Discovery of a Dusty Ring in the Coalsack: A Dense Core Caught in the Act of Formation?

We present a new infrared extinction study of Globule 2, the most opaque molecular cloud core in the Coalsack complex. Using deep near-infrared imaging observations obtained with the ESO New Technology Telescope, we are able to examine the structure of the globule in significantly greater detail than previously possible. We find the most prominent structural feature of this globule to be a strong central ring of dust column density that was not evident in lower resolution studies of this cloud. This ring represents a region of high density and pressure that is likely a transient structure. For a spherical cloud geometry, the ring would correspond to a dense inner shell of high pressure that could not be in dynamical equilibrium with its surroundings, since there appear to be no sources of pressure in the central regions of the cloud that could support the shell against gravity and prevent its inward implosion. The timescale for the inward collapse of the ring would be less than 2 × 10 5 yr, suggesting that this globule is in an extremely early stage of evolution, and is perhaps being caught in the process of forming a centrally condensed dense core or Bok globule. Outside its central regions, the globule displays a well-behaved density profile whose shape is very similar to that of a stable Bonnor-Ebert sphere. Using the Swedish ESO Submillimeter Telescope, we also obtained a C 18 O spectrum toward the center of the cloud. The CO observation indicates that the globule is a gravitationally bound object. Analysis of the CO line profile reveals significant nonthermal gas motions likely due to turbulence. As a whole, the globule may be evolving to a global state of quasi-static dynamical equilibrium in which thermal and turbulent pressure balance gravity.