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

Charles J. Lada, Tracy L. Huard, Lionel J. Crews, João F. Alves

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 2×105 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 C18O 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.

External organisation(s)
Harvard-Smithsonian Center for Astrophysics, University of Tennessee at Martin, European Southern Observatory (Germany)
The Astrophysical Journal: an international review of astronomy and astronomical physics
Publication date
Peer reviewed
Austrian Fields of Science 2012
103004 Astrophysics
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