Wolf-Rayet stars as tracers of gamma-ray emission: Isolated stars and stellar clusters/associations

TL;DR

This study assesses Wolf-Rayet stars and stellar clusters as potential sources of gamma-ray emission, identifying promising targets and spatial correlations with unidentified gamma-ray sources.

Contribution

It introduces a ranking method based on wind power and distance to identify promising gamma-ray emitters among clusters and isolated WR stars.

Findings

Hint of correlation between WR-hosting clusters and gamma-ray sources

Identified 11 clusters with new spatial associations

Found 4 isolated WR stars coinciding with gamma-ray sources

Abstract

Context: Recent gamma-ray observations of young star clusters revealed that stellar wind termination shocks accelerate particles, with the energy reservoir provided by the mechanical power of massive-star winds. Aims: Our goal is to identify promising targets for 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 rival the cumulative wind power of the most massive clusters, we also investigate isolated WR stars, many of which are indeed isolated. Methods: We ranked a large sample of stellar clusters and associations according to the number of member WR stars divided by the distance squared, a quantity proportional to the expected gamma-ray signal, and searched for spatial correlations with known gamma-ray sources. We repeated the same procedure for individual WR stars with known wind mechanical powers and distances. Results: We found a hint ($\lesssim 3 \sigma$ confidence) for a correlation between WR-hosting clusters and unidentified GeV gamma-ray sources, and identified new spatial associations for 11 clusters. We also found spatial coincidences between 4 isolated WR stars (WR110, WR114, WR111, and WR14) and unidentified gamma-ray sources. Although no significant correlation is found for isolated WR stars as a population, these 4 objects exhibit particularly large wind-power-to-distance-squared ratios, a necessary condition for detectability with current instruments. Assuming the gamma-ray emission is powered by WR winds, it can be interpreted as arising from interactions between particles accelerated at the wind termination shock and ambient matter or radiation fields. Conclusions: Since the wind power of an individual WR star can rival that of an entire stellar cluster, we provide a ranking of stellar clusters and isolated WR stars that may constitute potential gamma-ray emitters.