While the classical paradigm emphasises supernova explosions as the dominant source of cosmic rays, it has been speculated that CRs are also accelerated in stellar winds around massive young stars before they explode as supernovae, or in star clusters. Indeed, there is growing evidence in existing gamma-ray data for a population of sources related to young stellar clusters and environments with strong stellar winds. However, the current instruments’ sensitivity prevents the detailed study of these sources of gamma radiation. The Carina Nebula is one of the most active HII regions. It harbours 8 open stellar clusters with more than 60 O-type stars, 3 Wolf Rayet (WR) stars, and η Carinae, the only colliding-wind binary firmly established to emit gamma rays. Scientists of the Astronomy Department of UniGe have measured the periodic modulation of its gamma-ray emission during two entire orbits of the system. Confrontation of these data with MHD simulations suggests that the X-ray to GeV emission is of leptonic origin while the GeV-TeV photons are likely associated with hadronic emission generated by proton-proton collisions. These γ rays are produced via diffuse shock acceleration along the shocks formed in the wind-collision system. The huge luminosity of the primary star (5 million times larger than the luminosity of the Sun), provides a very large pool of soft photons as a target for γ-γ absorption. The variability of that absorption will be very well observable by the southern CTAO array on a daily/weekly timescale and will allow to confirm the physical mechanisms at play and constrain system parameters.

Spectral energy distribution of Cassiopeia A, a 320 yr-old SNR. The pion decay model tracing protons (blue line) best fits the MAGIC data [arXiv:1707.01583]