Observer: Jean Lecacheux
Location: Pic du Midi Observatory, France
Date: February 2-10, 1997
Ten equidistant shells of dust were detected on February 10.2 (and on previous days) in the sunny side of the coma, extending to 150 000 km from the nucleus. Shells were missing apparently in a paraboloidal volume opposite to sun. Likely the shells are propelled by active area on the nucleus submitted to periodic variations of insolation.
This auto-differential image was obtained by dividing, after convenient normalisation, a 16 mn exposure around 6 h 20 UT by a 16 mn exposure around 4 h 30 UT. The bandpass was 830-1000 nm. Two similar images of a steady comet should cancel exactly and produce a flat featureless field. Here radial expansion of the parabolic shells becomes evident owing to relief effect : forward movement faces the apparent illumination. In addition two inner clouds of dust have erupted outwards during the 110 mn interval and gained in brightness. In this image the white level corresponds to +17 % of flux and the black level to -39 %. The small flat ellipse at the center indicates saturation area. Note that the decrease of distance of -0.08 % to the Earth has been taken in account. Position measurements on original images show quasi-isotropical expansion of the dust complexes at 0.8 arc sec. /hour approximately, or 300 m/s.
The orientation of this bright curved jet changes at hour scale and from one night to another. A rotation of the nucleus with the 11.47 +/- 0.05 hours period seems explain all the Pic-du-Midi observations between January 12 and February 10. By perspective shortening, the nucleus appears quasi-stellar when the jet is pointing toward observer (Feb.07 image). The jet is bright and narrow when the active area on the nucleus passes near the subsolar point (Feb.04 image). It is wide and fuzzy at the opposite phase of low sun elevation (Feb.02 image).
All the images were taken with the 105 cm telescope at Pic du Midi Observatory (Station de Planetologie des Pyrenees)
Observer : Jean Lecacheux
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