Mark Kidger
mrk@ll.iac.es
Over the last few weeks there have been many reports that the nucleus of Comet Hale-Bopp has split or may be splitting. Most have come from visual observers who have obviously seen something within the inner coma. The following are some comments which may help them to understand what they are seeing.
Many observatories are taking regular high-resolution images of the near-nucleus region and it is not too likely that these observations would miss an important nucleus splitting.
We have compiled a movie (http://www.iac.es/comet/hbet.html, 88K Mpeg) of images taken with the 1m Jacobus Kapteyn Telescope in La Palma between February 15th and February 23rd (UT dates) an interval which covers various reports of suggested splittings or ejections. The images were taken as part of the Comet Hale-Bopp European Team's monitoring efforts (http://www.iac.es/Hale-Bopp/hbitp.html).
This movie shows various near-nucleus structures expanding away. A whole series of bright shells of material are being ejected. These appear to explain most, if not all of the sightings. These shells are very bright (they can easily be seen on the guide tv of the 1m telescope with no image processing aat all) and are also very clumpy. Frame 1 of the movie shows a typical clump in the jet which is so bright that it could easily be mistaken for a fragment of the nucleus. As the frames develop one can see that this is not a discrete "piece" of material, but is actually just a small part of a large shell which is slowly expanding away from the nucleus. These shells are not parabolic, but are actually more like the typical child's representation of a sea-gull, looking like a highly flattened "M". The frame is approximately 130 arcseconds across which gives an idea of the scale of the features and their separation.
We see how a new jet develops and, in the last frame of the movie, this splits away from the "nucleus" forming a new shell. Once again, the centre of this shell has a very bright clump which could easily be mistaken for a piece of the nucleus being ejected. At least six shells are visible stacked on top of each other from the nucleus to the edge of the frame.
As these clumps of material (condensations within the shell) develop, they expand slowly and lose surface brightness, becoming indistinguishable from the rest of the shell.
In other words, when a visual observer detects a "false nucleus" of this kind, the most likely thing that he is observing is the formation of a new shell of material.