Probing hadronic gamma-ray and high-energy neutrino emission from Gaia DR2 star clusters

Abstract

Young and massive stellar clusters are a potential source of galactic cosmic rays due to at least two acceleration mechanisms. Collective stellar winds from massive stars form a wind-blown bubble with a termination shock at which particle acceleration to PeV energies may be achieved. Furthermore, shock acceleration may occur at supernova remnants (SNRs) expanding inside the bubble. We apply a model of cosmic ray acceleration at both the collective wind termination shock and SNR shocks to the catalog of known stellar clusters derived from the Gaia DR2. Predictions for the secondary fluxes of gamma-ray and neutrino emission are derived based on hadro-nuclear interactions with the surrounding medium. We compare our modelling under baseline and optimistic scenarios to available data, finding consistent results. An anticipated flux range is provided for a shortlist of the most promising stellar clusters. Approximately 10 clusters may be detectable with future facilities, and 1-3 could be currently operating as PeVatrons. Among these, data from three gamma-ray detected clusters can be consistently described by our model. Several further as-yet-undetected stellar clusters offer promising targets for future gamma-ray observations, although the flux range allowed by our model can be broad (>~factor 10). The large angular size of the wind-blown bubble may pose a challenge, leading to low surface brightness emission, thus exacerbating the problem of source confusion. Nevertheless, we discuss how further work will help to constrain stellar clusters as PeVatron candidates.