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J. Crovisier, P. Colom, E. Gérard, D. Bockelée-Morvan (Observatoire de Paris-Meudon)
Since 1974, the 18 cm lines of the OH radical were observed in some 60 comets with the Nançay radio telescope. This allowed us to evaluate the water production rates of these comets and their evolution with time, as well as to study several physical processes such as the excitation mechanisms of the OH radio lines, the expansion of the cometary atmospheres, their anisotropy in relation with asymmetric outgassing and non-gravitationnal forces, the Zeeman effect in relation with the cometary magnetic field. These observations are now organized in a data base. Some statistical aspects of this data base will be presented.
The Nançay radio telescope has been recently upgraded. The design of a new focal system resulted in an improvement of the sensitivity by about a factor of two. This will allow us to pursue the OH survey on weaker comets, with water production rates well below 1028 molec. s-1. We will thus have access to comets at larger heliocentric distances and to smaller comets from the Jupiter family.
C/1999 S4 (LINEAR) was the first comet to be observed with the new system at Nançay in July 2000. These first results will be presented.