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Overview Of Comet Hale-Bopp Conference

ESO Education and Public Relations Dept.

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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, email: rwest@eso.org).

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 group at 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 cristallization 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, SO2, H2CS, HC3N, HNCO, NH2CHO, HCOOH, CH3OCHO).
Observations of isotopes (now also including DCN, HC15N and C34S 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/H2O 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 
etected 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 CO2, 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.