The effect of the virial state of molecular clouds on the influence of feedback from massive stars

A set of Smoothed Particle Hydrodynamics simulations of the influence of photoionising radiation and stellar winds on a series of 10^4 solar-mass turbulent molecular clouds with initial virial ratios of 0.7, 1.1, 1.5, 1.9 and 2.3 and initial mean densities of 136, 1135 and 9096 cm^−3 are presented. Reductions in star formation efficiency rates are found to be modest, in the range 30% − 50% and to not vary greatly across the parameter space. In no case was star formation entirely terminated over the ≈ 3 Myr duration of the simulations. The fractions of material unbound by feedback are in the range 20 − 60%, clouds with the lowest escape velocities being the most strongly affected. Leakage of ionised gas leads to the HII regions rapidly becoming underpressured. The destructive effects of ionisation are thus largely not due to thermally–driven expansion of the HII regions, but to momentum transfer by photoevaporation of fresh material. Our simulations have similar global ionisation rates and we show that the effects of feedback upon them can be adequately modelled as a steady injection of momentum, resembling a momentum–conserving wind.