The origin of Galactic cosmic rays as revealed by their composition

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, ionization 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 greater than or similar to 2 MK, which is typical of the hot medium found in Galactic superbubbles energized by the activity of massive star winds and supernova explosions. Another GCR component, which is responsible for the overabundance of Ne-22, 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 arc 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 Ne-22.

Automated summary

This study delves into the composition of Galactic cosmic rays (GCRs) using recent measurements from various experiments, revealing that the volatile elements of CR material are accelerated from a hot plasma typical of Galactic superbubbles. It also suggests that the overabundance of certain elements in GCRs may originate from specific sources, such as Ne-22 from wind termination shocks. The study estimates ion acceleration efficiency in supernova shocks and wind termination shocks to be around 10^(-5), and highlights the preferential acceleration of refractory elements contained in interstellar dust in the GCR source composition.