Star clusters in the gamma-ray sky

TL;DR

This paper investigates whether winds from massive star clusters (SCs) can accelerate Galactic cosmic rays (CRs) to PeV energies, potentially complementing supernova remnants (SNRs). It reviews gamma-ray observations of young to evolved SCs, showing that emission often aligns with wind-driven HII shells and can be modeled as hadronic processes powered by wind termination shocks, with typical CR acceleration efficiencies around $\eta_{CR} \sim 0.5$%–1% and a conservative Galactic contribution of at least ~1%. The analysis highlights the importance of environmental parameters (gas density, radiation fields) and diffusion in wind bubbles, and notes that distinguishing hadronic from leptonic components requires MeV-band measurements near the pion bump. While a few SCs show TeV to PeV signals, the overall evidence supports SC winds as a modest, non-negligible source of Galactic CRs, with upcoming facilities like CTA and ASTRI-MiniArray needed to robustly test PeVatron prospects; SNRs inside clusters remain a plausible complementary path to high-energy CR production.

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.

Figures (3)

• Figure 1: The regions around the Heart and Soul nebulae, namely the Hii regions blown by the embedded clusters IC 1805 and IC 1848, respectively. For an illustrative purpose, the expected size of the termination shock and of the wind-blown bubble are shown. The action of the wind creates typical filaments known as “fingers” visible the surrounding dust. The thick layer of gas and dust around the bubble serves as target for gamma-ray production [Background image by https://unsplash.com/it/foto/un-primo-piano-di-una-formazione-stellare-nel-cielo-_4a3ROTYL5k]
• Figure 2: On the left: the Hii region surrounding NGC 3603 as seen by WISE at 22$\mu$m; the size of the wind-blown bubble calculated using Weaver formula and $\eta_M=100\%$ or $\eta_M=1\%$ are shown as a dashed and a solid turquoise circle, respectively. The position of unidentified Fermi-LAT sources are indicated as red circles with an inner cross. The contours indicate significance detection levels obtained with Fermi-LAT observations at 9, 10 and 12$\sigma$ as reported in Peron2025Ricap. On the right: the radial distribution of the gas and of the CRs for increasing distance from the center of the cluster Peron2025.
• Figure 3: Broad-band emission expected from the SC NGC 6611. The GeV emission is predominantly hadronic, while the leptonic emission is modeled starting from electrons with the same injection spectrum as protons, but assuming a reduced normalization of $K_{ep}=0.015$, and the appropriate energy losses, considering a magnetic field of $\sim 10 \mu G$. Peron2025.