On the origin and composition of Galactic cosmic rays

TL;DR

This paper investigates the origin of Galactic Cosmic Rays, focusing on their composition, especially the Ne22/Ne20 ratio, and proposes that they are accelerated during the early Sedov phase of supernova shocks, challenging the superbubble hypothesis.

Contribution

It demonstrates that the observed Ne22/Ne20 ratio in GCR can be explained by supernova shocks during the early Sedov phase, providing a quantitative model and refuting the superbubble acceleration hypothesis.

Findings

GCR Ne22/Ne20 ratio explained by supernova shock acceleration during early Sedov phase

Acceleration efficiency of GCR particles is approximately 1e-6 to 1e-5

Superbubble acceleration hypothesis does not account for high Ne22/Ne20 ratio in GCR

Abstract

The composition of Galactic Cosmic Rays (GCR) presents strong similarities to the standard (cosmic) composition, but also noticeable differences, the most important being the high isotopic ratio of Ne22/Ne20 which is about 5 times higher in GCR than in the Sun. This ratio provides key information on the GCR origin. We investigate the idea that GCR are accelerated by the forward shocks of supernova explosions, as they run through the presupernova winds of the massive stars and through the interstellar medium. We use detailed wind and core yields of rotating and non-rotating models of massive stars with mass loss, as well as simple models for the properties of the forward shock and of the circumstellar medium. We find that the observed GCR Ne22/Ne20 ratio can be explained if GCR are accelerated only during the early Sedov phase, for shock velocities >1500-1900 km/s. The acceleration efficiency is found to be of the order of 1.e-6 - 1.e-5, i.e. a few particles out of a million encountered by the shock escape the SN at GCR energies. We also show quantitatively that the widely publicized idea that GCR are accelerated in superbubbles fails to account for the high Ne22/Ne20 ratio in GCR