7 Conclusion and prospects

We have attempted to present a most complete and comprehensive view of all the valuable OH cometary data taken at Nançay since 1973. The scientific exploitation of the data base is quite incomplete and it is hoped that the present paper will stir up new ideas and collaborations.

Clearly, some studies are lagging because the physics of OH cometary masers is not fully understood (e.g., the quenching mechanism and the non-thermal excitation of the OH transitions). This is also the case of the relation between the anisotropic outgassing and the non-gravitational forces, the average cometary magnetic field and its relation with the solar magnetic field structure. It is also true that the interest of cometary researchers has strongly shifted to millimetre wavelengths where many fundamental microwave lines have been discovered.

The Nançay radio telescope has undergone a major upgrade which started in 1995 and ended in 2000 (van Driel et al. 1996). The old Hoghorn feeds were replaced by a double Gregorian wide-band system with corrugated horns and sensitivity improved by a factor of 2.2. Furthermore, the horns can be rotated by $\pm 90\hbox{$^\circ$ }$ and provide full polarization capability. Thus, if the OH cometary maser is linearly polarized with an expected minimum along the comet-Sun direction (Mies 1976), the position angle of the orthogonal polarizations can be set in this direction. A new autocorrelator has also been constructed which provides for cometary observations 8 banks of 1024 channels in 195 kHz (190 Hz channel separation, corresponding to 0.035 km s^-1, or 0.07 km s^-1 after Hanning smoothing). As in the past, a frequency-switching mode is used and subsequent folding results in a gain of a factor of 1.4. It is now possible to observe simultaneously the four OH transitions at 1665 and 1667 MHz (main lines) and 1612 and 1721 MHz (satellite lines) in both left- and right-hand circular polarizations, to detect possible Zeeman shifts and departures from the LTE line intensity ratios.

The new system has been successfully used already since July 2000 to detect four comets in 2000, namely C/1999 S4 (LINEAR) in early July 2000, before its disintegration (Bockelée-Morvan et al. 2001), C/1999 T1 (McNaught-Hartley) in November-December 2000, 73/P Schwassmann-Wachmann 3, and C/2000 W1 (Utsunomiya-Jones) in December 2000. These new observations will described in future publications.

With an increased sensitivity by a factor 2.2, one can either start observing "usual" comets at larger heliocentric distances or detect intrinsically fainter objects, or perform a more continuous monitoring of these objects. Alternately, one may wish to study the long- and short-term variations of the OH production rate associated with the outbursts, break-up of the cometary nuclei and correlate them with the production rates variations of other species and/or with the visual magnitude. Whatever the future strategy, the new system should allow to handle several cometary observations daily much more easily than in the past.

Acknowledgements

This paper is dedicated to the memory of Odile Franquelin and Gabriel Bourgois, who died during the course of this work.

Thanks are due to other observers in the early days of this programme (F. Biraud, I. Kazès, D. Despois and N. Hallet) for their help and to other members or visitors of the groupe comètes at Meudon (L. Jorda, N. Biver and H. Rauer) for their support.

The observations at Nançay would not have been possible without the help and support of the operators and of the technical staff of the radio telescope. These observations blindly rely on cometary ephemeris; we are very grateful to those who provided timely astrometric positions and orbital elements (B. Marsden and D. Green at the Central Bureau for Astronomical Telegrams, D. Yeomans at the Jet Propulsion Laboratory, and P. Rocher at the Bureau des longitudes).

The Nançay Radio Observatory is the Unité scientifique de Nançay of the Observatoire de Paris, associated as Unité de service et de recherche (USR) No. B704 to the French Centre national de la recherche scientifique (CNRS). The Nançay Observatory also gratefully acknowledges the financial support of the Conseil régional of the Région Centre in France.