Young massive star clusters as teraelectronvolt emitters: Constraints from H.E.S.S. and LHAASO

Young massive star clusters (YMSCs) have been proposed as excellent candidates for the main sources of Galactic cosmic rays (CRs) up to the petaelectronvolt range. The detection and study of gamma rays in the very-high-energy (VHE, E>100 GeV) range has added to arguments in favour of this hypothesis. To date, observations with current instruments have detected only a few YMSCs. Next-generation observatories are expected to significantly increase this number, providing a larger sample that will improve our ability to constrain the role of YMSCs in the origin of CRs. We studied the population of YMSCs detected in the TeV range and their properties, confronting simulations of the YMSC population to the current observed sample, to address fundamental questions regarding particle acceleration at YMSC shocks concerning the spectrum of accelerated particles, the efficiency of the CR production, and the fraction of the wind luminosity that is converted into turbulent magnetic fields. We used Monte Carlo methods to simulate the Galactic population of YMSCs in the gamma--ray domain and confront our simulations to the catalogue of sources of the systematic survey of the Galactic plane performed by H. E. S. S. (HGPS) and the catalogue from the all-sky instrument LHAASO, the First LHAASO Catalogue of Gamma-Ray Sources. We systematically explored the parameter space of our model, including, for example, the slope of accelerated particles, α; the CR efficiency, η_̊m CR; the fraction of the wind luminosity converted into turbulent magnetic field, η_̊m b; and the diffusion regime. In particular, we found five possible sets of parameters for which ≳ 75% of Monte Carlo realisations are found to be in agreement with the combined data from the HGPS and LHAASO first catalogue: α =4. 5, η_ ̊m CR = 10^ -2. 0, and η_ ̊m b = 10^ -2. 0, L_ ̊m inj = 1 pc---assuming the Kraichnan diffusion regime---and α = 4. 4, η_ ̊m CR = 10^ -2. 5, ηb = 10^ -3. 0, with L_ ̊m inj = 0. 1 pc, assuming the Kolmogorov diffusion regime. Certain scenarios and regions of parameter space are strongly disfavoured, such as the Bohm diffusion regime at YMSCs. Our model successfully reproduces the YMSC population observed in both the HGPS and the First LHAASO Catalogue of Gamma-Ray Sources. With future systematic surveys, such as those by the Cherenkov Telescope Array Observatory (CTAO), this approach will help break degeneracies and improve our understanding of particle acceleration at YMSC shocks in the Galaxy.