We have observed the comet Hale-Bopp on February 22nd from 4 h 14 UT (at an elevation of 14 degrees) to 6 h 51 UT (14 minutes after sunrise). 106 individual exposures in the z near-infrared filter were binned in 16 intervals of ten minutes and then combined to produce the 16 final images.
We present two animations .FLI of 16 frames. North is up.
The first animation is made from unprocessed raw images, just corrected from flat-field. To obtain the second one, we have first applied to each image a 13x13 median filter, then substracted the result to the original image in order to remove all the low spatial frequencies and to flatten the main bump of the comet, and finally we have log-log transformed the pixel intensities. The three last images were affected by cirrus clouds and daylight, producing some disturbing noisy appearance. We recommand to play the second .FLI sequence at 25 images/sec for detailed perception of the turbulent motion.
Here we have measured outward flow at 300-350 meter / sec, a good confirmation of our preliminary value reported on February 17 in IAUC 6360. Hale-Bopp coma parallel shells in front of the sun were first discovered at Haute- Provence observatory by O.Lardiere and L.Arnold last January 30. We have followed the phenomenon since February 2 at Pic-du-Midi, where day after day, good "seeing" permits to resolve one arc-sec. at 10 degrees elevation or 0.3 arc sec. near zenith. Here have a good "seeing" near horizon is a main advantage to extend the observation window to more than two hours. The appearance of such concentric dust layers, named "shells" or "hoods", is a classical feature in internal coma of great comets (see by example old drawings of comet Donati in 1858 or CCD images of comet Hyakutake last year). The nuclear spin is basically the cause of this time modulated activity, one layer beeing emitted at each rotation, but many properties of the phenomenon still remain unexplained. In our opinion hydrodynamic waves seem required to try to understand the persistence of this coherent layering. Indeed we are able to follow full delineated waves up to 100 000 km (out of the present field), about 100 hours after they have been ejected from the nucleus. The present observation shows for the first time a compact cloud of dust leaving the nucleus at 300 meters / sec near p.a. 115 degrees. The main nuclear condensation was of magnitude I=9, and this receding object of magnitude 10. So they were both easily detected some minutes after sunrise. This dust cloud could be at the origin of the waves system in the south-eastern quadrant.
Pic du Midi Animation (FLI, 272K, Raw Images)
Pic du Midi Animation (Animated GIF, 261K, Raw Images)
Pic du Midi Animation (FLI, 774K, Processed Images)
Pic du Midi Animation (Animated GIF, 512K, Processed Images)
Scientific team : Laurent Jorda, Jean Lecacheux, François Colas and Pierre Laques
All the images were taken with the 105 cm telescope at Pic du Midi Observatory (Station de Planetologie des Pyrenees)
Comments have to be send to Francois Colas
Francois.Colas@bdl.fr
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