Between the cosmic-ray `knee' and the `ankle': Contribution from star clusters

Abstract

We show that massive young star clusters may be possible candidates that can accelerate Galactic cosmic rays (CRs) in the range of $10^7–10^9$ GeV (between the `knee' and `ankle'). Various plausible scenarios such as acceleration at the wind termination shock (WTS), supernova shocks inside these young star clusters, etc. have been proposed,since it is difficult to accelerate particles up to the $10^7–10^9$ GeV range in the standard paradigm of CR acceleration in supernova remnants. We consider a model for the production of different nuclei in CRs from massive stellar winds using the observed distribution of young star clusters in the Galactic plane. We present a detailed calculation of CR transport in the Galaxy, taking into account the effect of diffusion, interaction losses during propagation, and particle re-acceleration by old supernova remnants to determine the all-particle CR spectrum. Using the maximum energy estimate from the Hillas criterion, we argue that a young massive star cluster can accelerate protons up to a few tens of PeV. Upon comparison with the observed data, our model requires a CR source spectrum with an exponential cutoff of $5\times 10^{7}Z$ GeV ($50Z$~PeV) from these clusters together with a cosmic-ray injection fraction of $\sim 5\%$ of the wind kinetic energy. We discuss the possibility of achieving these requirements in star clusters, and the associated uncertainties, in the context of considering star clusters as the natural accelerator of the `second component' of Galactic cosmic rays.

18 pages, 6 figures, accepted for publication in ApJ

References in corpus (17)

• Detection of the Characteristic Pion-Decay Signature in Supernova Remnants
• Cosmic-Ray Proton and Helium Spectra from the First CREAM Flight
• Measurement of the cosmic-ray proton spectrum from 40 GeV to 100 TeV with the DAMPE satellite
• Measurement of the Flux of Ultra High Energy Cosmic Rays by the Stereo Technique
• Potential PeVatron supernova remnant G106.3+2.7 seen in the highest-energy gamma rays
• GeV-TeV cosmic-ray spectral anomaly as due to re-acceleration by weak shocks in the Galaxy
• Non-linear diffusive acceleration of heavy nuclei in supernova remnant shocks
• The Origin of Galactic Cosmic Rays as Revealed by their Composition
• Massive star cluster origin for the galactic cosmic ray population at very-high energies
• Realistic modeling of wind and supernovae shocks in star clusters: addressing ${}^{22}\mathrm{Ne}{/}^{20}\mathrm{Ne}$ and other problems in Galactic cosmic rays
• Turbulence and particle acceleration in collisionless supernovae remnant shocks: II- Cosmic-ray transport
• Long way to go: how outflows from large galaxies propagate through the hot halo gas
• Cosmic Ray Acceleration by a Versatile Family of Galactic Wind Termination Shocks
• Search for magnetic fields in particle-accelerating colliding-wind binaries
• Observational constraints on the maximum energies of accelerated particles in supernova remnants: low maximum energies and a large variety
• Cosmic rays from massive star clusters : A close look at Westerlund 1
• Diffuse Galactic Gamma-rays from star clusters