cross sections differ by less than 20%.

We note that the observed CR energy density and the galactic SN statistics require that $\eta_p \gsim 10$% over the lifetimes of isolated SNRs to power galactic CRs. However, the instantaneous efficiency may be much larger during early stages of the SNR evolution [@blasiAARv13]. In the case of a SNR colliding with a wind, we expect the efficiency to be even higher than in a young isolated SNR and assume it can reach $\eta_p \gsim$ 0.5 for the CSF stage lasting for a few hundred years [@MNRAS_BGO13]. Furthermore, since CSFs produce very hard spectra when ${t_\mathrm{acc}}\ll {t_\mathrm{esc}}$, i.e., $N(\gamma) \propto \gamma^{-1}$, most of the CR energy lies in the high-energy tail just below the upper break which occurs when ${t_\mathrm{acc}}$ is greater than the escape time, ${t_\mathrm{esc}}$. The essential properties of CSF acceleration in young stellar clusters are high overall acceleration efficiency, spectra harder than $N(\gamma) \propto \gamma^{-2}$, and maximum proton energies $\gsim 40$PeV achieved in a few hundred years.

![The small dashed circles (not to scale) indicate the inner CR acceleration region in the massive young cluster . The inner CR acceleration region has a radius of 3-4pc (shown in a Chandra 