Massive Quiescent Cores in Orion: Dynamical State Revealed by High-Resolution Ammonia Maps

TL;DR

This study uses high-resolution ammonia maps to analyze the physical and dynamical states of quiescent cores in Orion, revealing many are gravitationally supercritical and prone to collapse, unlike typical low-mass star-forming cores.

Contribution

It provides detailed measurements of temperature, mass, and virial ratios of Orion cores at high resolution, highlighting their supercritical state and potential for collapse, which was not well characterized before.

Findings

Many Orion cores are gravitationally supercritical.

Core velocities are similar to low-mass regions but with larger masses.

Some cores are truly supercritical, indicating imminent collapse.

Abstract

We present combined VLA and Green Bank Telescope 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 of $11\,\Ms$, radii of $0.039 \rm{}pc$, virial mass ratio $\bar{R_{vir}}$ = 3.9, and critical mass ratio $\bar{R_{C}}$ = 1.5. Twelve sources contain \textit{Spitzer} protostars. The thus defined starless and protostellar subsamples have similar temperature, line width, but different masses, with an average of $7.3\,\Ms$ for the former and $16\,\Ms$ for the latter. Compared to others Gould Belt dense cores, mores Orion cores have a high gravitational-to kinetic energy ratio and more cores have a larger thant unity critical mass ratio. Orion dense cores have velocity dispersion similar to those of cores in low-mass star-forming regions but larger masses for fiven size. Some cores appear to have truly supercritical gravitational-to-kinetic energy ratios, even when considering significant observational uncertainties: thermal and non-thermal gas mothins alone cannot prevent collapse.