Mid-infrared spectra of comets P/Borrelly, P/Faye, and P/Schaumasse

A 10 mu m silicate emission feature has been discovered in the spectra of comets P/Borrelly and P/Faye at R similar to 1.5 AU. These are the first short period comets in which silicate emission has definitely been detected. The broad emission features are about 25% above the continuum. No emission feature was present in the spectrum of P/Schaumasse; it is possible that the nucleus of P/Schaumasse was directly detected. If all of the observed flux originated from the nucleus, then the effective radius is about 3 km; the observed color temperature is consistent with a rapidly rotating nucleus. We present models that show how the shape of the silicate feature can depend on the way in which silicate and absorbing material are mixed in the grains. (C) 1996 Academic Press, Inc.