Massive star clusters in the gamma-ray sky: the role of HII regions

TL;DR

This paper investigates the role of massive star clusters in gamma-ray emission and cosmic-ray acceleration, analyzing Fermi-LAT data to isolate their contribution and exploring future observational prospects with CTAO and ASTRI.

Contribution

It provides new analysis of gamma-ray data from young star clusters, isolating their contribution to cosmic rays and assessing their significance compared to other sources.

Findings

Star clusters can accelerate particles up to PeV energies.

Young clusters (<3 Myr) allow isolation of stellar contribution without supernova influence.

Statistical correlation between gamma-ray sources and star clusters is quantified.

Abstract

Massive Star Clusters (SCs) have been proposed as important CR sources, with the potential of explaining the high-energy end of the Galactic cosmic-ray (CR) spectrum, that Supernova Remnants (SNRs) seem unable to account for. 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 the energetics is enough to account for a large fraction of the Galactic CRs, if the system is efficient enough. A handful of star clusters have been detected in gamma-rays confirming the idea that particle acceleration is taking place in this environment. However, contamination by other sources often makes it difficult to constrain the contribution arising from SCs only. Here we present a new analysis of Fermi-LAT data collected towards a few massive young star clusters. The young age (< 3 Myr) of the clusters guarantees that no SN has exploded in the region, allowing us to determine the power contributed by the stellar component alone, and to quantify the contribution of this type of sources to the bulk of CRs. Moreover, we will present a recent statistical investigation that quantifies the degree of correlation between gamma-ray sources and these astrophysical objects and briefly discuss the observational prospect for ASTRI and CTAO.