CONSTRAINTS ON THE NUCLEUS AND DUST PROPERTIES FROM MIDINFRARED IMAGING OF COMET HYAKUTAKE

We use Mie scattering theory to determine the expected thermal emissio n from dust grains in cometary comae and apply these results to mid-in frared images of comet Hyakutake (C/1996 B2) obtained preperihelion in 1996 March. Calculations were performed for dust grains in the size r ange from 0.1 to 10 mu m for two different compositions: amorphous oli vine (a silicate glass) and an organic residue mixture. The resulting emission efficiencies are complicated functions of wavelength and part icle size and are significantly different for the two materials in que stion. The Hyakutake data set consists of three nights of high-resolut ion imaging (100-150 km pixel(-1) at the comet) of the inner coma at 8 .7, 11.7, 12.5, and 19.7 mu m. Attempts to fit the observed colors (ra tios of fluxes at different wavelengths) using a single grain composit ion failed. However, fits to the data were achieved for all three nigh ts using a mixture of -1 mu m olivine grains and similar to 7 mu m org anic grains. The resulting olivine mass fraction was between 8% and 16 % of the total dust mass-loss rate. We also estimate the radius of the nucleus to be r = 2.1 +/- 0.4 km.