The 10 micron silicate feature of comet C/1996 Q1 (Tabur)

We present 7.6-13.2 μm infrared spectrophotometry (R ≃ 120) of the inner coma of comet C/1996 Q1 (Tabur), obtained 1996 October 8-10 UT. At this epoch, the comet was at a heliocentric distance of r_h = 0.96 AU. The local 10 μm continuum is fitted with a 300 ± 10 K blackbody, revealing a weak silicate feature. Our analysis suggests that comet Tabur most likely contains large grains of radii a ≈ 1-3 μm. Synthetic spectra derived from laboratory measurements of amorphous pyroxene and amorphous and crystalline olivine of grain sizes (radii) between 1 and 5 μm are fitted to Tabur's silicate emission feature. A mixture of crystalline olivine (1 μm radii) and amorphous pyroxene (3 μm radii) grains provides the best model fit to the observed 10 μm spectrum. We also compare the spectra of Tabur with the mid-IR spectra of C/1995 O1 (Hale-Bopp) and other comets with and without silicate features. In particular, we discuss the mid-IR spectrum of comet C/1998 K5 (LINEAR) obtained on 1998 June 28-30 UT with the same instrument and spectral resolution. In contrast to comet Tabur, the spectrum of comet K5 LINEAR does not exhibit 10 μm silicate emission in excess of a featureless continuum characterized by a 310 ± 10 K blackbody.