Proper motion and excitation structure of the expanding ionized rings around RY Scuti

Multiepoch HST/WFPC2 images of the massive eclipsing binary RY Scuti reveal that the compact ionized rings in its circumstellar nebula are expanding. Radio images of the free-free continuum at 15 GHz obtained with the Very Large Array confirm the expansion rate and extend the temporal baseline. Combined Hα and radio proper-motion measurements of the nebula's outer edge across the major axis suggest that the nebula is roughly 120 yr old, although complications exist due to nonuniform motion. The linear expansion rate supports a shell ejection hypothesis for the origin of RY Scuti's double-ring nebula and has important implications for the mass-loss history and evolutionary state of the stellar components. Historical observations lead us to speculate that at least one of the components in the binary system is an S Doradus variable or a close relative of this class of stars. [S III] λ9532 emission traces higher excitation than Hα, but it shows only minor intensity variations with position in the nebula. The width of the ionization fronts at the limb-brightened edges of the nebula are nearly unresolved in our WFPC2 images, validating proper-motion measurements that combine both Hα and radio continuum emission.