The rotational parameters of Hale-Bopp (C/1995 O1) from observations of the dust jets at Pic du Midi Observatory

We observed comet Hale-Bopp (C/1995 O1) at Pic du Midi Observatory in 1997 from February 2.24 UT to March 31.89 UT with the 1.05-m telescope equipped with a CCD camera and broad- and narrow-band IHW filters. A total of 30000 images were acquired both during night- and day-time. The images were automatically reduced and all the images obtained within 10 min. were co-added to give a set of similar to 1000 images used during the analysis. We can identify two jets on the images. The position angle of the brightest jet from February 2.24 UT to March 5.22 UT is measured using an automatic routine which searches for the averaged position angle of the maximum of brightness at a projected distance of 3200-6100 km from the optocenter. A preliminary model of nucleus rotation is used to fit the data and retrieve the rotational parameters of the nucleus. The best fit is found for a source located at a latitude of 64 +/- 3 degrees, a sidereal rotation period of 11.35 +/- 0.04 h and a right ascension and declination of the North pole of 275 +/- 10 degrees and -57 +/- 10 degrees. This preliminary analysis shows no evidence for a precession. Grains with velocities of 450-600 m s(-1) and radii < similar to 1 mu m dominate the optical scattering cross section in the jets.