Proper Motion and Excitation Structure of the Expanding Ionized Rings around RY Scuti

Abstract

Multiepoch HST/WFPC2 images of the massive eclipsing binary RY Scutireveal that the compact ionized rings in its circumstellar nebula areexpanding. Radio images of the free-free continuum at 15 GHz obtainedwith the Very Large Array confirm the expansion rate and extend thetemporal baseline. Combined H{$α$} and radio proper-motionmeasurements of the nebula's outer edge across the major axis suggestthat the nebula is roughly 120 yr old, although complications exist dueto nonuniform motion. The linear expansion rate supports a shellejection hypothesis for the origin of RY Scuti's double-ring nebula andhas important implications for the mass-loss history and evolutionarystate of the stellar components. Historical observations lead us tospeculate that at least one of the components in the binary system is anS Doradus variable or a close relative of this class of stars. [S III]{$λ$}9532 emission traces higher excitation than H{$α$}, but itshows only minor intensity variations with position in the nebula. Thewidth of the ionization fronts at the limb-brightened edges of thenebula are nearly unresolved in our WFPC2 images, validatingproper-motion measurements that combine both H{$α$} and radiocontinuum emission. Based on observations made with the NASA/ESA HubbleSpace Telescope, obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555.