Observations of OH in P/Swift-Tuttle and in several recent weak comets with the Nançay radio telescope

Abstract

The OH radio lines at 18 cm were monitored in six comets with the Nançay radio telescope in 1992 and early 1993: Zanotta-Brewington 1991g 1, Bradfield 1992b, Mueller 1991h 1, Shoemaker-Levy 1991a 1, P/Swift-Tuttle 1992t and P/Schaumasse 1992x. Neither Bradfield 1992b nor Mueller 1991h 1 could be detected. Zanotta-Brewington 1991g 1, Shoemaker-Levy 1991a 1 and P/Schaumasse 1992x were relatively weak comets with OH production rates of the order of 10 28 s −1 and required several hours of integration to be detected. P/Swift-Tuttle was observed and detected as early as mid-October 1992. The signal was much stronger than for the other comets, especially at the beginning of December 1992, when P/Swift-Tuttle was observed against galactic background radiation. The OH production rate was several 10 29 s −1. All along the observations, the OH radio lines exhibited unusually high blueshifts, corresponding to gas bulk motions of 0.5–1.0 km s −1 towards the observer. This undoubtedly corresponds to the strong jets observed in optical images and to the important nongravitational forces that affect the orbit of this comet. P/Swift-Tuttle was still observed at Nançay at the beginning of January 1993 when the small solar elongation precluded optical observations. A summary of these observations and of the estimations of the OH production rates is presented.