Between the cosmic-ray ‘knee’ and the ‘ankle’: Contribution from star clusters

It is believed that Cosmic Rays (CRs) up to PeV (10⁶ GeV) are accelerated by supernova shocks, and extra-Galactic CR contribution dominates in the range above ∼ 10⁹ GeV. Therefore, some other yet unknown sources must act as sources for the CR spectrum in between. Our recent work shows that stellar winds from massive young star clusters can explain Galactic CRs in the intermediate range. The wind termination shock (WTS) in these star clusters is strong enough to accelerate particles in this energy range, which is difficult to reach in the standard paradigm of CR acceleration in supernova remnants. We present a model for producing different nuclei in CRs from massive stellar winds using the observed spatial distribution of young star clusters in the Galactic plane and the elemental abundances in the stellar wind. We present a detailed calculation of CR transport in the Galaxy, considering the effect of diffusion, interaction losses, and re-acceleration by older supernova remnants to determine the all-particle CR spectrum. Using the estimated magnetic field values instar clusters, we argue the WTS can accelerate protons up to a few tens of PeV. To match the observations with our model, an exponential energy cutoff of (60 − 70) × Z PeV and a cosmic-ray injection fraction of ∼ (5 − 6)% from the clusters are needed. We, therefore, argue that this CR component originating from star clusters is the natural ‘second component’ of Galactic cosmic rays.