Acceleration of Cosmic Rays in Supernova Shocks: Elemental Selectivity of the Injection Mechanism

Precise measurements of galactic cosmic rays test mechanisms of their selective acceleration in supernova shocks. According to one mechanism, the mass-to-charge ratio A/Z is the key to selection. Striking similarities between He/$p$, C/$p$, and O/$p$ rigidity spectra, recently measured by AMS-02, confirms the premise. We study this mechanism by simulating a collisionless shock. A selection rate $\eta$ increases with A/Z, saturates, and peaks as a function of Mach number $M$. The dependence $\eta\left(M,A/Z\right)$ obtained from simulations and convoluted with the shock time dependence through $M\left(t\right),$ accurately reproduces the AMS-02 data.