The dense ring in the Coalsack: the merging of two subsonic flows?

JM Rathborne, CJ Lada, W Walsh, M Saul… - The Astrophysical …, 2008 - iopscience.iop.org
JM Rathborne, CJ Lada, W Walsh, M Saul, HM Butner
The Astrophysical Journal, 2008iopscience.iop.org
ABSTRACT A recent high angular resolution extinction map toward the most opaque
molecular globule, Globule 2, in the Coalsack Nebula revealed that it contains a strong
central ring of dust column density. This ring represents a region of high density and
pressure that is likely a transient and possibly turbulent structure. Dynamical models suggest
that the ring has formed as a result of a sudden increase in external pressure which is
driving a compression wave into the Globule. Here we combine the extinction …
Abstract
A recent high angular resolution extinction map toward the most opaque molecular globule, Globule 2, in the Coalsack Nebula revealed that it contains a strong central ring of dust column density. This ring represents a region of high density and pressure that is likely a transient and possibly turbulent structure. Dynamical models suggest that the ring has formed as a result of a sudden increase in external pressure which is driving a compression wave into the Globule. Here we combine the extinction measurements with a detailed study of the C 18 O (1–0) molecular line profiles toward Globule 2 in order to investigate the overall kinematics and, in doing so, test this dynamical model. We find that the ring corresponds to an enhancement in the C 18 O nonthermal velocity dispersion and nonthermal pressure. We observe a velocity gradient across the Globule that appears to trace two distinct systematic subsonic velocity flows that happen to converge within the ring. We suggest, therefore, that the ring has formed as two subsonic flows of turbulent gas merge within the Globule. The fact that the outer layers of the Globule appear stable against collapse, and yet there is no centrally condensed core, suggests that the Globule may be evolving from the outside in and has yet to stabilize, confirming its youth.
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