(larger JPG file; 20k)
This image is a 60sec exposure of Comet Hale-Bopp taken with the 2.2m telescope of the University of Hawaii telescope at Mauna Kea, through a filter centered on the emission line of the CN molecule, in the near Ultra-Violet.
The field of view of this portion of the image is approx. 2.5 Arcmin wide, or about 85 000km (53 000 miles) at the distance of the comet. The Comet was at 1.18AU from the Sun and 1.68AU from Earth.North is up and East is Left.
The contrast of the very complex jet structure has been enhanced by subtracting a by subtracting a combination of 1/rho and of a mean profile of the northern part of the coma. The main jet is pointing toward the Sun. Another image has been processed even further: all the coma has been removed using the "unsharp masking" filtering technique, leaving just the jet structure; the result is shown below.
The image was taken through a broad R filter (which captures the light from both dust and gas). The image has been "enhanced" to show the fine detail near the nucleus of the image by heavily smoothing the data and subtracting off the steep brightness gradients from the core, leaving only the detailed fine structure visible. What is seen are multiple jets of material emanating from the near-nuclear region. This most probably caused by uneven outgassing of volatiles from the nucleus, from a series of active "vents". Z. Sekanina (JPL) and H. Boehnhardt (Univ. Munich) have performed computer simulation experiments on images from 1996 to show that there are most likely 3-4 active regions on the nucleus (IAUC 6542). More recent observations which show changing jet structure (IAUC 6560) can be used to estimate that the nucleus is rotating with a period near 11.47 hours. As active regions move into and out of sunlight the escaping material can form concentric shell-like structures. The structure in this broad-band R image looks similar to the structure in the image taken through a CN filter (cyanide gas at 3880 angstroms), suggesting that the gas and dust are correlated in this near-nucleus region. Once the dust leaves the surface of the comet, it quickly no longer feels the gravity from the nucleus and the dust grains move in their own orbits around the sun, and much of the curvature can be explained by the radiation pressure from the sun pushing the dust tailward. In the case of comet P/Halley, there was evidence that not all of the CN was originating at the nucleus, but rather also from the dust grains themselves. The structures in the inner coma of comets are a very complex combination of the position of the active vents outgassing material, the velocity at which the material leaves, the rotation of the nucleus, the sun's radiation pressure and any secondary sources of material - such as dust in the coma. Without detailed analysis and modelling of these images we cannot yet say specifically what is occuring on the nucleus of Hale-Bopp."
Observers: K. Meech, O. Hainaut and J. Bauer.
If you want to observe Hale-Bopp yourself (you should, it is beautiful), maps and ephemerides are available on this site.
Update: Mon Feb 24 19:31:07 1997
Update: Mon Feb 24 19:31:07 1997 -- Hits: