Realistic modeling of wind and supernovae shocks in star clusters: addressing ${\rm ^{22}Ne/^{20}Ne}$ and other problems in Galactic cosmic rays

TL;DR

This paper presents a theoretical model showing that wind termination shocks in star clusters significantly contribute to Galactic cosmic rays, explaining isotopic ratios and high-energy acceleration, thus enhancing the supernova paradigm.

Contribution

It quantifies the contribution of wind termination shocks in superbubbles, demonstrating their importance in cosmic ray production and isotopic signatures, which was previously uncertain.

Findings

WTSs contribute at least 25% to CRs, up to 50% in young clusters.

The model explains the observed ^22Ne/^20Ne ratio in CRs.

WTSs can accelerate CRs to PeV energies, addressing longstanding issues.

Abstract

Cosmic ray (CR) sources leave signatures in the isotopic abundances of CRs. Current models of Galactic CRs that consider supernovae (SNe) shocks as the main sites of particle acceleration cannot satisfactorily explain the higher ${\rm ^{22}Ne/^{20}Ne}$ ratio in CRs compared to the interstellar medium. Although stellar winds from massive stars have been invoked, their contribution relative to SNe ejecta has been taken as a free parameter. Here we present a theoretical calculation of the relative contributions of wind termination shocks (WTSs) and SNe shocks in superbubbles, based on the hydrodynamics of winds in clusters, the standard stellar mass function, and stellar evolution theory. We find that the contribution of WTSs towards the total CR production is at least $25\%$, which rises to $\gtrsim 50\%$ for young ($\lesssim 10$ Myr) clusters, and explains the observed $^{22}{\rm Ne}/^{20} {\rm Ne}$ ratio. We argue that since the progenitors of apparently isolated supernovae remnants (SNRs) are born in massive star clusters, both WTS and SNe shocks can be integrated into a combined scenario of CRs being accelerated in massive clusters. This scenario is consistent with the observed ratio of SNRs to $\gamma$-ray bright ($L_\gamma \gtrsim 10^{35}$ erg s$^{-1}$) star clusters, as predicted by star cluster mass function. Moreover, WTSs can accelerate CRs to PeV energies, and solve other longstanding problems of the standard supernova paradigm of CR acceleration.