Wolf–Rayet stars as tracers of gamma-ray emission

Context. Recent gamma-ray observations of young star clusters revealed that stellar wind termination shocks accelerate particles. The energy reservoir for particle acceleration is provided by the wind mechanical power of cluster member stars. Aims. Our goal is to provide a list of promising targets to guide future gamma-ray studies of stellar clusters and associations powered by massive stars. As the wind power of a single Wolf-Rayet (WR) star can be of the same order as the cumulative wind power of the most massive star clusters, we also extend our study to isolated WR stars. This is particularly interesting as a large fraction of WR stars are indeed isolated. Methods. We first ranked a large sample of star clusters and associations according to the number of member WR stars over their distance squared, a quantity that is proportional to the expected gamma-ray signal. We then searched for possible spatial correlations between these objects and known gamma-ray sources. We repeated the same procedure for a sample of individual WR stars for which wind mechanical power and distance are known. Results. We found a hint (≲3 σ confidence) of a correlation between star clusters hosting WR stars and unidentified giga-electronvolt gamma-ray sources and we list new spatial associations for 11 clusters. Moreover, we found spatial coincidences between four isolated WR stars (WR110, WR114, WR111, and WR14) and unidentified gamma-ray sources. While we do not find a correlation between isolated WR stars and gamma-ray sources, we stress that these four isolated stars are characterized by extremely large values of the wind power divided the distance squared, which is a necessary condition to have gamma-ray emission detectable by current instruments. Assuming that the gamma-ray emission is powered by the WR stellar wind, we interpret it as the result of interactions between particles accelerated at the wind termination shock and ambient matter or photon fields. Further studies on the most promising targets (both clusters and isolated WR stars) are encouraged in order to validate or confute the associations. Conclusions. Based on the fact that the wind power of an individual WR star can rival that of a full stellar cluster, we present a ranking of stellar clusters and isolated WR stars that may represent potential gamma-ray emitters.