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F. Henry, D. Bockelée-Morvan, J. Crovisier (Obs. Paris), J. Wink (IRAM)
We have observed comet Hale-Bopp on October 5 and 25, 1996 and from March 6 to March 22, 1997 with the IRAM interferometer at Plateau de Bure. Millimeter lines of HCN, HNC, CO, H2CO, CH3OH, H2S, CS and SO were mapped with spatial resolutions of 1.5-3.5 arcsec. The observations presented here are those of the 2-1 transition of CO. They have been performed on March 11, 1997, producing 60x60 arcsec maps with a spectral resolution of 0.11 km/s. Interferometric observations were interrupted every hour for short on-off observations in single-dish mode.
Asymmetrical patterns are noticed in the maps. They cannot be explained only by the elliptical instrumental beam. They are evidence of the anisotropy of the gas emission, due to the existence of gas jets. The spectral analysis of the maps shows that the blueshifted line side is stronger than the redshifted one. This proves the existence of a gas jet toward the Earth. The disturbances in the redshifted signal show that there may be other jets. The on-off spectra show that the mean velocity of the line is moving from blue to red velocities with a period which is that of the nucleus rotation (11.4 h): we are observing here a jet rotating in a plane almost perpendicular to the plane of the sky.
Jets of dust or radicals were already observed in comets in the visible. But it is the first time that rotating jets are observed for parent molecules.
The interpretation of these interferometric data is tricky because two rotations are involved: the comet rotation itself and the Earth rotation. We are developping a 3-d model of gas jets where the comet outgassing is an isotropic distribution plus a conical jet of given orientation and opening angle. Then, visibilities are computed for the real antenna configuration. And maps are synthetised with the same procedures as that used for the observations of comet Hale-Bopp. We will present here a comparison of the observations with the first results of this model.
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