A&A 451, 141-146 (2006)
DOI: 10.1051/0004-6361:20054634

A dynamical model for the dusty ring in the Coalsack

P. Hennebelle¹, A. P. Whitworth² and S. P. Goodwin<sup>3

1</sup>  École Normale Supérieure, Laboratoire de Radioastronomie Millimétrique, 24 rue Lhomond, 75231 Paris Cedex 05, France
    e-mail: Patrick.Hennebelle@ens.fr
²  School of Physics & Astronomy, Cardiff University, 5 The Parade, Cardiff CF24 3YB, Wales, UK
    e-mail: Anthony.Whitworth@astro.cf.ac.uk
³  Department of Physics & Astronomy, University of Sheffield, Hicks Building, Housfield Road, Sheffield S3 7RH, UK
    e-mail: S.Goodwin@sheffield.ac.uk

(Received 18 July 2005 / Accepted 2 February 2006)

Abstract
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 . It is shown that even a low initial level of turbulence completely destroys the coherence of the ring, as seen in projection. In the scenario of strong external compression that we are proposing this implies that the initial turbulent energy in the globule was low, . Protostar formation should occur in about 40 000 years.

Key words: ISM: clouds -- stars: formation -- hydrodynamics -- instabilities -- shock waves

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