DYNAMICS OF CONDUCTIVE COOLING FRONTS - CLOUD IMPLOSION AND THERMAL SOLITONS

We investigate the evolution of interfaces among phases of the interstellar medium with different temperatures. It is found that, for some initial conditions, the dynamical effects related to conductive fronts are very important even if radiation losses, which tend to decelerate the front propagation, are taken into account. We also explored the consequences of the inclusion of shear and bulk viscosity, and we have allowed for saturation of the kinetic effects. Numerical simulations of a cloud immersed in a hot medium have been performed; depending on the ratio of conductive to dynamical time, the density is increased by a huge factor and the cloud may become optically thick. Clouds that are highly compressed are able to stop the evaporation process even if their initial size is smaller than the Field length. In addition to the numerical approach, the time-dependent evolution has been studied also analytically. Simple techniques have been applied to the problem in order to study the transition stages to a stationary state. The global properties of the solution for static and steady fronts and useful relations among the various physical variables are derived; a mechanical analogy is often used to clarify the physics of the results. It is demonstrated that a class of soliton-like solutions are admitted by the hydrodynamical equations appropriate to describe the conduction/cooling fronts (in the inviscid case) that do not require a heat flux at the boundaries. Solitons are shown to result from the exact balance of convective and conductive energy transport, and we demonstrate that they can exist only as associated with the fast velocity mode (analogous to the positive Riemann invariant) of the system. Some astrophysical consequences are indicated along with some possible applications to the structure of the Galactic ISM and the extragalactic objects.