Dr Thierry Montmerle
In massive star-forming regions, the evolution of massive stars (M > 20 Msol) forming "OB associations" is so rapid that they explode as supernovae close to their birthplaces, the molecular clouds. Before reaching this final stage, they lose a significant fraction of their mass via dense stellar winds. The result is a feedback effect, in the form of vast hot plasma bubbles filling the star-forming region. When supernovae collide with molecular clouds, there is observational evidence that they are powerful cosmic-ray accelerators : at high energies, they induce GeV-TeV gamma-ray emission; at low energies, they induce enhanced ionization and peculiar meV chemistry. But they can also explode in the pre-existing wind cavities, in which case cosmic rays are inefficiently accelerated or convected away by the hot gas. As a result, the high-energy life of an OB associations is made up of successive phases of long (Myr) quiescent, wind-dominated phases, interrupted by temporary (< 0.1 Myr) episodes of supernova-dominated phases, possibly characterized by an intense, localized acceleration of cosmic rays.