Supernova remnants in super bubbles acting as cosmic ray accelerators

Supernova remnants (SNRs) are often considered the main sites of acceleration of cosmic rays in our Galaxy, possibly up to the ‘knee’. However, their ability to accelerate particles to reach PeV energies is questionable and lacks observational evidence. Theoretical predictions suggest that only a small sub-class of very young SNRs evolving in dense environments could potentially satisfy the necessary conditions to accelerate particles to PeV energies. Most theoretical investigations of this type have been carried out either in the standard interstellar medium or in the wind of the progenitor. Since most core-collapse supernovae (CC SNe) occur in star clusters, it is important to extend such investigations to SNRs taking place within a star cluster. In this work, we focus on an SNR shock propagating in the collective wind of a compact star cluster. We studied the acceleration process as a function of time, with a special emphasis on the maximum energy of accelerated particles. Using both analytic and numerical approaches, we investigated the spectrum of accelerated particles and maximum achievable energy in the case of pre-existing turbulence in the collective wind and self-generated magnetic perturbations. We find that similarly to the case of isolated SNRs, the acceleration to PeV energies is plausible only in extreme conditions that are achievable only for a small sub-set of SNRs.