Spectroscopic Results from Comets Hale-Bopp and Hyakutake

D. Bockelée-Morvan (Obs. Paris)

Two bright comets made recently spectacular apparitions. C/1996B2 (Hyakutake) approached at only 0.1 AU from the Earth in March 1996. C/1995O1 (Hale-Bopp) was discovered exceptionnally active at 7 AU from the Sun in July 1995 and intensively observed during 2 years. These two comets were the object of extensive spectroscopic studies in all wavelength ranges, and benefited of new Earth- and space-based instrumentations. An overview of the first results of these spectroscopic investigations will be presented.

This review will emphasize the major progresses made in our knowledge of cometary composition. Many molecular species have been identified for the first time in a comet and important results on the nature of cometary grains have been obtained. The ratios of several stable isotopes have been measured in gas species, with strong implications on the origin of cometary material. The outgassing of a number of species has been monitored as comet Hale-Bopp approached the Sun from 7 AU to perihelion, bringing unprecedented insights on the processes governing the activity of cometary nuclei and clues on the physical state of cometary ices. The spectroscopic observations of these two bright comets brought also new informations on a number of chemical and physical processes occuring in the coma.