The Comet Hale-Bopp European Team is a group of mostly European cometary scientists which was constituted in early 1996 with the aim of coordinating their studies of the unusual comet Hale-Bopp. For more information, you can go to http://www.iac.es/Hale-Bopp/hbitp.html
Technical details:
The animation is made up of 12 frames taken between April 1st and 21st 1997 using the 51-cm University of Mons Telescope, at Teide Observatory, Tenerife, Spain. This movie covers almost the whole post-perihelion period to date. There is one sizable gap due to very poor weather for several nights in the week following perihelion.
The images were taken by a number of observers from the Comet Hale-Bopp European Team, some resident in the Canaries, other visiting to help out with observations:
Pablo Santos (University of La Laguna), Javier Sánchez (amateur helper), Dave Osip (University of Florida), Nanci Sabalisck (Instituto de Astrofísica de Canarias) and Richard West (ESO) all contributed to the observations shown here.
The data is reduced by Pablo Santos, using routines developed by Miquel Serra (Instituto de Astrofísica de Canarias) and the final animation was compiled by Miquel Serra.
A standard ST-6 CCD camera was used at the cassegrain focus of the telescope. This is our "spare" camera as the telescope's ST-8 is in the USA for repairs.
Each image is the result of taking as many as 100 exposures of 5-10 seconds duration between about 20:30 and 22:15 UT each night (the exact number of exposures and duration of the observations depends on the conditions) through a V filter. These frames are flat-fielded recentered on the nucleus; this recentering means that the edges of some of the frames show a noise pattern due to the fact that not all frames cover the whole field of view. Each frame is examined individually and bad frames are excluded. The remaining good images are co-added, although we are experimenting with median filtering to try to take out seeing variations in the innermost coma.
The co-added frame is then filtered using a Laplacian filter which looks for changes in gradient (again, we are experimenting with other processing methods as we know that this one has its disadvantages as well as its advantages) and the coma substracted. This gives us the "final" frame which is seen in the animation, where the jets and other fine structure show up. In most cases the brightest of these structures can already be seen in the raw frames, although with much lower contrast.
The field of view of the CCD on the telescope is about 3 arcminutes. The image is resampled as part of the reduction process, hence the apparent pixel size is rather smaller than the true one.
Description of the movie:
The first frame was taken about 18 hours after perihelion. The animations shows the evolution of the spiral jets over three weeks post-perihelion. Two bright jets (a jet and a fainter counter-jet) are seen in the image. These are due to different active areas on opposite sides of the nucleus. Other images taken by the Comet Hale-Bopp European Team show that there are, without doubt, more active areas than just these two.
Each jet produces as spiral of gas and dust as it rotates. In some of our images the full spiral can be followed for at least two full rotations around the nucleus, although at the point opposite the main jet (top left), the trail of material is quite faint. In frame one we see clearly how the faint tail of the counter-jet joins up with the spiral from the main jet.
Further from the nucleus we see just the sunward part of the spiral forming a series of expanding hoods. Four of these are clearly seen to the edge of the frame. These hoods appear to expand only slowly, although this is actually an illusion caused by the data sampling.
Note that the rotation period of the nucleus, according to the latest determination by the Comet Hale-Bopp European Team, based on results of long daylight sequences of observations from the 1.5-m Carlos Sánchez Telescope in Teide Observatory, is of 11.20 hours. This has two interesting consequences. First, two full rotations of the nucleus are only slightly less than one day, so we see a similar point in the rotation each night. This makes the jets and the shells appear to move only very slowly from night to night. The second consequence is that, on some nights, where we have as much as 2 hours of data, this is a significant part of the rotation period of the nucleus. On summing all of the images the effects of the rotation can be seen as strange structures coming out of the nucleus (eg: the nearly horizontal bar in frame 2). This rotation somewhat blurs the structures that we see.
Diagonal streaks cross some of the frames from top right to lower left. These are just star trails. More interesting is the streaks which run from the nucleus towards the upper left (eg: frame 7). This is the start of the ion tail and can only be seen in a very few images.
Some frames, such as frame 10, show some very complex structures in the inner shell. Here we see the spirals from the two jets not quite overlapping and, as a result, producing some complicated interactions. This shows that the two active areas are probably not quite separated by 180 degrees on the nucleus, although they are close being exactly on opposite sides.
Images taken in the infrared over a full rotation show how the two jets completely disappear at some times due to perspective effects. Frame 10 is a good example of this where only one of the two jets is apparantly visible at the moment that the frame was taken. Once again, this shows that the two main active areas are not quite on opposite sides of the nucleus. In frame 9 we are quite close to losing both of the jets completely, with just one rather blurred structure visible.
As the rotation progresses the spiral unwinds like a corkscrew. Although the overall effect appears to be stationary, one day's jet has reached the first shell by the following day, the second the next and is completely out of the frame by the fifth day.
These shells eventually spiral out until they are swept up into the tail. Images from some observatories (eg: the Pic du Midi data) have a rather larger field of view and are able to show how as many as 15 hoods expanding away from the nucleus; the most distant of these fifteen would have been emitted almost exactly one week beforehand. These shells are later responsible, as they expand outwards, for the astonishingly beautiful structures seen in photographs of the tail.
The attached movie is a compilation of images taken with the 1 m. Jacobus Kapteyn Telescope at the Observatorio del Roque de los Muchachos, La Palma, Canary Islands, Spain.
It is made up of images taken by the European Hale-Bopp Team (observer Mark Kidger) and by Simon Hodgekin of Leicester University (United Kingdom), these last images were kindly offered to the European Hale-Bopp Team and are used with his permission. These images have been reduced by Javier Licandro at the IAC and animated by Miquel Serra. More images will be added as they are reduced.
The images have been taken in R over a period of a week and a half from the morning of February 15th 1997 and show how shells of material are being expelled from the the nucleus of the comet. The expansion velocity of these shells is only about 100 m/s (note that this is a rough estimate only, a more accurate figure will be given later). The jet that gives rise to the shells is extraordinarily bright and, as can be seen in some of the images, its clumpy form has led to some erroneous reports by visual observers that the nucleus of the comet has fragmented.
Please note that the doughnut-like forms seen in some of the exposures are due to dust grains on the window of the CCD camara. These form the characteristic out of focus doughnut images. On other images lines of dots can be seen (stars crossing the field of view behind the comet) or isolated dots (cosmic ray impacts). None of these are real features.
In the sequence of images we see how a bright jets leaves the nucleus to the north and bends through a right angle at a certain distance from the nucleus. A prominent knot of material can be seen above the point where the jet bends, this knot of material is the brightest part of a jet which was thrown off the nucleus some days earlier, it is NOT a fragment of the nucleus. As the jet expands away from the nucleus, the rotation of the comet's nucleus separates it off and it begins to form an expanding shell, moving away slowly from night to night. A new jet is seen forming immediately to replace the one which has separated. This new jet grows until it too will separate from the nucleus and form another new shell of material.
The shells are not the symmetric parabola forms seen in Comet Hyakutake, but rather are very twisted and somewhat assymetric. The form is strikingly reminiscent of a sea gull in flight. At least four shells can be seen to the north of the nucleus and deeper exposures reveal that there are more shells right out to and beyond the edge of the field of view.
An enormous amount of structure can be seen in the images. Various reports have suggested that changes are seen in the inner jet in the space of a few hours or less. These reports indicate that the rotation of the nucleus is very much faster than previously suspected and a period of well under 12 hours cannot be ruled out. The structure is at least in part the result of the rapid rotation of the nucleus.
The interval between the formation of shells is much longer than the suspected rotation period. Archival images show that these shells form approximately every 10 days, hence this may be a precession period, or the rotation may be complex like that of Comet Halley.
Big colour changes can be seen within the coma with REDDER colours being seen further away from the nucleus on some nights. This indicates that finer grains of ice and dust are being thrown off on some occasions. No significant difference is seen though between the images in filters that include strong spectral lines and those that are mainly reflected continuum light.
Mark Kidger,
For the Hale-Bopp European Team
and the IAC Comet Team.
