The contribution of winds from star clusters to the Galactic cosmic-ray population

Cosmic rays (CR) are energetic nuclei that permeate the entire Galactic disk. Their existence requires the presence of powerful particle accelerators. While Galactic supernova explosions may supply the required energy, there is growing evidence that they cannot explain all of the observed properties of cosmic rays, such as their maximum particle energy and isotopic composition. Among Galactic objects, winds from stellar clusters meet the energetic requirement and provide a suitable environment for particle acceleration. The recent detection of some of these objects in gamma rays confirms that they indeed harbour high-energy particles. However, as most supernovae explode inside stellar clusters, it is difficult to distinguish the contribution of winds to particle acceleration. Here we report the detection of young star clusters in the nearby Vela molecular ridge star-forming region. The young age of the systems guarantees an unbiased estimate of the stellar CR luminosity free from any supernova or pulsar contamination and allows us to draw conclusions on the acceleration efficiency and the total power supplied by these objects. We demonstrate that much more than 1% of the wind mechanical power is converted into CRs and consequently conclude that a small but non-negligible fraction, similar to 1-10% of the CR population, is contributed by stellar clusters.