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Astron. Astrophys. 348, 1020-1034 (1999)

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1. Introduction

Since the appearance of Comet Arend-Roland (1957 III) it has been tried to detect radio continuum radiation of comets. Hobbs et al. (1975) reported the radio detection of Comet Kohoutek (1973 XII) with the Green Bank interferometer; the strong, transient signal was explained by an icy grain halo model. The signal of Comet IRAS-Araki-Alcock (1983 VII), found by Altenhoff et al. (1983) could be explained by the nuclear emission. The signal of Comet P/Halley, monitored over a longer time in 1985/6 by Altenhoff et al. (1989), showed in addition to the nuclear emission a steady halo contribution. After the Halley campaign a detailed review of cometary radio observations was given by Crovisier & Schloerb (1991). Jewitt & Luu (1992) added a series of submillimeter detections with the JCMT. Most of these cometary observations were done with sensitive bolometers near 300 GHz, giving little spectral information for physical interpretations.

When Comet Hale-Bopp (C/1995 O1) was detected, it seemed highly probable that it would become a bright radio comet near perigee, allowing observations over a wide frequency range. The comet signal was expected to be variable, e.g. by its changing geo- and heliocentric distance, possibly also by some intrinsic variability, or an outburst, or even by an icy grain halo (IGH) event. Therefore it was planned that the observations should be done simultaneously, covering the mm- and submm-range over an extended period of time to derive the structure of the comet.

Fortunately Comet Hyakutake (C/1996 B2) appeared in the set up phase of observations; its close passage to earth promised a strong cometary signal, allowing to check out the combined observing network: the Heinrich-Hertz-Telescope (HHT) of the SMTO 1 between 250 and 870 GHz, the IRAM 30m telescope at 250 GHz, the IRAM Plateau de Bure Interferometer (PdBI) near 90 and 230 GHz, and the VLA of NRAO 2 at 22 GHz. At this time the 100 m telescope of the MPIfR was out of operation for a major repair. The observations ran between March 19 and April 4, 1996.

The performed trial observations were successful, facilitating the proposed coordinated observations of Comet Hale-Bopp from February 1 to April 27, 1997. This time the 100m telescope of MPIfR was part of the network.

Intermediate reports of the observations of Comet Hyakutake have been given by Altenhoff et al. (1996), and for Comet Hale-Bopp by Bieging et al. (1997) and by Wink et al. (1999).

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199