Impressions from the Hale-Bopp Meeting on Tenerife (February 1998)

This is a provisional overview of some of the discussions that took place at the First International Hale-Bopp Conference at Tenerife in February 1998. It was prepared by R. M. West (ESO).

I. Introduction

Ten months after the perihelion passage, the First International Meeting about Comet Hale-Bopp was held at the Conference Centre in Puerto de la Cruz (Tenerife, Canary Islands, Spain) on February 2-5, 1998. Nearly 150 specialists from all major comet research groups in the world participated. During 4 days of intensive debates and with the presentation of approximately 150 papers, the participants surveyed the current status of the many research programmes related to this most unusual comet.

The Local Organising Committee, headed by Mark Kidger and Monica Murphy (IAC) had done a great job and the frame was excellent. The conference provided a good opportunity for a discussion about some of the fundamental issues connected to this spectacular astronomical event. For instance: why was this comet so bright and in which respect(s) did it differ from other comets observed with modern equipment? Although many new results were presented and some main lines can be perceived, those present were left with the impression that there are still many open questions. There is no doubt that the associated research will continue for some time. It is also obvious that further meetings on these subjects will be held in due time.

The Hale-Bopp event provided observers with a long lead time, thanks to the early discovery in July 1995 by Alan Hale and Tom Bopp who were both at the conference. Thus, it was possible for the scientists to obtain a substantial amount of observing time at the world's major observational facilities and to prepare their runs well. Moreover, the Comet was visible in the sky for an extremely long period. It was very bright and in the end, a large number of telescopes and instruments were used at all wavelengths from X-ray to radio. It is therefore no surprise that all the work by so many scientists during the past months has resulted in important new knowledge, as exposed at this meeting.

In what follows, some of the highlights of the conference will be reviewed. They are arranged roughly in the order they were presented at the meeting. Kindly note that not all contributions mentioned here are attributed to individual speakers and various information by others has been left out in order to keep this survey within a reasonable size. However, a complete version of the conference summary, with full references and more details, will ultimately appear in the Conference Proceedings.

II. Motion and Early Observations

The meeting began with some basic information about the comet.

The Orbit

Based on more than 2600 astrometric observations from 1993-98, Brian Marsden has calculated a new and improved orbit, now taking into account non-gravitational forces arising from the jet effect associated with the Comet's vigorous activity. He found that the original period was 4211 years and that the future period will be 2392 years with a formal uncertainty of a few months only. However, the limited knowledge about the future development of the Comet's activity may still change this period somewhat.

Had it arrived about four months earlier this time, it would have passed the Earth nearly as close as did Comet Hyakutake one year earlier. In that case it would have been an incredible view. Interestingly, it appears that Comet Hale-Bopp may have passed very close to Jupiter on June 7, 2216 BC. In view of the rather unstable orbit, it is unlikely that there have been more than a few earlier, close perihelion passages.

Early Observations

Alan Fitzsimmons reviewed the various signs of very early activity which are typical for this comet. In particular, investigations of early images of the dust tail by Hermann Boehnhardt and Marco Fulle have shown that the Comet most probably was active already 4-5 years before discovery, that is at pre-perihelion distance 18-20 AU. In addition to a UK Schmidt pre-discovery image obtained in April 1993, an image of the Comet may possibly be present on another photographic plate taken with the same telescope in September 1991; this will now be investigated.

III. The Nucleus

Size

Harold Weaver and Philippe Lamy surveyed 7 different methods which have led to reasonably consistent estimates of the size of Comet Hale-Bopp's nucleus. Most of these lie in the interval between 20 and 40 km radius (i.e., 40 and 80 km diameter), but a few are somewhat larger. There is also a possible indication that the nucleus may have elongated shape. Particularly impressive among these observations were those performed at radio wavelengths with the VLA in New Mexico and which lasted more than 6 days - they pointed towards a diameter of approximately 50 km.

Interestingly, there may be more than one component of the nucleus. By very careful analysis of high-resolution HST images obtained in 1996, Zdenek Sekanina believes that the primary nucleus may have a lesser companion of approximately half the size. This issue is still somewhat controversial, but observations with the Adonis adaptive optics camera at the ESO 3.6 m telescope in November 1997 and January 1998 by three ESO astronomers also appear to show a double nucleus. More observations with this facility in the coming months and/or with the HST scheduled for later this month are expected to clarify this issue.

Rotation

In a review talk on this subject, Dave Jewitt listed the observational possibilities for measuring the rotation period of Comet Hale-Bopp's nucleus. With the nucleus hidden inside the coma already at the moment of discovery, they include periodic fluctuations of that part of the light at the centre of the coma which supposedly comes from the nucleus itself, and also periodic changes in the coma structure (orientation of jets, outward motion of shells, etc.).

Many such observations are available; the longest series was apparently obtained by Mark Kidger and his groupat the Teide Observatory on Tenerife, right above the site of the conference. At this moment, there is good agreement among the values published by 8 different groups and the true rotation period of the nucleus must be close to 11.34 +/- 0.03 hours.

Although there were originally some signs of precession (wobbling of the rotation axis), this is now less sure. The direction of the polar axis has also not been unambiguously determined yet, but this may become possible after further analyses.

Composition and Structure

Dominique Bockelee-Morvan and Hans Rickman surveyed the many new observations which will ultimately allow a better `look' into the still unknown interior of a cometary nucleus. This is first of all due to the very extensive observations which were made of the production rates of various molecules, as the comet came closer to the Sun. These observations show that not all of these species emerge in parallel and there seem to be certain `transitory' periods during which changes in the production rates can be observed. They are indicative of the composition and structure of the upper layers of the nucleus.

For instance, a slowing down of the rate of increase of CO production was observed at about the time when the water production started at a heliocentric distance of approximately 3.5 AU. The production rates of some, less abundant molecules, showed a very steep dependence on heliocentric distance. All in all, the observed behaviour seems to follow quite well what is predicted by the models which have been put forward and which were described at the meeting by Dina Prialnik - they include in particular heat release by sub-surface crystallization of amorphous ice in the nucleus.

It is also well established that the unusually great activity of Hale-Bopp which was observed while it was still far from the Sun is mostly caused by the outgassing of CO from its interior; this process pushed large amounts of dust into space. The more dust there is around the nucleus, the more sunlight is reflected and the brighter will the comet appear.

IV. The Gas Phase

Many gaseous molecules and atoms were observed in the coma. Some of these are electrically uncharged (neutrals), others have lost one or more electrons (ions). Sodium, a neutral atom observed extensively in Hale-Bopp, plays a particular role and was discussed in a special session.

Neutrals

Didier Despois reviewed extensive radio observations which have led to the discovery of a total of 8 new molecules never seen before in a comet (SO, SO 2 , H 2 CS, HC 3 N, HNCO, NH 2 CHO, HCOOH, CH 3 OCHO). Observations of isotopes (now also including DCN, HC 15 N and C 34 S for the first time) indicate that this comet is similar to Comet Halley and that it was formed in the solar system. In particular, the HDO/H 2 O ratio was found to be twice that measured in the Earth's oceans, and 10 times larger than the protosolar value.

Thanks to great technological advances, it has now become possible to produce detailed maps of the distribution of individual molecules in the coma. This has led to very interesting research which will ultimately help to understand the extremely complex chemistry of a cometary coma. In particular, this may allow to determine which of the molecules observed really come from the nucleus itself (as parents ) and which are secondary products (daughters ).

Jacques Crovisier reported equally exciting new observations in the infrared spectral region, from the ground with several of the largest infrared telescopes and from space (ISO). This includes hydrocarbons (organic molecules) and also water for which the ortho-to-para ratio was equal to that measured at Halley and indicates the very low spin temperature of 25 K. It is not clear whether this is also the temperature of formation.

Unfortunately, at least for this type of research, the very large dust-to-gas ratio observed in Comet Hale-Bopp made observations of spectral emission lines difficult since they were recorded on top of a very strong continuum spectrum of solar light reflected from the dust in the coma. Partly for this reason, it appears that it has not been possible to gain new knowledge about the interesting emission lines from organic molecules seen in the 3.2 - 3.6 micron band. Nevertheless, many new mineral bands were seen in the infrared region (see below).

Many spectral observations in the optical region were reported by Claude Arpigny . They generally show that Hale-Bopp is similar to other long-period comets. Several groups have reported detailed, very high-resolution spectroscopic monitoring of the various emission lines in this wavelength region. There is obviously still much work to be done on all of these high-dispersion spectra.

In the ultraviolet spectral region observations were made with a number of spacecraft and also with several sounding rockets. Paul Feldman described the spectra obtained with HST and the IUE Space Observatories which include many atomic lines. Further towards shorter wavelengths, a line of singly ionized oxygen (O + ) has been detected by the EUVE satellite at 538 A, but unexpectedly, neon (Ne) was not detected in the same spectral region. This points to a very low neon-to-oxygen ratio in this comet, at least 25 times less than the solar value.

An enormous Lyman-alpha halo of hydrogen, about 150 million km diameter, that is the distance from the Sun to the Earth, was observed by the SOHO Observatory when the comet was near perihelion. It was also possible to view the comet in the ultraviolet light of various atoms; when compared to images obtained at other spectral wavelengths, they will contribute to the understanding of the processes in the coma.

Ions

Heike Rauer reported that most of the ions known in earlier comets have also been observed in Comet Hale-Bopp. Strangely, emission from CO + was first detected quite late (at a heliocentric distance of 3.6 AU); the reason for this is still unclear. Very complex coma and tail structures were observed by Steve Larson and others in the light of CO + and other selected ions, indicating an exceedingly complex interaction between the solar wind and the cometary ions (streamers, sunward arcs, etc.). In this respect, the detailed mapping of the spatial distribution in the coma and the corresponding velocity field of HCO + by groups in Europe and the USA provided very valuable observational information.

There has clearly been tremendous progress in the modelling of the solar wind/comet interaction in recent years.Tamas Gombosi showed that new and very complex computer software running on the fastest machines available now make it possible to reproduce in quite some detail the observed structure (distribution of ions, magnetic field lines, cavities, sheets, etc.). In this context, the discovery by the Ulysses Spacecraft that the solar wind moves faster at high ecliptic latitudes and therefore interacts stronger with the comet when it is far from the ecliptic plane, has provided an important breakthrough in this field.

Sodium

While sodium has been seen since 1910 in comets that come close to the Sun, the first signs of a sodium tail was reported in 1957 from an objective prism spectrum obtained of the unusual Comet Mrkos. However, it was in mid-April 1997 that the now famous third cometary tail of neutral sodium atoms and measuring more than 50 million km was discovered by Gabriele Cremonese and his colleagues of the European Comet Hale-Bopp Team. Already at that time, the correct interpretation was brought forward, that is fluorescence acceleration of sodium atoms released in the coma.

Meanwhile, this and other groups have also reported the presence of neutral sodium in the normal dust tail, demonstrating that these atoms are also released from the dust in this tail. It is still unclear, however, from where the sodium in the inner coma comes. Interestingly, no NaOH (soda) or NaCl (salt) was found in gaseous form in the coma (but may still be present in the dust grains).

V. Dust Phase

Observations of new minerals

Klaus Jockers reported on extensive observations of the dust in Comet Hale-Bopp. These concern direct imaging, the distribution of colours within the coma and the tail and also the polarization. This comet had a somewhat higher degree of polarization when observed at large phase angles than other comets, indicating differences in the dust component.

A true breakthrough has occurred in the field of remote observing of cometary minerals, as discussed by Martha Hanner and others. Ground-based and space-based observations of the detailed infrared spectrum of Comet Hale-Bopp have revealed for the first time many new spectral features which can be assigned to particular minerals with a great degree of certainty. They include above all cristalline olivines, in particular the magnesium-rich forsterite, and also pyroxene-rich minerals.

In fact, it seems that the composition of some of the grains observed in Comet Hale-Bopp are very similar to those of two main types of interplanetary dust particles which have been collected in the Earth's atmosphere and subsequently analysed in great detail in terrestrial laboratories.

Dust production

The dust production of Comet Hale-Bopp was enormous, especially when compared to other comets, for instance 100 times more than in Comet Halley. Similarly, the dust-to-gas ratio was very high, from most measurements estimated as between 2 and 5. The dust production at the maximum reached about 400 tonnes/sec, but since the nucleus is so large, the entire mass loss at this passage is probably still less than 0.1 percent of its total mass.

Similarities with circumstellar dust

It is also very interesting to compare the infrared spectra of Comet Hale-Bopp obtained with the ISO Observatory with spectra of stars which are surrounded by circumstellar dust. As Christoffel Waelkens pointed out, there are great similarities, but also some differences. For instance, the spectrum of the star HD 100546 also displays the minerals mentioned above, as well as cristalline water, but contrary to the Comet, it also has strong spectral features of organic components in the 3.5 micron band.

There may thus be a close relationship between comets like Hale-Bopp and the material observed in circumstellar disks, e.g. around the southern star Beta Pictoris. All of this may provide very valuable new information about the formation of the cometary reservoirs in the solar system (Kuiper Belt and Oort Cloud).

VI. Dust-Gas Interactions

As mentioned above, the gas chemistry of cometary comae is extremely complicated and when the dust component is also taken into account, everything becomes even more complex. Thus, it is most promising to see that it has now become possible to model in significantly greater detail what is going on in a cometary coma by means of very elaborate three-dimensional computer models. The report by Mike Combi and others proved that, when taken together with the new observations which have become available and which have been mentioned above, we may expect to reach a much better understanding of the various interactions in cometary comae in the future.

X-rays

An intensive debate is still raging about the origin of the soft X-ray emission that has now been observed in a total of 10 comets, including Hale-Bopp. No less than 5 different explanations (models) have been put forward and none has yet been ruled out. It now seems that two of these may both play particularly important roles. The first is based on a charge exchange between heavy ions in the solar wind and light atoms in the cometary coma. By excitation of the inner atomic levels of these atoms, X-rays are released from these. The other is based on the presence of large numbers of extremely fine dust grains (so-called "atto-dust" seen in Comet Halley) which reflect the solar X-rays. It is obvious that more observations of more comets are needed before this controversy can be resolved.

VII. Some Conclusions

Comet Hale-Bopp has indeed proven to be a bonanza for researchers in this field. Never has a comet been observed so extensively at such large heliocentric distances and not even in the case of Comet Halley was it possible to obtain such detailed information about the progressive changes that took place in the coma of Comet Hale-Bopp as it approached the Sun.

This gives substantial hope that it will now be possible to understand better the structure and composition of cometary nuclei, before the first cometary space missions perform in-situ measurements. The newly found, clear similarities between the cometary dust and the dust around certain stars also promise to give new insights into the origin and formation of the comets in the solar system.

Some participants in this very successful meeting expressed that the appearance of Comet Hale-Bopp was such an important event in the history of cometary research that it may later be considered almost a par with the Halley encounter in 1986.

Observations of Comet Hale-Bopp will continue for quite some time. On the spectroscopic front, astronomers with access to large telescopes will follow the steady decrease of gas production which, with the exception of CO and CO 2 , is likely to cease during the next years. Images will be made which will show structural changes in the coma and allow to study the decreasing dust activity. Perhaps it will later be possible to observe directly the naked nucleus and to get an accurate understanding of its spin state. Continued astrometric observations will gradually improve the orbit so that very accurate predictions can be made for the Comet's next return, some 24 centuries from now.

But work will also continue on other fronts. Much of the enormous amount of data has not yet been thoroughly studied and there important new information may still be uncovered. At the same time, it is obvious that the modelling of the Comet's coma, the processes therein and the interaction with the solar wind will advance greatly in the coming years. Progress is also likely for the modelling of the cometary nucleus itself.