IRAS LOW-RESOLUTION SPECTRAL OBSERVATIONS OF THE 10 AND 18 MICRON SILICATE EMISSION FEATURES

The IRAS LRS Atlas contains 8-23-mu-m spectra of 1816 stars with silicate emission features. In order to study the shapes of the silicate dust features, the 117 stars whose emission features appear to be optically thin and which have the best signal-to-noise ratio at the longest wavelengths were analyzed. Simple spherical dust shell models were calculated in both the optically thin and the slightly optically thick approximations. From the comparison of the predicted spectra of the dust plus stellar continuum, the emissivity function kappa-lambda was derived. In the different stars, the shape of the 10-mu-m feature is either narrow or broad and it is peaked either at approximately 9.7-mu-m or at 10-mu-m. Either particle size effects (particles greater-than-or-similar-to 0.75-mu-m in radius) or optical depth effects (central optical depth approximately 1) could broaden the 10-mu-m feature. Chemical composition differences no doubt are also important, particularly as regards the position of the peak of the 10-mu-m feature. The stars with the peak at 10-mu-m are more closely confined to the Galactic plane than the stars with the peak at 9.7-mu-m. The shape of the 18-mu-m feature is essentially the same in all stars, and can be used to extend the interstellar extinction curve past 13-mu-m to 22-mu-m.