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J. Crovisier, D. Bockel\'ee-Morvan, T. Encrenaz, E. Lellouch (Obs. Paris), B. Altieri, K. Leech, A. Salama (ESA, Villafranca), M. Griffin (Queen Mary and Westfield College, London), T. de Graauw (SRON, Groningen), E. van Dishoeck (Leiden Obs.), R. Knacke (Penn. State Erie), T.Y. Brooke (J.P.L.)
Two Infrared Space Observatory programmes (guaranteed time and open time) were devoted to high-resolution spectroscopic observations of short-period comets. 22P/Kopff was observed on October--December 1996 with the short- and long-wavelength spectrometers. Due to the weakness of the object, which was at 1.9~AU from the Sun at that moment, only the \nu3 ro-vibrational lines of water were detected, with SWS. They allowed a determination of the production rate of water in this comet and an analysis of its physical conditions. Comet 103P/Hartley 2 was observed close to its perihelion (at 1.04~AU from Sun and 0.82~AU from Earth) on January 1998 with the two spectrometers and ISOCAM. The bands of H2O and CO2 at 2.7 and 4.3 \mum are detected, with [CO2]/[H2O] = 20~%. The 2.7 \mum band of H2O is observed with a high signal-to-noise ratio, which permits to evaluate the rotational temperature of water averaged within the field of view to 20~K, and its ortho-to-para ratio to \approx 2.7, corresponding to a spin temperature of 35~K. The 5--17 \mum spectrum of comet P/Hartley 2 observed with CAM-CVF shows the 9--12 \mum signature of silicates. A weak emission at 11.3 \mum (at a level of about 15~% of the continuum) indicates the presence of crystalline silicates. This is the first time crystalline silicates are found in a short-period comet. The ISO observations of the short-period, Jupiter-family comet P/Hartley 2, presumably originating from the Edgeworth-Kuiper belt, are to be compared to those of comet Hale-Bopp which came from the Oort cloud. They both show crystalline silicates, a high content of CO2, and a low water ortho-to-para ratio. However, the dust-to-gas ratio of comet Hale-Bopp was much higher than that of P/Hartley 2.
The author(s) of this abstract have provided an email address for comments about the abstract: crovisie@obspm.fr