A broadband view of the thermal and nonthermal emission from the embedded massive star cluster RCW 38

Abstract

Gamma-ray emission has been detected from a variety of source classes in the Galaxy, including clusters of young massive stars. RCW 38, a very young embedded massive star cluster, is of particular interest: its gamma-ray emission detected up to hundreds of GeV, provides the first observational evidence of high-energy particle acceleration powered exclusively by stellar winds. In this work, we aim to characterize the emission mechanisms responsible for the gamma-ray flux in RCW 38 and to estimate the acceleration efficiency, as well as the fraction of accelerated electrons compared to protons, K_ ep. We present the most comprehensive multiwavelength study of a single star cluster to date. Our analysis ranges from megahertz radio observations obtained with the GLEAM-X survey from the Murchison Widefield Array (MWA) to GeV gamma-ray data from Fermi -LAT, and includes gigahertz and terahertz measurements from Parkes, Planck , and IRAS. We modeled the thermal and nonthermal emission of RCW 38, including synchrotron, free-free, dust, bremsstrahlung, pion-decay, and inverse Compton processes, using an eight-parameter model constrained by the Markov chain Monte Carlo method. Our results support an interpretation in which hadronic interactions with the host molecular cloud produce the gamma-ray emission from RCW 38. We derive robust constraints on the electron-to-proton ratio, with K_ ̊m ep < 10^ -2 , and on the acceleration efficiency, which we estimate to be ≳ 1%. These results are consistent with those required to explain the cosmic-ray composition, particularly its 22 Ne anomaly. These results strengthen the idea that stellar clusters significantly contribute to cosmic-ray protons in our Galaxy, at least up to energies of a few TeV. Future investigations with the next generation of ground-based detectors will determine whether stellar clusters also play a relevant role at higher energies, particularly in the context of the cosmic-ray knee.