On the ionization of clouds and the formation of stars

This paper studies the interaction of ionization fronts with a density discontinuity across which the density increases. The motivation for this work has been the recent suggestion that stars form from the gravitational collapse of the layer of shocked neutral gas produced as the ionization front-shock system moves into a cloud. During the ionization front-cloud interaction one can detect the appearance of an isothermal shock moving towards the star (usub(s) approximately 9 km/s). This affects the rest of the evolution by slowing down the ionization front and its associated shock. Thus, the question is whether or not sufficient material can be compressed in the layer for this to undergo gravitational collapse. Several points that are stressed should be taken into consideration when one seeks star formation with such a mechanism. (U.K.)