Feedback by massive stars and the emergence of superbubbles I. Energy
  efficiency & Vishniac instabilities
  We study the evolution of isolated and merging interstellar bubbles of three
stars (25, 32 and 60 M\odot) in a homogeneous background medium with a density
of 10 mp /ccm via 3D-hydrodynamic simulations with standard ISM thermodynamics
(optically thin radiative cooling and photo-electric heating) and time
dependent energy and mass input according to stellar evolutionary tracks. We
vary the position of the three stars relative to each other to compare the
energy response for cases of isolated, merging and initially cospatial bubbles.
Due to mainly the Vishniac instability, our simulated bubbles develop thick
shells and filamentary internal structures in column density. The shell widths
reach tens of per cent of the outer bubble radius, which compares favourably to
observations. More energy is retained in the ISM for more closely packed
groups, by up to a factor of three and typically a factor of two for
intermediate times after the first supernova. Once the superbubble is
established, different positions of the contained stars make only a minor
difference to the energy tracks. For our case of three massive stars, the
energy deposition varies only very little for distances up to about 30 pc
between the stars. Energy injected by supernovae is entirely dissipated in a
superbubble on a timescale of about 1 Myr, which increases slightly with the
superbubble size at the time of the explosion. The Vishniac instability may be
responsible for the broadening of the shells of interstellar bubbles. Massive
star winds are significant energetically due to their - in the long run - more
efficient, steady energy injection and because they evacuate the space around
the massive stars. For larger scale simulations, the feedback effect of close
groups of stars or clusters may be subsumed into one effective energy input
with insignificant loss of energy accuracy.
