Observations of unusually small dust grains in the coma of comet Hale-Bopp C/1995 O1

We present 1.2-18.3 μm photometry of comet 1995 01 (Hale-Bopp). Our observations spanned nearly 4 months pre- and post-perihelion. The dust grain size-dependent parameters of superheat, 10 μm silicate excess, and albedo are used to make determinations about the Hale-Bopp coma grain population. The evolution of the coma grain size distribution is tracked by measuring these parameters over a range of heliocentric distances. We also observed several regions in the Hale-Bopp coma revealing significant gradients in the measured color temperature suggesting differences in the dust grain size distribution. Finally, we compare the grain size - dependent parameters to those of 11 other bright comets. The extreme values of these parameters displayed by Hale-Bopp suggest it had a smaller mean dust grain size than those associated with the other comets.