array and Green Bank Telescope images of NH3 inversion transitions (1, 1) and (2, 2) toward OMC2 and OMC3. We focus on the relatively quiescent Orion cores, which are away from the Trapezium cluster and have no sign of massive protostars or evolved star formation. The 5'' angular resolution and 0.6 km/s velocity resolution enable us to study the thermal and dynamic state of these cores at ∼0.02 pc scales, comparable to or smaller than those of the current dust continuum surveys. We measure temperatures for a total of 30 cores, with average masses of 11 M☉, radii of 0.039 pc, virial mass ratio = 3.9, and critical mass ratio = 1.5 . Twelve sources contain Spitzer protostars. The thus defined starless and protostellar subsamples have similar temperature, line width, but different masses, with an average of 7.3 M☉ for the former and 16 M☉ for the latter. Compared to other Gould Belt dense cores, more Orion cores have a high gravitational-to-kinetic energy ratio and more cores have a larger than unity critical mass ratio. Orion dense cores have velocity dispersions similar to those of cores in low-mass star-forming regions but larger masses for given size. Some cores appear to have truly supercritical gravitational-to-kinetic energy ratios, even when considering significant observational uncertainties: thermal and non-thermal gas motions alone cannot prevent collapse.