  Cosmic rays 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
harbor 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 ~
1-10% of the CR population is contributed by stellar clusters.
