The energy and momentum input of supernova explosions in structured and ionised molecular clouds

We investigate the early impact of single and binary supernova (SN) explosions on dense gas clouds with three-dimensional, high-resolution, hydrodynamic simulations. The effect of cloud structure, radiative cooling, and ionising radiation from the progenitor stars on the net input of kinetic energy, f_kin=E_kin/E_SN, thermal energy, f_therm=E_therm/E_SN, and gas momentum f_P=P/P_SN to the interstellar medium (ISM) is tested. For clouds with n=100 cm^{-3}, the momentum generating Sedov and pressure-driven snowplough phases are terminated early (~ 0.01 Myr) and radiative cooling limits the coupling to f_therm ~ 0.01, f_kin ~ 0.05, and f_P ~ 9, significantly lower than for the case without cooling. For pre-ionised clouds these numbers are only increased by ~ 50%, independent of the cloud structure. This only suffices to accelerate ~ 5% of the cloud to radial velocities >30km/s. A second SN might further enhance the coupling efficiencies if delayed past the Sedov phase of the first explosion. Such very low coupling efficiencies cast doubts on many galaxy-scale sub-resolution models for supernova feedback, most of which are validated a posteriori by qualitative agreement of galaxy properties with observations. Ionising radiation appears not to significantly enhance the immediate coupling of SNe to the surrounding gas as it drives the ISM into inert dense shells and cold clumps, a process which is unresolved in galaxy scale simulations. Our results support previous conclusions that supernovae might only drive a wind if a significant fraction explodes in low-density environments or if they are supported by processes other than ionising radiation.