D. Li, J. Kauffmann, Q. Zhang, W. Chen
We present combined VLA and GBT images of \ammonia\ 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 nor evolved star formation, such as IRAS source, water maser, and methanol maser. The 5\arcsec\ angular resolution and $0.6 \rm{}km s^{-1}$ velocity resolution of these data enable us to study the thermal and dynamic state of these cores at $\sim{}0.02 \rm{}pc$ 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 and radii of $11 \Ms$ and $0.039 \rm{}pc$, respectively. Compared to other Gould Belt dense cores, the Orion cores have an unusually high gravitational-to-inetic energy ratio (virial mass ratio $R_{vir} > >1$), resembling results for other clouds forming high--mass stars. This results from Orion cores having velocity dispersions similar to those in, e.g., Perseus and Ophiuchus, but higher masses for given sizes. 12 out of 30 cores are associated with embedded YSOs, which are identified by Spitzer. These Prostellar cores have a mean mass of $16.5 \Ms$ versus $7.3 \Ms$ for that of the starless cores. 14 of starless massive Orion cores are supercritical (mass-to-critical-mass ratio R$_c > 1$). These massive Orion starless cores will likely collapse or fragment relatively quickly and can thus be considered direct precursors to protostars.
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http://arxiv.org/abs/1207.1178
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