Pic du Midi Observatory Images of Comet Hale-Bopp

Observers: Francois Colas, Jean Lecacheux
Location: Pic du Midi Observatory, France
Date: February 4, 1997 06:00 UT

1997, February 4th, 6H UTC

The comet is becoming brighter and brighter, the nucleus is now brighter than that of comet Hyakutake. Las year we used narrow band filters centered on emission lines of molecules. As the observing time was quite short we used only one filter centered on C2 emision at 0.514 micron and an other in a spectum region where there are quite no molecular or atomic emissions. This second filter is called "dust" because the light which is not blocked by itself comes only from solar light reflected by the dust ejected from the nucleus. So an image taken with this kind of filter permits to get informations about dust density on the viewing line. The C2 emission at 0.514 micron is responsible of the blue-green color of many coma.

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The C2 image is really smoother than the "dust" one. In fact the gaz is ejected from the comet nucleus in a quasi isotopic way, in an other hand dust is ejected preferentially in a few directions. This can be explained by the existence of active regions on the nucleus surface. Jets are produced when the sun illuminates these regions. We have drawn on each image the direction of the sun. The dissymmetry between illuminated and shadow sides is then obvious.

It is difficult to see details in the coma because of the tremendous luminosity contrast between the center and the edge of the coma. So we have processed these images with the rotational gradient algorithm to reduce this contrast, it is then possible to see small structures in the coma. The C2 image shows quite no details, but the "dust" one shows a very complex structure! Radial structures are well explain by the jets theory, but the "waves" in front of the comet (toward the sun) are more enigmatic. This fact can be explain by an active region with a pulsating jet synchronised on the nucleus rotation. Is this explanation is true, there is one wave every nucleus round.

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The rotational gradient is very powerful to detect small structure in the coma, but the result is difficult to understand. In a first time we have made a zoom on the nucleus, two important jets are visible on each side of the nucleus. There is also a "blob" in front of the sun. This "blob" seems not to be a comet fragment like that of Hyakutake. It should be some dust cloud. The last image is important to understand the problem, the image is processed by unsharp masking. A "blob" is clearly visible in front of the sun and two "waves" systems on each sides of the sun direction. The blob was seen on images two days before and could be motionless.

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Observers : Jean Lecacheux, Frangois Colas

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