§26 Background to shock-cloud collisions

The problem of the collision of a shock wave with a cloud has been intensively investigated in the past by several authors (see for example Klein et al. 1994 and references therein). The simplest assumption is to consider a cloud for which gravitational effects can be neglected, magnetic fields are non-important and radiative losses are negligible. The fact that gravity is not taken into account, makes it possible to consider the density of the cloud to be uniform. The complete 3D hydrodynamical problem is extremely complicated, even under the simplifications mentioned above. However, numerical simulations have been carried out for this case which ultimately give rise to instabilities causing a complete disruption of the cloud (Klein et al., 1994).

This chapter describes how the solution of the one dimensional problem can be obtained. It has been argued in the past that at least for the case in which the density contrast is high, i.e. the ratio of the cloud's density to that of the external environment is high, the problem has to be very similar to that found in the problem of a collision of a plane parallel shock with with a solid wall (Spitzer 1982, McKee 1988).

With these considerations in mind, the present discussion aims to give a simple approach to solve a particular case of the whole problem. In the description of the solution of the problem of the collision between a plane parallel shock and a plane parallel cloud, it is assumed that the specific volume in the cloud is a first order quantity. In other words solutions are given for the case in which the density of the cloud is much greater than that of the surrounding environment.