High-energy cosmic rays have been studied for more than a century since their discovery, yet their origin and acceleration mechanisms remain not fully understood. While supernova remnants have long been considered the dominant accelerators of the Galactic cosmic-ray population up to the knee, increasing observational and theoretical evidence suggests that this picture is incomplete, particularly considering composition an maximal energies. Stellar clusters and associations of massive stars constitute natural complementary candidates, as they host powerful stellar winds and successive supernova explosions that can sustain extended regions of particle acceleration over million-year timescales. Cosmic rays accelerated in these environments can interact with the surrounding gas and radiation fields, producing high-energy gamma-ray emission, with nearby molecular clouds providing particularly sensitive probes of their presence and activity.
Within this context, this work investigates massive-star environments as potential sources of high-energy emission, from the identification of promising stellar clusters, and isolated massive stars to the parametric characterization of cosmic-ray propagation and their interaction with the surrounding medium. The resulting framework is applied to selected stellar systems and molecular clouds, using gamma-ray observations to investigate the connection between massive stars, cosmic-ray acceleration, and transport properties.