Shock-triggered formation of magnetically-dominated clouds

To understand the formation of a magnetically dominated molecular cloud out of an atomic cloud, we follow the dynamical evolution of the cloud with a time-dependent axisymmetric magnetohydrodynamic code. 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. As a fast-mode shock propagates through the cloud, the gas behind it becomes thermally unstable. The $\beta$ 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.