Constraints on the nucleus and dust of comet Hale-Bopp

J. R. Sarmecanic (UCSD), D. J. Osip, S. M. Lederer (U. Florida), M. N. Fomenkova, B. Jones (UCSD)

We present analysis of quasi-simultaneous mid-infrared and narrow-band visible imaging of comet C/1995 O1 (Hale-Bopp) taken in early March 1997. Thermal infrared imaging was obtained at the Mt. Lemmon Observing Facility using the UCSD infrared camera while visible CCD imaging through narrow-band continuum filters was obtained at Lowell Observatory. Differences in the dust morphology at visible and infrared wavelengths are highlighted by albedo maps of the near-nuclear region. The rotation period of tex2html_wrap_inline11 11.3 hr on UT March 5 is determined by tracking the dust structures during nearly 12 hours of continuous mid- infrared imaging. The thermal fluxes abserved at 8.7, 10.3, 11.7, and 12.5 tex2html_wrap_inline13 m can be explained by modeling the dust grains as Mie spheres composed of either amorphous olivine or an organic residue mixture. The best fit is achieved for an olivine mass fraction of 0.35, using olivine and organic grains whose average radii are 0.8 and 5.0 tex2html_wrap_inline13 m, respectively. Finally, we will provide an estimate of the nucleus size by subtracting the modeled dust emission from the data and interpreting the residual flux as thermal emission directly from the nucleus itself.