Particle acceleration in winds of star clusters

The origin of cosmic rays in our Galaxy remains a subject of active debate. While supernova remnant (SNR) shocks are often invoked as the sites of acceleration, it is now widely accepted that the difficulties of such sources in reaching PeV energies are daunting and it seems likely that only a subclass of rare remnants can satisfy the necessary conditions. Moreover, the spectra of cosmic rays escaping the remnants have a complex shape that is not obviously the same as the spectra observed at the Earth. Here, we investigate the process of particle acceleration at the termination shock that develops in the bubble excavated by star clusters' winds in the interstellar medium. While the main limitation to the maximum energy in SNRs comes from the need for effective wave excitation upstream so as to confine particles in the near-shock region and speed up the acceleration process, at the termination shock of star clusters the confinement of particles upstream is guaranteed by the geometry of the problem. We develop a theory of diffusive shock acceleration at such shock and we find that the maximum energy may reach the PeV region for powerful clusters in the high end of the luminosity tail for these sources. A crucial role in this problem is played by the dissipation of energy in the wind to magnetic perturbations. Under reasonable conditions, the spectrum of the accelerated particles has a power-law shape with a slope 4/4.3, in agreement with what is required based upon standard models of cosmic ray transport in the Galaxy.

Automated summary

The origins of cosmic rays in our Galaxy remain a subject of ongoing debate, with supernova remnant shocks considered as possible sites of acceleration. However, it is widely accepted that reaching PeV energies from such sources is challenging. Accelerated particles escaping remnants have complex spectra not matching with those observed at Earth. Diffusive shock acceleration at the termination shock of star clusters is investigated, where maximum energy may reach the PeV region for powerful clusters. Energy dissipation in the wind to magnetic perturbations plays a crucial role in shaping the spectrum of accelerated particles. The possibility of a power-law spectrum with a slope of 4/4.3 is found, in agreement with standard models of cosmic ray transport in the Galaxy.