The structure of circumstellar shells

Speckle-interferometric measurements are reported for the brightness distributions of the OH/IR stars OH 26.5 + 0.6 and IRC + 10420 at wavelengths which are near the center of and just outside the 10-micron absorption/emission feature produced by circumstellar dust. For OH 26.5 + 0.6, the angular size within the absorption feature is 0.50 + or - 0.02 arcsec, while the angular size outside the feature is less than 0.2 arcsec. For IRC + 10420, the angular sizes inside and outside the emission feature are both 0.42 + or - 0.02 arcsec. Simple models of the circumstellar shells are calculated which can account for the measured angular sizes and flux distributions of the objects. The models give the wavelength dependence of the opacity of the circumstellar material, which is quite different for the two objects.