Nonthermal element abundances at astrophysical shocks

Abstract

The nonthermal source abundances of elements play a crucial role in the understanding of cosmic ray phenomena from a few GeV up to several tens of EeV . We present a first systematic approach to describe the change of the abundances from the thermal to the nonthermal state via diffusive shock acceleration by a temporally evolving shock. We consider hereby not only ionization states of elements contained in the ambient gas, which we allow to be time dependent due to shock heating, but also elements condensed on solid, charged dust grains which can be injected into the acceleration process as well. Our generic parametrized model is then applied to the case of particle acceleration by supernova remnants in various ISM phases, for which we use state-of-the-art computation packages to calculate the ionization states of all elements. The resulting predictions for low energy cosmic ray (LECR) source abundances are compared with the data obtained by various experiments. We obtain excellent agreement for shocks in warm ionized ISM environments, which include HII regions, if dust grains are injected into the diffusive shock acceleration process with a much higher efficiency than ions. Less dependence of the fit quality is found on the mass-to-charge ratio of ions. For neutral environments, assuming that there are shocks in the weakly ionized component, and for the hot ionized medium we obtain generally inferior fits, but except for the cold neutral medium we do not exclude them as subdominant sites of Galactic cosmic ray production. The key challenge is found to be putting the LECR abundance of pure gas phase elements like neon and the (semi-)volatile elements phosphorus, sulfur and chlorine into the right balance with silicon, calcium and elements of the iron group. We present a brief outlook to the potential consequences of our results for the understanding of the composition around the second knee or the cosmic ray spectrum, or for the viability of explaining ultra-high energy cosmic rays with a dominant contribution by radio galaxies.