During several observations with the MAMBO bolometer array at the IRAM 30m telescope we detected thermal emission from comet C/1999 S4. Our data imply that before the comet's breakup on 23 July 2000, its radio photometric diameter was 4.7 km and its nucleus had a diameter of about 0.9 km. Comet S4 is the smallest comet ever detected at radio wavelengths.
Optical observations had shown that after the comet's break-up a bright, compact dust cloud appeared which expanded and faded with time. After the breakup we were unable to detect the comet's radio emission, supporting the implication of the optical observations that the dust production was reduced.
Our radio observations allow to estimate upper limits of the mass of mm-sized particles in the (Kidger) dust cloud and to consider the changing brightness temperatures of the new fragments.
Acknowledgements
We would like to thank Dr. H. Ungerechts of IRAM for help with early trial measurements, Dr. D. W. E. Green at the CfA for providing undisturbed orbital elements, Dr. B. G. Marsden at the CfA for advice on the usage of orbital elements combined with non-gravitational parameters, Dr. J. Crovisier of Observatoire de Paris-Meudon for new informations and helpful advice, Dr. J. Schmidt at MPIfR for help with Dr. Stumpff's n-body-programme, and Drs. J. Neidhoefer and W. Reich at MPIfR for introduction to the not yet commissioned new module of the 32 GHz system.
Copyright ESO 2002