Gamma rays from star clusters and implications for the origin of Galactic cosmic rays

Abstract

Context. Star clusters are often invoked as contributors to the flux of Galactic cosmic rays and as sources potentially able to accelerate particles to $\sim$PeV energies. The gamma radiation with $E\gtrsim$ TeV recently observed from selected star clusters has profound implications for the origin of Galactic cosmic rays. Aims. We show that if the gamma rays observed from the Cygnus cocoon and Westerlund 1 are of hadronic origin, then the cosmic rays escaping the cluster at energies $\gtrsim$ 10 TeV must cross a grammage inside the cluster that exceeds the Galactic grammage. At lower energies, depending on the model adopted to describe the production of gamma rays, such grammage may exceed or be comparable with the grammage inferred from propagation on Galactic scales. Methods. The flux of gamma rays is analytically computed for a few models of injection of cosmic rays in star clusters, and compared with the flux measured from selected clusters. Results. In all models considered here, comparing the inferred and observed gamma ray fluxes at $E\gtrsim$ TeV, we conclude that CRs must traverse a large grammage inside or around the cluster before escaping. Clearly these implications would not apply to a scenario in which gamma rays are produced due to radiative losses of leptons in the cluster. Leptonic models typically require weaker magnetic fields, which in turn result in maximum energies of accelerated particles much below $\sim$ PeV. Conclusions. We conclude that if gamma ray emission in SCs is a generic phenomenon and if this radiation is due to hadronic interactions, either star clusters cannot contribute but a small fraction of the total cosmic ray flux at the Earth, or their contribution to the grammage cannot be neglected and the paradigm of Galactic transport should be profoundly revisited.