The Rotation Period of Comet Hale-Bopp

T.L. Farnham, D.G. Schleicher (Lowell Obs.), E. Ford (MIT), E.A. Blount (Beloit College)

Hale-Bopp was a very active comet that produced dust at a rate greater than has been measured in any other comet at any distance in the past 20 years. As early as the Fall of 1995, Hale-Bopp was exhibiting dust features that changed with time. We have images obtained on 40+ nights between June 1996 and May 1997 at the 42'' Hall telescope at Lowell Observatory.

Images were obtained using both the IHW narrowband filters the new Hale-Bopp narrowband filter set. The comet was monitored each night using the IHW Red Continuum filter (6840 Å) or the HB Red Continuum filter (7128 Å) to isolate scattered light from the dust particles near the nucleus. Images were also obtained with other filters in each set to isolate gas and ion emission bands and measure the continuum at other wavelengths. The dust morphology was essentially the same in all of the continuum filters during a single night's observations. After varying only slowly during 1996, the dust structure showed variations from night to night in 1997, with drastic changes taking place in the month-to-month time frame. At large heliocentric distances, the structure was primarily radial, with spiral-shaped arcs becoming apparent in February. Closer to perihelion, the arcs were the dominant structure near the nucleus.

We will use the changing morphology throughout the apparition to measure the rotation period of the nucleus and to obtain an estimate of the orientation of its rotation axis. The results of this rotation analysis will be presented, along with constraints on the locations of active regions on the surface of the nucleus.

This research is supported by NASA and NSF.