Keck LWS Images of the Compact Nebula around RY Scuti in the Thermal Infrared

Abstract

We report new 3 to 20 {$μ$}m Keck Telescope images of the massiveeclipsing binary RY Scuti with ~0.25'' spatial resolution. These imagesshow complex structure near the diffraction limit of the 10 m telescopein a nebula less than 2'' across. The limb-brightened structure isconsistent with optically thin emission from a circumstellar torus. At 3to 5 {$μ$}m, the extended emission is predominantly scatteredphotospheric light from the central stars. The 8.9, 11.7, 12.5, and 18.7{$μ$}m images are dominated by thermal emission from warm carbon andsilicate dust in the outer regions of the torus. The new observationsare consistent with the geometric models previously proposed by Gehrz etal. and Smith et al. to explain the structures observed in thecircumstellar environment of RY Scuti. For example, the inside of thetorus is ionized by the central star and shows strong emission from [NeII] at 12.8 {$μ$}m. Double-ring structure similar to that seen in HubbleSpace Telescope images is partly resolved at the limb-brightened edgesof the torus in the new mid-infrared continuum images, and the new IRimages are consistent with two parallel dust rings adjacent to theionized gas rings and farther from the star. The new mid-infrared imagesat 8.9, 11.7, and 12.5 {$μ$}m show faint emission from a thin diskoutside the previously detected torus. We estimate a dust mass in thetorus of 1.4{\times}10^{-6} M_{solar} and a neonabundance in the ionized gas ofn_{Ne}/n_{H}{\gt}~1.77{\times}10^{-4} from the12.8 {$μ$}m [Ne II] emission. The data presented herein were obtained atthe W. M. Keck Observatory, which is operated as a scientificpartnership among the California Institute of Technology, the Universityof California, and the National Aeronautics and Space Administration.The Observatory was made possible by the generous financial support ofthe W. M. Keck Foundation.