1997 Hale-Bopp

Mark Kidger, For the Hale-Bopp European Team and the IAC Comet Team

Movie of jet formation in Comet Hale-Bopp. The 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.

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. 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. 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.

Alberto Quijano Vodniza, Jose Alonso, Mayaguez, University of Puerto Rico

The image (09:20:51 UT) was taken with a ST-6 CCD camera through a 16 inch, F10 reflector. 5 sec exposure. The field of view is 7.3 X 5.5 . The image is unprocessed. We watched again a shade on the left side of Hale's coma today. We watched that shade for first time at 02/22/97 ( 10:14:40 UT ).

GAS AND DUST WAVES EXPANDING AWAY FROM HALE-BOPP's NUCLEUS

Pic du Midi Observatory, France: Laurent Jorda, Jean Lecacheux, Frangois Colas, Pierre Laques

We have observed 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 Gunn-z near-infrared filter were co-added to produce 16 final images each separated by 10 minutes. We present two animations (.FLI and GIF) of 16 frames. From these images, we have measured an outward flow at 300-350 m/s. The parallel dust shells towards the Sun are classical features in the coma of great comets. The nuclear spin is basically the cause of this time modulated activity, one layer beeing emitted at each rotation. However, many characteristics of the phenomenon still remain unexplained. In our opinion, hydrodynamic waves might be required to explain the persistence of the layers. Indeed, we are able to follow very well-delimited waves up to 100 000 km (out of the present field), i.e. 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 m/s near p.a. = 115 degrees. The main nuclear condensation was of magnitude I=9, and this receding "blob" of magnitude 10. They were both easily detected a few minutes after sunrise. The waves system in the south-eastern quadrant could originate from this periodically emitted dust cloud.

WHAT WE HOPE TO LEARN (Sky & Telescope)

The Hubble Space Telescope will not be able to view Hale-Bopp until July, four months after its closest approach to Earth, because the comet will be positioned too near the Sun.

"Comet Hale-Bopp is just another piece in the comet jigsaw puzzle," says Charles S. Morris (Jet Propulsion Laboratory). "The fact that this long-period comet was detected so far out has extended our knowledge of what happens to a comet as it comes in toward the Sun."

Bortle adds, "If the past few months are any indication, we will discover more about the complete range of chemical species in comets over the next six months than in the last six decades. One might also say that we will be getting a glimpse into the composition of those distant and obscure trans-Neptunian bodies that have been detected at the outer edge of our solar system in recent years. Certainly Comet Hale-Bopp must have quite a bit in common with them."

"I would like to learn more about the comet's nucleus," says Brian G. Marsden (Central Bureau for Astronomical Telegrams). "For example, is it really as large as 40 km across? But that may not be possible. Soon after Comet Hale-Bopp's discovery, some enlightened people wondered about actually sending a space probe to intercept the object. This would in fact have been feasible. It's really too bad it wasn't done, for we are unlikely to see a comparable object in the inner solar system for a long time to come."

Jean Lecacheux, Pic du Midi Observatory (105 cm telescope), France

NUMEROUS DUST SHELLS: Ten equidistant shells of dust were detected on February 10.2 (and on previous days) in the sunny side of the coma, extending to 150,000 km from the nucleus. Shells were missing apparently in a paraboloidal volume opposite to sun. Likely the shells are propelled by active area on the nucleus submitted to periodic variations of insolation.

RADIAL EXPANSION OF DUST SHELLS DETECTED ON 1997 FEBRUARY 10.2: Auto-differential image was obtained by dividing, after convenient normalisation, a 16 mn exposure around 6 h 20 UT by a 16 mn exposure around 4 h 30 UT. The bandpass was 830- 1000 nm. Two similar images of a steady comet should cancel exactly and produce a flat featureless field. Radial expansion of the parabolic shells becomes evident owing to relief effect : forward movement faces the apparent illumination. In addition two inner clouds of dust have erupted outwards during the 110 mn interval and gained in brightness. Position measurements on original images show quasi-isotropical expansion of the dust complexes at 0.8 arc sec. /hour approximately, or 300 m/s.

EVIDENCE OF NUCLEAR SPIN REVEALED BY A WOBBLING JET (see image): The orientation of this bright curved jet changes at hour scale and from one night to another. A rotation of the nucleus with the 11.47 +/- 0.05 hours period seems explain all the Pic-du- Midi observations between January 12 and February 10. By perspective shortening, the nucleus appears quasi-stellar when the jet is pointing toward observer (Feb.07 image). The jet is bright and narrow when the active area on the nucleus passes near the subsolar point (Feb.04 image). It is wide and fuzzy at the opposite phase of low sun elevation (Feb.02 image).

Richard M. West (European Southern Observatory's)

Some images of the tail structure clearly show the presence of a strong ion tail, the result of the interaction between the solar wind and the charged particles (ions) in the cometary coma. On at least one of these pictures, an impressive image [JPEG, 60k] obtained on February 4, 1997 at the Crni Vrh Observatory in Slovenia, a strong kink, or perhaps a disconnection event, is visible. Such tail features appear when the comet crosses magnetic boundaries in interplanetary space, resulting in a disruption of the plasma in the tail.

As is the case for the tail, the past days have also seen the development of a complex structure in the coma. While dust jets have been visible since a few weeks after the discovery, these structures have now reached an almost unprecedented degree of complexity. The first indications of this appear to have come from a group of American astronomers who reported the appearance of changing structure in jets of Comet Hale-Bopp from CCD images obtained with two telescopes (the 1.2-m Whipple Observatory telescope and the 0.36-m telescope at the Goodricke-Pigott Observatory) on February 1 and 3 (IAU Circular 6555; February 5, 1997): Bright linear jets are seen at p.a. 25, 166, and 208 deg and fainter ones at 91 and 306 deg. An additional bright but curved jet is seen close to and south of the central condensation on both days with different orientations... (the) southern jets contain at least five periodic brightness enhancements 8 arcseconds apart extending along each jet. Each enhancement in both of these jets has curved tail-like extensions fading towards the approximate anti-solar direction.

New, detailed coma images have also just become available from the Wendelstein Observatory (Bavaria, Germany) (UT 5:18 February 7, 1997 [GIF,41k] and UT 5:20 February 7, 1997 [GIF,40k]). Hermann Boehnhardt (University of Munich, Germany) provides the following information about them: These two R filter exposures of Comet Hale-Bopp were taken with the MONICA camera at the the 80cm Mt. Wendelstein Observatory of the University of Munich, Germany, and processed by Laplace filtering with box width 15 pixels. North is up and east to the left. The field of view is about 260 x 260 arcsec. A strong and slightly curved jet with variable morphology appears at position angle (PA) 25 deg, a much fainter jet points towards PA = 320 deg. In the south-western quadrant, a rather complicate wavy pattern with 7 - 8 shell-like wavefronts and embedded jets and knots may indicate the daily motion of the dust outflow from one (or maybe several) rotating active region(s) on the surface of the nucleus. The projected distance between the wavefronts in this fan-shape geometry is about 15000 km. Another much fainter wave pattern "flows" into the south-east quadrant and may even partially overlap with the former one. Observer: C. Ries; Processing: H. Barwig, H. Boehnhardt, A. Fiedler and W. Mitsch.

Francois Colas, Jean Lecacheux, Pic du Midi Observatory (France)

105 cm. Telescope, 06:00 UT. We have utilized a filter "poussières" to detect only the sun photons reflected by the comet’s matter. The rotational gradient method is used. The last image is elaborated by the "masque flou" technique. The sun position is indicated on each image.

The dust concentration may be explained by the presence of more active zones on the comet nucleus. The Jets are generated when the sun lights up these active zones. The waves in front of the comet (toward the sun) are of more difficult explanation. It could be supposed the existence of a zone that is active during sun lightning and not active when the shade comes. Then, a wave is generated each revolution of the nucleus. Two relevant side jets and a frontal bright zone of the nucleus have been observed. The "dust" is not a nucleus fragment. It is a dust concentration. It can be seen the "dust" is in the sun direction and the two wave systems are at each side respect to the sun direction. The "dust" has been observed on the two days ago images. It seems to be quasi-stationary. (Translation from French)

Olivier Lardiere, Haute-Provence Observatory (France)

I have observed Hale-Bopp (06:00 UT) with the 80cm at F/D=15 telescope of Haute-Provence Observatory (France). The CCD images shown a series of concentric hoods. To enhance these hoods, I use the rotational gradient algorithm (S.M.Larson and Z. Sekanina, Astr.J. April 1984). A similar concentric structure has already been observed in Hyakutake comet. Some specialists think that shock waves are the origin of this concentric morphology.

Misato Observatory (Japan)

We detected a close-up R-band image of Comet Hale-Bopp on Jan. 16 20h 59m UT. We used a LN2-cooled CCD camera (AstroCam) equipped with the Cassegrain focus of the 1.05-m reflector (F7.6) at Misato Observatory.

1998 to present Unusual Observations Synthesis

1997 Unusual Observations Synthesis

• 1997 April - December

• 1997 March

• 1997 January - February

1996 Unusual Observations Synthesis

1995 Unusual Observations Synthesis

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