Ionization-induced star formation - II. External irradiation of a turbulent molecular cloud

In this paper, we examine numerically the difference between triggered and revealed star formation. We present smoothed particle hydrodynamic (SPH) simulations of the impact on a turbulent 10(4) M-circle dot molecular cloud of irradiation by an external source of ionizing photons. In particular, using a control model, we investigate the triggering of star formation within the cloud. We find that, although feedback has a dramatic effect on the morphology of our model cloud, its impact on star formation is relatively minor. We show that external irradiation has both positive and negative effects, accelerating the formation of some objects, delaying the formation of others and inducing the formation of some that would not otherwise have formed. Overall, the calculation in which feedback is included forms nearly twice as many objects over a period of similar to 0.5 freefall times (similar to 2.4 Myr), resulting in a star formation efficiency approximately one-third higher (similar to 4 per cent as opposed to similar to 3 per cent at this epoch) as in the control run in which feedback is absent. Unfortunately, there appear to be no observable characteristics which could be used to differentiate objects whose formation was triggered from those which were forming anyway and which were simply revealed by the effects of radiation, although this could be an effect of poor statistics.