Shock-triggered formation of magnetically-dominated clouds

¹ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK e-mail: svenvl@ast.leeds.ac.uk
² School of Mathematics, University of Leeds, Leeds LS2 9JT, UK
³ Astrophysics Research Institute, Liverpool John Moores University, Birkenhead, CH41 1LD, UK

Received: 7 March 2007
Accepted: 4 June 2007

Abstract

Aims.Our aim is to understand the formation of a magnetically dominated molecular cloud out of an atomic cloud.

Methods.A thermally stable warm atomic cloud is initially in static equilibrium with the surrounding hot ionised gas. A shock propagating through the hot medium interacts with the cloud. We follow the dynamical evolution of the cloud with a time-dependent axisymmetric magnetohydrodynamic code.

Results.As a fast-mode shock propagates through the cloud, the gas behind it becomes thermally unstable. The β value of the gas also becomes much smaller than the initial value of order unity. These conditions are ideal for magnetohydrodynamic waves to produce high-density clumps embedded in a rarefied warm medium. A slow-mode shock follows the fast-mode shock. Behind this shock a dense shell forms, which subsequently fragments. This is a primary region for the formation of massive stars. Our simulations show that only weak and moderate-strength shocks can form cold clouds which have properties typical of giant molecular clouds.