Modeling gamma-ray signatures of particle acceleration in stellar clusters from GeV to PeV

Abstract

Young massive stellar clusters (YMSCs) have recently regained interest as PeVatron candidates, potentially accounting for the cosmic-ray (CR) knee as alternatives to isolated supernova remnants (SNRs). LHAASO's unique capability to detect photons above 0.1 PeV, hence tracing multi-PeV CRs, can provide critical constraints on galactic acceleration models when combined with H.E.S.S. and Fermi-LAT data. We investigate the transport of particles from YMSCs acceleration sites, namely wind termination shocks (WTS) or embedded SNRs, to nearby dense molecular clouds where proton-proton interactions produce high-energy gamma rays. We determine the necessary conditions, such as the distance between the acceleration site and the target, or the cluster's power and age, for detectable gamma-ray excesses and identify viable systems through parameter space exploration. By comparing with observations, we can constrain key physical parameters including WTS efficiency, diffusion coefficient and injection slope. Our analysis also examines whether some of LHAASO's unidentified sources might correspond to such cluster-cloud systems.