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

Abstract

We report new 3 to 20 μm Keck Telescope images of the massive eclipsing binary RY Scuti with ~0.25" spatial resolution. These images show complex structure near the diffraction limit of the 10 m telescope in a nebula less than 2" across. The limb-brightened structure is consistent with optically thin emission from a circumstellar torus. At 3 to 5 μm, the extended emission is predominantly scattered photospheric light from the central stars. The 8.9, 11.7, 12.5, and 18.7 μm images are dominated by thermal emission from warm carbon and silicate dust in the outer regions of the torus. The new observations are consistent with the geometric models previously proposed by Gehrz et al. and Smith et al. to explain the structures observed in the circumstellar environment of RY Scuti. For example, the inside of the torus is ionized by the central star and shows strong emission from [Ne II] at 12.8 μm. Double-ring structure similar to that seen in Hubble Space Telescope images is partly resolved at the limb-brightened edges of the torus in the new mid-infrared continuum images, and the new IR images are consistent with two parallel dust rings adjacent to the ionized gas rings and farther from the star. The new mid-infrared images at 8.9, 11.7, and 12.5 μm show faint emission from a thin disk outside the previously detected torus. We estimate a dust mass in the torus of 1.4×10^-6 M_solar and a neon abundance in the ionized gas of n_Ne/n_H>~1.77×10^-4 from the 12.8 μm [Ne II] emission. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.