Gamma-ray production in young open clusters: Berk 87, Cyg OB2 and Westerlund 2

Abstract

Young open clusters are likely sites of cosmic ray acceleration as indicated by recent detections of the TeV gamma-ray sources in the directions of two open clusters (Cyg OB2 and Westerlund 2) and their directional proximity to some unidentified EGRET sources. In fact, up to now a few different scenarios for acceleration of particles inside open clusters have been considered, i.e. shocks in massive star winds, pulsars and their nebulae, supernova shocks, massive compact binaries. Here we consider in detail the radiation processes due to both electrons and hadrons accelerated inside the open cluster. As a specific scenario, we apply the acceleration process at the shocks arising in the winds of WR type stars. Particles diffuse through the medium of the open cluster during the activity time of the acceleration scenario defined by the age of the WR star. They interact with the matter and radiation, at first inside the open cluster and, later in the dense surrounding clouds. We calculate the broad band spectrum in different processes for three example open clusters (Berk 87, Cyg OB2, Westerlund 2) for which the best observational constraints on the spectra are at present available. It is assumed that the high energy phenomena, observed from the X-ray up to the GeV-TeV gamma-ray energies, are related to each other. We conclude that the most likely description of the radiation processes in these objects is achieved in the hybrid (leptonic-hadronic) model in which leptons are responsible for the observed X-ray and GeV gamma-ray emission and hadrons are responsible for the TeV gamma-ray emission, which is produced directly inside and in dense clouds surrounding the open cluster.