MPI/ESO 2.2-m + EFOSC II (May 14, 1996)

This is a summary of recent developments around this comet; the previous was published on the ESO Web on August 2, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Observations of C/1995 O1 (Hale-Bopp) continue at many observatories and the development of the comet is still such that, unless there is an abrupt and unexpected change, it will become one of the brightest comets in this century. Of particular current interest is the intense jet activity around the nucleus; on some images, no less than seven straight features can be discerned. This is a most unusual sight, even in bright comets, and some observers have remarked upon the similarity with a porcupine !

Current appearance and activity

Comet Hale-Bopp continues to be very active (e.g. IAU Circular 6463; August 28). Recent images show up to seven straight jets emanating from the nucleus; one of these is particularly strong and points to the North. It can be traced over a distance of more than 10-15 arcsec. The distance from the comet to the Earth is now about 420 million kilometres (2.83 AU) and this angular size corresponds to a projected distance of no less than 30,000 kilometres. It is expected that images obtained with La Silla telescopes and showing this phenomenon will become available in a few days' time.

A spectacular image [GIF, 142k] of the jets was obtained in the R-band on the night of the 27th August 1996 by Iwan Williams (Queen Mary and Westfield College, London University), Martin Cartwright and Alan Fitzsimmons (Queen's University of Belfast), using the 1-m Jacobus Kapteyn Telescope on La Palma (Canary Islands). In the version here displayed, the original image has had most of the surrounding dust coma subtracted to bring out the unusual spatial structure. Six jets can be seen. This field spans 84 arcseconds, or roughly 170,000 km at the comet.

Visual observers at dark sites now see a two-degree tail (14 million km projected) pointing south-east, as well as a broad fan, spanning over a position angle of 40-50 degrees and extending up to 10 arcmin (1.2 million km) towards the north. The coma diameter is more than half a degree.

The visual magnitude estimates range from about 5.5 to 5.2 at this moment. This nominally corresponds to an absolute magnitude of about -2, or slightly down from some earlier estimates. The corresponding, predicted magnitude at the perihelion is still impressively bright. It would be very strange if this comet would not reach a negative magnitude.

Dust and gas

Observations of the dust in the coma of Comet Hale-Bopp have been made with various telescopes. Of special interest are here the infrared, spectral observations of the silicate band in the 10 micron spectral region (IAU Circulars 6444 of August 5 and 6448 of August 12). These measurements allow to determine the dust temperature which was found to be relatively high (200 +- 10 K or -70 C) on July 22 (Infrared Telescope Facility, Hawaii) and also to estimate the composition of the dust. For instance, observations with the Canada-France-Hawaii 3.6-m telescope on July 8 indicated the presence of crystalline olivine.

Yet another organic molecule, methyl cyanide (CH3CN) was detected in this comet on August 14-17 by the IRAM IR facility in Spain (IAUC 6458 of August 22). This molecule was also detected in Comet Hyakutake when it passed near the Earth in late March 1996.

Occultation on October 5, 1996

When, as in this case, a comet is involved, we may expect a very exciting opportunity to investigate the structure of the coma; as the comet moves, the amount by which the starlight is dimmed at any time is a direct measure of the density of dust particles in the corresponding area of the coma. Moreover, this event may, at least in theory and with a lot of luck! - provide a direct measure of the nucleus diameter, if an observer is so placed that the nucleus passes directly between him/her and the star. This, however, will only happen in a band of the same width as the nucleus, i.e. perhaps 20 (40??) kilometres in the case of Comet Hale-Bopp . The ground velocity of the comet is about 5 km/sec, so an occultation by the nucleus (that is, when the light from the star is not seen at all) may last several seconds and should be clearly seen.

This time, the ground track is located in the western part of USA. According to the IAU Circular, more details about this event are availble at http://www.lowell.edu/users/buie/occ/predict.html. However, there will most certainly be other occultations later, where observers in other geographical regions may become involved.

Comet Hale-Bopp and the Astronomy On-Line project

The orbit of Hale-Bopp and astrometric measurements

A new set of orbital elements for Comet Hale-Bopp has been computed by Don Yeomans (JPL). It is based on somewhat over 1212 individual position measurements, most of which were performed by amateur astronomers equipped with CCD cameras.

By integrating the new orbit forward and backward in time, the original orbital period (before entering planetary system) was found to be 4161 years; after the perihelion passage, the future orbital period (after exiting planetary system) is 2364 years.

This new orbital calculation also allows to estimate the mean accuracy of the submitted Hale-Bopp positions as somewhat better than 0.8 arcsec (r.m.s.). This is excellent and corresponds to what was obtained by many professional astronomers just some years ago. It is interesting that these values do not directly represent the measuring accuracy on the CCD frames, this number also includes significant systematic errors which are inherent in the astrometric catalogue which provides the reference stars needed to determine the comet's position. If these catalogue errors could be removed, the work by the amateurs would in principle be even more accurate, and lead to even more precise orbital determinations. Fortunately, this situation is likely to change in a not-too-far future.

The catalogue which is currently most used by amateurs for the measurements of positions of moving objects is the Guide Star Catalogue (GSC) . This catalogue is by far the largest astrometric catalogue in existence and it contains more than 40 million stars with listed sky positions. This corresponds to about 1000 stars per square degree, so many that there will always be some in a sky field recorded with a CCD camera at an amateur telescope. This ensures that the comet's actual position can then be easily interpolated. However, this catalogue has systematic errors of up to one arcsecond or even more in some sky zones, meaning that the measured position may be offset by the same amount from the true one. This is because the GSC was not primarily prepared for high-accuracy astrometric uses.

However, there will soon be important developments in this area. Just a few days ago, the European Space Agency (ESA) announced that two new astrometric catalogues will become available next year which are virtually free of systematic errors and which will thus form a superior basis for accurate astrometric measurements by professional and amateur astronomers.

Following several years of hard work, the results from the astrometric ESA Hipparcos mission will soon be published. A summary [GIF,15k] of the achievements shows that the primary Hipparcos catalogue with about 118,000 stars will have an positional accuracy that is better than 0.001 arcsecond and the so-called Tycho Catalogue , with over 1 million stars, will reach about 0.025 arcsec. The latter catalogue has a sufficient sky density to ensure that in most cases it can be used in the same way as GSC is now used, and any systematic errors are negligible for this purpose.

Thus there is little doubt that when it is released next year, many astrometric observers will want to use the Tycho Catalogue.