Star clusters in the gamma-ray sky

TL;DR

This paper reviews gamma-ray observations of young massive star clusters to evaluate their potential role in accelerating cosmic rays up to PeV energies, addressing observational challenges and implications for Galactic cosmic ray sources.

Contribution

It provides an analysis of gamma-ray data from young star clusters, highlighting their possible contribution to Galactic cosmic rays and discussing observational constraints.

Findings

Gamma-ray detections suggest potential particle acceleration in star clusters.

Young clusters (<few million years) are key to isolating wind-driven cosmic ray contributions.

Observational contamination complicates definitive attribution to stellar winds.

Abstract

Massive Star Clusters (SCs) have been proposed as additional contributors to Galactic Cosmic rays (CRs), to overcome the limitations of supernova remnants (SNRs) to reach the highest energy end of the CR spectrum. Thanks to fast mass losses due to the collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies, and their energetics is enough to account for a non-negligible fraction of the Galactic CRs. Anyhow, the theoretical expectations need to be corroborated by clear observations. Despite the increasing number of detections at different energies, the contamination of other sources often makes it difficult to constrain the contribution arising from stellar winds only, unless one selects objects younger than a few million years, namely before stars start to explode inside clusters. I will review the results obtained with gamma-ray data towards a few massive young star clusters and discuss what implications these result have, especially concerning their contribution to the bulk of Galactic CRs.