Wolf-Rayet stars in young massive star clusters as potential sources of Galactic cosmic rays

TL;DR

This paper explores how Wolf-Rayet stars in young massive star clusters could be significant sources of Galactic cosmic rays, especially explaining the observed neon isotope ratios in cosmic rays.

Contribution

It introduces a model where cosmic rays are accelerated by shocks from Wolf-Rayet star winds in young clusters, linking stellar nucleosynthesis to cosmic ray composition.

Findings

Wolf-Rayet star winds can produce neon isotope ratios matching cosmic ray observations.

Cluster shock acceleration can account for a significant fraction of Galactic cosmic rays.

The model suggests WR stars as potential primary sources of certain cosmic ray components.

Abstract

For most elements, the isotopic ratios seen in cosmic rays (CRs) are similar to those in the solar wind. The most important exception to this is $^{22}$Ne/$^{20}$Ne where the CR value is $\sim 5$ times that of the solar wind. According to most recent models of nucleosynthesis, a large amount of $^{22}\mathrm{Ne}$ is generated in Wolf-Rayet (WR) stars. In the winds of carbon sequence WR stars, i.e., WC stars, the isotopic ratio $^{22}\mathrm{Ne}$/$^{20}\mathrm{Ne}$ can be much larger than in the solar wind. Here, we consider CRs produced by $^{22}\mathrm{Ne}$-enriched WR winds in compact clusters of young massive stars like Westerlund~1. We assume that efficient CR acceleration in clusters occurs over the WR lifetime in an ensemble of shock waves from multiple massive star winds. We estimate the fraction of all Galactic CRs such sources may produce for a given set of parameters.