Thermal-to-nonthermal element abundances in different Galactic environments

TL;DR

This paper presents a systematic model describing how element abundances transition from thermal to non-thermal states in cosmic ray sources, considering ionization and dust grain injection, with applications to supernova remnants and Wolf-Rayet winds.

Contribution

It introduces a novel parametrized model that accounts for dust grain injection and time-dependent ionization in cosmic ray acceleration, applied to various Galactic environments.

Findings

Dust grains are the dominant injection channel for heavy elements.

Wolf-Rayet explosions produce a harder cosmic ray spectrum.

Model explains spectral hardening observed in cosmic ray fluxes.

Abstract

The non-thermal 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. In this work a first systematic approach is presented that describes the change of the abundances from the thermal to the non-thermal state via non-linear diffusive shock acceleration by a temporally evolving shock. Hereby, not only time-dependent ionization states of elements contained in the ambient gas are considered, but also elements condensed on solid, charged dust grains, which not only can be injected into the acceleration process as well, but are from our findings even the dominant injection channel for most heavy elements. This generic parametrized model is then applied to the case of particle acceleration by supernova remnants in various ISM phases as well as Wolf-Rayet (WR) wind environments. We show that the overall low to medium energy cosmic ray distribution by WR explosions yield a significantly harder, which makes this contribution quite promising in order to explain the spectral hardening of the flux of certain elements, such as helium, observed by AMS-02 and other experiments at rigidities of about 1 TV, which would also be an important test for the potential role of WR-progenitor supernovae as the sources of Galactic cosmic rays around the second knee.