Comets with ISOPHOT

S. B. Peschke (MPI für Kernphysik), E. Grün (MPI für Kernphysik), ISOPHOT Comet Team

The Infrared Space Observatory (ISO) provided a unique opportunity to study the thermal dust emission from cometary comae. We used the spectrophotometer ISOPHOT on board the satellite to do multi-filter and multi-aperture photometry and low resolution spectroscopy. We observed P/Kopff, P/Schwassmann-wachmann 1 and Chiron within the last year at heliocentric distances 1.9-2.2 AU, 6.2 AU and 8.4 AU, respectively. Observations of P/Wild 2 are scheduled in July 97. Within the Joint Targert of Opportunity for an exceptionally bright comet, we were able to perform multiple Hale-Bopp Observations, at heliocentric distances ranging from 5 to 2.8AU.

The spectral energy distribution of these comets was sampled in nine filters between 3.6 and 160 tex2html_wrap_inline11 m. In addition, multi-aperture measurements around the expected maximum of thermal emission have been performed to get coma size estimates. For the bright comets, low resolution spectra ranging from 2.4-12.0 tex2html_wrap_inline11 m were obtained to get the best constraints on the Wien law side of the spectral energy distribution. For most of the ISO observations, it was possible to coordinate (quasi-) simultaneous ground-based observations in the optical and the near IR/mid IR.

After very complicated data reduction and despite calibration uncertainties, we were able to determine temperature estimates for the dust in the coma by fitting a simple black body to the data. The temperatures range from about 100 K for Chiron, 110 K for P/Schwassmann-Wachmann 1, 230 K for P/Kopff to 150-220 K for C/Hale-Bopp. We could further deduce model-dependent coma sizes and albedos. Applying grain size distribution models for fitting the spectral energy distributions, will allow us to constrain the size distribution and the composition of the dust grains in the cometary coma. First results will be presented.