We report interferometric observations of the semi-regular 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 8–12 \mu m. The interferometric observations allow a non-ambiguous 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 \sim 30 % to \sim 70 % of the overall mid-infrared flux - depending on the wavelength - originates from 7-8 stellar radii .
The derived shell opacity profile shows a broad peak around 11 microns ( \tau \simeq 0.06 ) , characteristic of Mg-rich silicate dust particles .