Tails of the unexpected: the interaction of an isothermal shell with a cloud

A new mechanism for the formation of cometary tails behind dense clouds or globules is discussed. Numerical hydrodynamical models show that when a dense shell of swept-up matter overruns a cloud, material in the shell is focused behind the cloud to form a tail. This mode of tail formation is completely distinct from other methods, which involve either the removal of material from the cloud or shadowing from a strong, nearby source of ionization. This mechanism is relevant to the cometary tails seen in planetary nebulae and to the interaction of superbubble shells with dense clouds.