Probing star clusters as cosmic ray factories

Stellar clusters are the fundamental building blocks of galaxies and are among the most studied astronomical objects in the Cosmos. The recent association of diffuse gamma-ray emission detected by different experiments with a dozen young clusters suggests that some acceleration process, able to accelerate particles at least up to hundreds of TeV, is at work. In this manuscript, we investigate the capability of young massive stellar clusters (YMSC) to produce cosmic rays under the scenario where particle acceleration is achieved at the cluster wind termination shock. The study is divided into three parts. First, we focus on the specific case of Cygnus OB2, one of the most massive young star clusters of the Milky Way. We model the observed gamma-ray emission assuming different models for particle propagation in the neighborhood of the cluster and that the observed emission is pure hadronic. We found that particles accelerated by Cygnus OB2 are able to reproduce both the gamma-ray spectrum and the radial morphology at very high energy if the particle diffusion coefficient close to the cluster has a spectral dependence intermediate between a Kraichnan and Bohm-like diffusion. However, the radial morphology observed by Fermi-LAT, characterized by a peaked profile, is in tension with our predictions characterized by a flat morphology. We conclude that a possible way to overcome such discrepancy is to include the inverse Compton emission from a population of freshly accelerated leptons. In the second part of the manuscript, we compute the diffuse gamma-ray emission expected by a synthetic population of galactic YMSC. For this purpose, we simulate a mock population of YMSC based on the properties of local (within ~2 kpc from the Sun) clusters. For every YMSC, we build the stellar population, so as to estimate the clusters wind properties. The expected gamma-ray emission is then compared to observations available in the literature in two specific regions of the Galactic Plane. Under the assumption of a pure hadronic emission, we found that YMSCs are potentially dominating the observed diffuse gamma-ray emission at a few TeV. In the final part of the work, we calculate the ionization rate induced by sub-GeV particles in molecular clouds close to YMSCs. We found that the ionization rate can significantly differ from the expected value in clouds positioned in the unperturbed interstellar medium. We show that the measured value of ionization rate, paired with gamma-ray observations, can be used to constrain particle diffusion in the vicinity of the stellar cluster, and we apply this method to the molecular cloud DR21, close to the YMSC Cygnus OB2. We found that the Bohm diffusion is favored compared to the Kraichnan one.