The Origin of Galactic Cosmic Rays as Revealed by their Composition

TL;DR

This paper investigates the origin of galactic cosmic rays by analyzing their composition, revealing they originate mainly from hot superbubble plasma and star wind shocks, with dust grain sputtering contributing to refractory element acceleration.

Contribution

It provides a detailed composition-based model linking cosmic ray origins to superbubbles, star winds, and dust sputtering, explaining observed abundance patterns and neon overabundance.

Findings

CRs mainly accelerated from hot plasma in superbubbles

Star wind shocks contribute to neon-22 overabundance

Refractory elements are preferentially accelerated from dust grains

Abstract

Galactic cosmic-rays (GCRs) are thought to be accelerated in strong shocks induced by massive star winds and supernova explosions sweeping across the interstellar medium. But the phase of the interstellar medium from which the CRs are extracted has remained elusive until now. Here, we study in detail the GCR source composition deduced from recent measurements by the AMS-02, Voyager 1 and SuperTIGER experiments to obtain information on the composition, ionisation state and dust content of the GCR source reservoirs. We show that the volatile elements of the CR material are mainly accelerated from a plasma of temperature higher than $\sim 2$ MK, which is typical of the hot medium found in galactic superbubbles energised by the activity of massive star winds and supernova explosions. Another GCR component, which is responsible for the overabundance of $^{22}$Ne, most likely arises from acceleration of massive star winds in their termination shocks. From the CR-related $\gamma$-ray luminosity of the Milky Way, we estimate that the ion acceleration efficiency in both supernova shocks and wind termination shocks is of the order of $10^{-5}$. The GCR source composition also shows evidence for a preferential acceleration of refractory elements contained in interstellar dust. We suggest that the GCR refractories are also produced in superbubbles, from shock acceleration and subsequent sputtering of dust grains continuously incorporated into the hot plasma through thermal evaporation of embedded molecular clouds. Our model explains well the measured abundances of all primary and mostly primary CRs from H to Zr, including the overabundance of $^{22}$Ne.