A dynamical model for the dusty ring in the Coalsack
  Lada et al. recently presented a detailed near-infrared extinction map of Globule G2 in the Coalsack molecular cloud complex, showing that this starless core has a well-defined central extinction minimum. We propose a model for G2 in which a rapid increase in external pressure is driving an approximately symmetric compression wave into the core. The rapid increase in external pressure could arise because the core has recently been assimilated by the Coalsack cloud complex, or because the Coalsack has recently been created by two large-scale converging flows. The resulting compression wave has not yet converged on the centre of the core, so there is a central rarefaction. The compression wave has increased the density in the swept-up gas by about a factor of ten, and accelerated it inwards to speeds of order $0.4 {\rm km} {\rm s}^{-1}$. It is shown that even small levels of initial turbulence destroy the ring seen in projection almost completely. In the scenario of strong external compression that we are proposing this implies that the initial turbulent energy in this globule is such that $E_{{\rm turb}} / E_{{\rm grav}} \le 2 %$. Protostar formation should occur in about $40,000 {\rm years}$. 
P. Hennebelle, A. P. Whitworth and S. P. Goodwin