How Fast the Nucleus of Comet Hale–Bopp was Rotating?

At the First International Conference on Comet Hale–Bopp(Canaries, 1998) many astronomers agreed with the value of theperiod of axial rotation of its nucleus, which was obtained byobservers by a comparison of the two sequential patterns of thedust arcs. It was noted that the pattern had been repeated each∼12 hours. The value 11.35^h was taken as the periodof axial rotation. However, alternative periods of 4 and 8 days havealso been suggested. The possible reasons for thisdiscrepancy are discussed. It is shown that a short period could be obtained by a misidentification of the same arc on twosequential images and, as a result, an overestimate of theradial velocity of dust in the arcs. To obtain the half day period one must take the projected velocity of the arc ∼ 1.6 km/s,which exceeds the H₂O gas expansion velocity. If one takes a lower expansion velocity a longer period would be obtained.     

The Spin Axis Position of C/1995 O1 (Hale–Bopp)

Monitoring of the near-nucleus activity of C/1995 O1 (Hale-Bopp) began in Teide Observatory in August 1995. During 1996 the comet was observed on 72 nights between March 26 and November 13. A permanent fan structure was observed towards the north during the whole period of observation. The position angle of the axis of this fan was measured and its variations with time were used to determine the position of the North Pole of the cometary nucleus. This revised version was published online in July 2006 with corrections to the Cover Date.