  We developed a new population synthesis code for groups of massive stars,
where we model the emission of different forms of energy and matter from the
stars of the association. In particular, the ejection of the two radioactive
isotopes 26Al and 60Fe is followed, as well as the emission of hydrogen
ionizing photons, and the kinetic energy of the stellar winds and supernova
explosions. We investigate various alternative astrophysical inputs and the
resulting output sensitivities, especially effects due to the inclusion of
rotation in stellar models. As the aim of the code is the application to
relatively small populations of massive stars, special care is taken to address
their statistical properties. Our code incorporates both analytical statistical
methods applicable to small populations, as well as extensive Monte Carlo
simulations. We find that the inclusion of rotation in the stellar models has a
large impact on the interactions between OB associations and their surrounding
interstellar medium. The emission of 26Al in the stellar winds is strongly
enhanced, compared to non-rotating models with the same mass-loss prescription.
This compensates the recent reductions in the estimates of mass-loss rates of
massive stars due to the effects of clumping. Despite the lower mass-loss
rates, the power of the winds is actually enhanced for rotating stellar models.
The supernova power (kinetic energy of their ejecta) is decreased due to longer
lifetimes of rotating stars, and therefore the wind power dominates over
supernova power for the first 6 Myr after a burst of star-formation. For
populations typical of nearby star-forming regions, the statistical
uncertainties are large and clearly non-Gaussian.
