Ring nebulae: what they tell us about Wolf-Rayet stars

Abstract

The environments of evolved massive stars provide an opportunity of obtaining information on the past, as well as current, condition of the stars themselves. In this review we will look at the incidence of ring nebulae around Wolf-Rayet stars, their differing morphologies at various wavelengths and the existence of multiple, concentric shells. We use this information to show that WRs are indeed evolved stars and that the various phases of evolution for a WR star are evidenced in their environments. Abundance measurements and kinematics show that complex forms of mass ejection are likely to have occurred in the evolution of WR stars providing clumpy structures of dust, and both ionized and neutral gas. Gas kinematics also provide estimates to the time-scales of each of the evolutionary phases of WR stars, which can be combined with estimates of nebular masses to provide the approximate values for such crucial parameters as total mass-loss and historical mass-loss rates. Overall, it is illustrated that studies of the environments of WR stars have the potential to provide important information about the mass-loss history of very massive stars, including estimates of the time period of each mass-loss phase, typical mass loss rates, total mass lost and likely evolutionary path. Some of the remaining problems relating to the use of ring nebulae as probes to the evolutionary history of WR stars are also discussed.