Acceleration of Heavy Ions at Nonrelativistic Collisionless Shocks

Abstract We investigate the process of diffusive shock acceleration of particles with mass number to charge number ratios A / Q > 1, e.g., partially ionized heavy ions. To this end, we introduce helium- and carbon-like ions at solar abundances into two-dimensional hybrid (kinetic ions–fluid electrons) simulations of nonrelativistic collisionless shocks. This study yields three main results: (1) Heavy ions are preferentially accelerated compared to hydrogen. For typical solar abundances, the energy transferred to accelerated helium ions is comparable to, or even exceeds, that of hydrogen, thereby enhancing the overall shock acceleration efficiency. (2) Accelerated helium ions contribute to magnetic field amplification, which increases the maximum attainable particle energy and steepens the spectra of accelerated particles. (3) The efficient acceleration of helium significantly enhances the production of hadronic γ- rays and neutrinos, likely dominating the one due to hydrogen. These effects should be taken into account, especially when modeling strong space and astrophysical shocks.