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

TL;DR

This study combines extinction mapping and molecular line analysis to reveal that a dense ring in Globule 2 results from merging subsonic turbulent flows, indicating a transient, young, and evolving structure in the Coalsack Nebula.

Contribution

It provides a detailed kinematic analysis linking the formation of a dense ring to merging subsonic flows, supporting a dynamical model of pressure-driven structure formation.

Findings

The ring shows enhanced non-thermal velocity dispersion and pressure.

A velocity gradient indicates two converging subsonic flows.

The globule is likely young and evolving from outside in.

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 C18O (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 C18O non-thermal velocity dispersion and non-thermal 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 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.