Star formation in shocked layers

We analyze the gravitational stability of a shocked interstellar gas layer and show how such a layer fragments into protostellar condensations whilst it is still confined mainly by ram pressure. As a consequence, the resulting protostars are massive and well separated. Our analysis is completely general and applies both to layers resulting from collisions between molecular cloud clumps, and to shells swept up by expanding nebulae. We present a numerical simulation of the former scenario, which produces a cluster of 35 massive stars resembling an OB subgroup, with most of the stars in binary systems.