Comparison of Star Clusters With and Without Wolf-Rayet Stars in Wolf-Rayet Galaxies

We compare the properties of young star clusters with and without Wolf-Rayet (W-R) stars in W-R galaxies using optical, near-infrared imagery and optical spectroscopy. Our work identifies the clusters with W-R stars in these galaxies for the first time. With this information, comparisons of clusters with and without W-R stars are now possible, enabling us to understand the chemical and morphological impact of massive stars on their environment and to constrain the parameters for modeling these systems. We find that clusters with W-R stars (W-R clusters) are systematically younger, bluer clusters. Knowing this age difference between the two cluster sets, we use an evolutionary scenario to interpret their other properties. Young clusters, typically W-R clusters, have a Strömgren sphere-like gas configuration. They also tend to have H - K colors redder than those of theoretical models. We interpret the H - K excess as a combination of thermal emission from hot dust, nebular emission, and molecular emission. Older clusters, typically clusters without W-R stars, have ionized gas in a superbubble configuration caused by the prolonged influence of stellar winds and supernovae. The H - K excess is generally absent for these clusters. The nitrogen-to-oxygen abundance ratio (N/O) does not appear to increase as a function of age over the first 10 Myr. Systems without W-R stars do appear to have a significant, elevated N/O over systems with W-R stars in the metallicity range 12 + log(O/H) = 7.7-7.9. For the entire metallicity range in our sample, this finding is only marginally significant. We concur with previous studies, which find no correlation between the sulfur-to-oxygen abundance ratio and metallicity.