Coordinated radio continuum observations of comets Hyakutake and Hale–Bopp from 22 to 860 GHz

We have observed both Comets Hyakutake and Hale–Bopp close to perigee with several telescopes at frequencies between 30 and 860 GHz for an extended period of time. The observed “light” curves can be described as a simple function of heliocentric and geocentric distances without any outburst or noticeable variability with time. Our most sensitive diameter estimate for C/Hyakutake resulted in an upper limit of 2.1 km. The nuclear diameter of C/Hale–Bopp was determined to 44.2 km after separation from the halo emission. The central part of both halos can be represented by a Gaussian with a linear size at half power points of 1870 and 11080 km for Hyakutake and Hale–Bopp, respectively. The spectral index for both comets is α = 2.8, indicating a similar particle size distributions in the halo of these comets. For Hale–Bopp the extended emission could be traced to more than 105 km from its nucleus. The derived masses, contained in the halo depend strongly on the assumed physical properties of the halo particles. With κ(1mm) = 75 cm2/g, possibly more appropriate for comets, a halo mass of 6 1010 g is derived for Hyakutake and of 8 1012 g for Hale–Bopp.