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 La rge Array confirm the expansion rate and extend the temporal baseline. Comb ined H alpha and radio proper-motion measurements of the nebula's outer edg e across the major axis suggest that the nebula is roughly 120 yr old, alth ough complications exist due to nonuniform motion. The linear expansion rat e 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 ev olutionary state of the stellar components. Historical observations lead us to speculate that at least one of the components in the binary system is a n S Doradus variable or a close relative of this class of stars. [S III] la mbda 9532 emission traces higher excitation than H alpha, but it shows only minor intensity variations with position in the nebula. The width of the i onization fronts at the limb-brightened edges of the nebula are nearly unre solved in our WFPC2 images, validating proper-motion measurements that comb ine both H alpha and radio continuum emission.