SIMBAD references

We analyze the fragmentation of shocked interstellar gas layers. Our formulation is designed to explore the consequences of radiative cooling behind the shock, to derive from first principles the basic phenomenology of the fragmentation of shocked interstellar gas layers, and to emphasize how little this phenomenology depends on the details of the process generating the layer. Two processes are considered: colliding clouds and expanding nebulae (i.e. HII regions, stellar wind bubbles and supernova remnants). In both these processes, there is a stage during the accumulation of the layer when it switches from being confined mainly by ram pressure to being confined mainly by self-gravity. Gravitational fragmentation of the layer into protostellar condensations occurs well before confinement of the layer becomes dominated by self-gravity. Consequently fragmentation of the layer results in protostellar condensations whose initial size and separation measured parallel to the layer is considerably greater than the thickness of the layer, and whose masses are significantly greater than would be predicted if fragmentation were delayed until the layer became confined mainly by self-gravity. Such fragments are likely to condense into massive stars in OB associations or subgroups.