MEASUREMENT OF SUBMICRON GRAINS IN THE COMA OF COMET-HALE-BOPP C 1995O1 DURING 1997 FEBRUARY 15-20 UT/

We present 1.20-18.5 mu m infrared (IR) spectrophotometric measurement s of comet C/1995 O1 (Hale-Bopp) during 1997 February 15 and 20 UT. Th e spectral energy distribution (SED) was dominated by scattering and t hermal emission from submicron sized dust grains that were unusually s mall, Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucl eus. The thermal emission continuum from the grains had a superheat of S = T-color/T-BB approximate to 1.84, the peak of the 10 mu m silicat e emission feature was 1.81 mag above the carbon grain continuum, and the albedo (reflectivity) of the grains was approximate to 0.41 at a s cattering angle of theta = 144 degrees. These are the highest values f or these empirical parameters ever observed in 20 years of optical/IR measurements of bright comets. The observations indicate that the opti cally important grains dominating the visual scattering and near-IR em ission from the coma had an average radius of a less than or equal to 0.4 mu m. The strong silicate feature is produced by grains with a sim ilar size range. These dust radii are comparable to the radii of the g rains that condense in the outflows of some novae (''stardust'') but s till about 10 times larger than the average radius of the grains that produce the general interstellar extinction.