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We study the evolution of isolated and merging interstellar bubbles of threestars (25, 32 and 60 M\odot) in a homogeneous background medium with a densityof 10 mp /ccm via 3D-hydrodynamic simulations with standard ISM thermodynamics(optically thin radiative cooling and photo-electric heating) and timedependent energy and mass input according to stellar evolutionary tracks. Wevary the position of the three stars relative to each other to compare theenergy response for cases of isolated, merging and initially cospatial bubbles.Due to mainly the Vishniac instability, our simulated bubbles develop thickshells and filamentary internal structures in column density. The shell widthsreach tens of per cent of the outer bubble radius, which compares favourably toobservations. More energy is retained in the ISM for more closely packedgroups, by up to a factor of three and typically a factor of two forintermediate times after the first supernova. Once the superbubble isestablished, different positions of the contained stars make only a minordifference to the energy tracks. For our case of three massive stars, theenergy deposition varies only very little for distances up to about 30 pcbetween the stars. Energy injected by supernovae is entirely dissipated in asuperbubble on a timescale of about 1 Myr, which increases slightly with thesuperbubble size at the time of the explosion. The Vishniac instability may beresponsible for the broadening of the shells of interstellar bubbles. Massivestar winds are significant energetically due to their - in the long run - moreefficient, steady energy injection and because they evacuate the space aroundthe massive stars. For larger scale simulations, the feedback effect of closegroups of stars or clusters may be subsumed into one effective energy inputwith insignificant loss of energy accuracy.

Feedback by massive stars and the emergence of superbubbles I. Energy efficiency & Vishniac instabilities