Superbubbles and Energetic Particles in the Galaxy. I: Collective effects of particle acceleration

TL;DR

This paper explores how the collective effects of clustered massive stars and supernovae within superbubbles influence particle acceleration, potentially explaining features of Galactic Cosmic Rays and supported by observational data.

Contribution

It investigates the collective impact of superbubbles on particle acceleration, highlighting effects not seen in isolated supernova remnants, and connects these to cosmic ray properties.

Findings

Repeated shock acceleration occurs inside superbubbles.

Clustering enhances particle acceleration efficiency.

Observational data supports superbubbles as cosmic ray sources.

Abstract

Observations indicate that most massive stars in the Galaxy appear in groups, called OB associations, where their strong wind activity generates large structures known as superbubbles, inside which the subsequent supernovae (SNe) explode, in tight space and time correlation. Acknowledging this fact, we investigate four main questions: 1) does the clustering of massive stars and SN explosions influence the particle acceleration process usually associated with SNe, and induce collective effects which would not manifest around isolated supernova remnants?; 2) does it make a difference for the general phenomenology of Galactic Cosmic Rays (GCRs), notably for their energy spectrum and composition?; 3) can this help alleviate some of the problems encountered within the standard GCR source model?; and 4) Is the link between superbubbles and energetic particles supported by observational data, and can it be further tested and constrained? We argue for a positive answer to all these questions. Theoretical, phenomenological and observational aspects are treated in separate papers. Here, we discuss the interaction of massive stellar winds and SN shocks inside superbubbles and indicate how this leads to specific acceleration effects. We also show that due to the high SN explosion rate and low diffusion coefficient, low-energy particles experience repeated shock acceleration inside superbubbles.