On the destruction of star-forming clouds

Type II supernovae (SNe), probably the most important contributors to stellar feedback in galaxy formation, explode within the very dense star-forming clouds, where the injected energy is most easily radiated away. The efficiency of type II SNe in injecting energy into the interstellar medium (ISM) and in re-heating a fraction of the original star-forming cloud is estimated with the aid of a two-phase model for the ISM of the cloud. We argue that when SNe explode the star-forming cloud has already been partially destroyed by ionizing light and winds from massive stars. SN remnants (SNRs) will first cause the collapse of most of the cloud gas into cold fragments, until the diffuse hot phase has a low enough density to make further radiative losses negligible. This is completed in ~3 Myr, with a modest energy loss of ~5 per cent of the total budget. We compute that a fraction ranging from 5 to 30 per cent of the cloud is reheated to a high temperature (from 10^5 to 10^7 K); these numbers are very uncertain, due to the very complicated nature of the problem, and to the uncertain role of thermal evaporation. Small star-forming clouds, less massive than ~10^4 Msun, will be destroyed by a single SN. In all cases, a high fraction of the energy from type II SNe (~80 per cent for large clouds, smaller but still significant for small clouds) will be available for heating the ISM.