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

Abstract A set of Smoothed Particle Hydrodynamics simulations of the influence of photoionizing radiation and stellar winds on a series of 104 M⊙ 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 of 30–50 per cent, and do 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 of 20–60 per cent, clouds with the lowest escape velocities being the most strongly affected. Leakage of ionized gas leads to the H ii regions rapidly becoming underpressured. The destructive effects of ionization are thus largely not due to thermally driven expansion of the H ii regions, but to momentum transfer by photoevaporation of fresh material. Our simulations have similar global ionization 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.