The Dusty AGB Star RS CrB: First MidInfrared Interferometric Observations with the Keck Telescopes

We report interferometric observations of the semiregular variable star RS CrB, a red giant with strong silicate emission features. The data were among the first long-baseline mid-infrared stellar fringes obtained between the Keck telescopes, using parts of the new nulling beam combiner. The light was dispersed by a low-resolution spectrometer, allowing simultaneous measurement of the source visibility and intensity spectra from 8 to 12 m. The interferometric observations allow a nonambiguous determination of the dust shell spatial scale and relative flux contribution. Using a simple spherically symmetric model, in which a geometrically thin shell surrounds the stellar photosphere, we find that 30% to 70% of the overall mid-infrared fluxdepending on the wavelengthoriginates from 78 stellar radii. The derived shell opacity profile shows a broad peak around 11 m ( 0.06), characteristic of Mg-rich silicate dust particles.