Coordinated observations of Comet Hale-Bopp between 32 and 860 GHz

The concept of simultaneous multifrequency continuum observations, successfully tested on Comet Hyakutake, was applied to Comet Hale-Bopp, using the Heinrich Hertz Submillimeter Telescope (HHT) with the four color bolometer between 250 and 870 GHz, the IRAM 30m telescope at 240 GHz, the MPIfR 100-m telescope at 32 GHz, and the IRAM interferometer near 90 and 240 GHz. Near-simultaneous measurements were done between February 15 and April 26, 1997, mainly concentrated in mid March shortly before perigee of the comet. The measurements gave the following preliminary results: 1. Interferometer detection of the nuclear thermal emission. If the signal at the longest interferometer spacing of 170 m is due to thermal emission from the nucleus only, its equivalent diameter is ~49 km. If, however, this signal contains a contribution from a strongly centrally peaked halo distribution (e.g., r^-2 density variation) the diameter may be as low as 35 km. 2. The emission found interferometrically was always 5″ north and 0.1 sec east from the position predicted by Yeoman's solution 55. 3. The comparison of the interferometric continuum emission with the simultaneously obtained molecular line observations (reported on this conference) shows the origin of the strongest line emission concentrated on the nucleus. 4. The 30-m observations show a radio halo with a gaussian FWHP of ~11, corresponding to a diameter of 11000 km at geocentric distance of 1.2 a.u. 5. A spectral index of ~3.0 for the total signal, which may indicate a smaller mean particle size than for Hyakutake. Assuming an average cometary density of 0.5 gcm^-3, the mass contained in the nucleus is ~1-3 10¹⁹ g and 10¹² g in the particle halo.