Thermal Infrared Imaging and Spectroscopy of Comet Hale‐Bopp (C/1995 O1)

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Abstract
We present λ = 3-5 and 8-13 μm images and long-slit spectra of comet Hale-Bopp (C/1995 O1) taken in 1997 February-April, near the time of perihelion passage. In agreement with other infrared observations, we find that the 3-13 μm spectral energy distribution was dominated by a 10 μm silicate feature with a strong underlying continuum. In the nuclear region, the feature's peak flux-to-continuum ratio varied from 2.8 on February 17 to 3.3 on April 19, and the color temperature of the 3-5 μm continuum was ~1.8 times the equilibrium blackbody temperature at the relevant heliocentric distance. Both quantities are larger than in any previous comet and indicate a high abundance of submicron dust particles. Spectral structure within the silicate feature reveals the presence of crystalline olivine and pyroxene grains. The physical properties of the grains, as evidenced by their infrared emission, correlated with the morphology. Within the visible jets and halos the silicate feature was up to 15% stronger, and the 8-13 μm color temperature ~1.1 times higher, than in the regions between the halos. Therefore, the halos were enriched in submicron particles relative to the background, as expected from the higher outflow velocities of these small grains. Within the halos, there was no measurable variation in the strength of the silicate feature or the color temperature with distance from the nucleus; that is, we detect no time-dependent evolution of dust particle properties (such as fragmentation) on a timescale of one rotational cycle (between ~2 and 13 hr after ejection). The spectral shape of the silicate feature was essentially the same everywhere in the inner coma in all of our 1997 spectra. Monte Carlo simulation of dust particle motions in the coma indicates that the observed patterns of jets and halos were dominated by submicron-sized grains. Modeling of the thermal emission from small grains demonstrates that the principal features of the 3-13 μm continuum and the 8-13 μm silicate feature can be synthesized from a mixture of amorphous carbon and amorphous and crystalline silicates. The overall shape of the silicate feature resembled that in other bright comets, indicating a similar mineral mix. However, the 10.0 μm peak was sharper in Hale-Bopp near perihelion than in any previously observed comet except comet Mueller (C/1993 A1) at 2 AU.