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 January 9, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Orbit and ephemeris

Improved orbital elements and the corresponding ephemerides have recently been computed by Syuichi Nakano (Japan) and Don Yeomans (USA), based on more than 1000 accurate positions available up to July 1996. The derived orbital elements agree very well and already now allow to predict the comet's positions early next year to an accuracy of the order of 10 arcsec. There is little doubt that further observations will reduce this uncertainty to about just a few arcsec in due time. The orbital calculations now definitively confirm that the image found on a photographic plate obtained in April 1993, does indeed belong to this comet.

The deduced orbital period (at perihelion) is around 2500 years. However, this period changes with time because of the gravitational attraction of the large planets and it was apparently longer in the past. At this moment, it is not yet possible to calculate with reasonable accuracy the time of the previous apparation, but further positional observations will gradually improve this. It is not likely, though, that it will be possible to identify historical observations of this comet from the earlier return.

The perihelion passage takes place at about 3:22 UT in the morning of Tuesday, April 1, 1997. The perihel distance will be about 136.8 million km (0.914 AU) and the comet will be placed on the opposite side of the Sun, as seen from the Earth, cf. the view of the orbit.

In 1996, the comet will make the closest approach to the Earth on August 3, at which time the distance will be about 409 million km (2.733 AU). This distance then increases and reaches a maximum of 455 million km (3.053 AU) in late October 1996 after which it will begin to decrease until it reaches its smallest value on March 22-23, 1997, at 197 million km (1.315 AU).

Comet Hale-Bopp will thus remain quite far away from the Earth, over 13 times more distant than bright Comet Hyakutake that came within 15 million km only, when it passed the Earth in late March 1996.

Comet Hale-Bopp is now located in the southern constellation of Serpens Cauda (The Serpent's Tail), well inside the bright band of the Milky Way. It moves steadily northwards and will pass the celestial equator on December 6. It will reach its northernmost position (+45 deg declination) a few days before the perihelion passage.

During the period when it is brightest, it will be best observable from the northern hemisphere. In February and most of March 1997 the best time will be in the morning twilight and later in the evening twilight.

Current appearance and activity

Recent reports, many of which are published on the List of Recent Comet Brightness Estimates by the International Comet Quarterly , indicate that Comet Hale-Bopp reached naked-eye brightness (visual magnitude 6) in late June 1996. This, however, is only true for perfectly dark sites; in most places, a small binocular telescope will still be needed to see the comet.

The size of the coma is now visually estimated at about 20 arcminutes and some observers have reported a short dust tail of up to 0.5 - 1 deg, possibly even 1.5 deg. This tail will undoubtedly develop further during the coming months. From the geometry, it is almost certain that there will be interesting tail displays in early 1997. A set of predictions by Klaus Beisser (Nuremberg) will be placed at this Website shortly.

Interesting CCD pictures have been published recently on the Web, showing the gradual development of the comet, e.g. at the Crni Vrh Observatory in Slovenia. Exposures with higher spatial resolution have also been obtained in the second half of July, for instance at the Calar Alto Observatory (Spain) and with the Mons telescope of the IAC on the Canary Islands, which show up to six jets in the inner coma.

Thus there is little doubt that the dramatic outflow of dust from Comet Hale-Bopp's nucleus continues. Moreover, an analysis of images obtained with the ESO Schmidt telescope in August 1995 ESO Press Photos 24a-c/95 show that some of the dust must have emanated from the nucleus more than three years earlier. Together with the April 1993 image which was obtained when Comet Hale-Bopp was still nearly 2,000 million km (13 AU) from the Sun, this proves beyond doubt that the comet was active already at a very large heliocentric distance.

The emission of gaseous species has also been vigorous. CO molecules were detected already at a heliocentric distance of 7 AU and the current production rate at 4 AU is about 10e29 mol/s. From measurements of the OH emission, the water production has been found to be of the same order. This is very high when compared to other comets at the same distance.

How bright will Comet Hale-Bopp become in 1997?

This is the most urgent question, now asked by all astronomers who prepare observations for next year. As is usual in such cases, no one can give an answer that is completely trustworthy. Nevertheless, the currently observed activity and its continued and steady increase, combined with the fact that this comet must have been near the Sun several times before, makes it rather unlikely that it will deceive its many fans.

A current, conservative brightness prediction is that Comet Hale-Bopp will reach a total visual magnitude of at least -1 at perihelion. In fact, by combining about 300 amateur observations up to the end of July 1996, Alan Fitzsimmons finds a perihelion magnitude of -2.6. This calculation also indicates that the absolute magnitude , i.e. the brightness the comet would have, it it were seen at exactly 1 AU geocentric and 1 AU heliocentric distance, is extremely bright, actually the second brightest cometary absolute magnitude since the year 1450, after Comet Sarabat of 1729. Nevertheless, Charles Morris finds a less steep brightening, although it still predicts a negative magnitude at perihelion.

It would be important to know the size of the nucleus of Comet Hale-Bopp to support these estimates. Unfortunately, no further measurements, direct or indirect, have been reported, since an upper limit of about 40 km was found from exposures with the Hubble Space Telescope. Most astronomers believe that it will turn out to be somewhat larger than that of Comet Halley (largest dimension 15 km), but we cannot be sure yet. In the same connection, it has not yet been possible to measure the rotation period of the nucleus. During the second half of 1995, outburst were seen to occur at a mean time interval of about 19.5 days, but it is not certain that this really represents the rotation period.

The further development of Comet Hale-Bopp will be followed with great interest. We shall probably have to wait until early 1997, before we will know how bright this comet will become at perihelion.

Molecules in Comet Hale-Bopp

Due to its unsual brightness at large heliocentric distance, it has already been possible to observe Comet Hale-Bopp with some of the best sub-millimetre telescopes in the world and several molecules have been found during the past months. As was the case for Comet Hyakutake, some of these molecules are 'parents', i.e. molecules which are emitted from the nucleus and which later break down into 'daughter' molecules in the coma.

Until the advent of infrared and sub-millimetre techniques, it was not possible to observe the 'parent' molecules directly. Now, however, their numbers and distribution, as well as the motions, can be measured and already yield interesting clues about the composition of the cometary nucleus.

Information about these observations have recently been published on several IAU Circulars. Another place to look for the latest news on this front is the Comet Hale-Bopp (1995 O1) Observers' Bulletin Board and Archives at the University of Maryland (USA). In addition to the above mentioned molecules, CO and H2O , which were detected earlier at larger heliocentric distances, the following have now been found: H2S (IAUC 6408; May 27) and CS, H2CO, HCN and CH3OH (IAUC 6421; June 19). A dramatic increase was observed in the H2S production between May and June. (So now we know that Comet Hale-Bopp does not smell good!)

By means of optical spectra, the CN molecule, presumable a 'daughter' of HCN , was detected already in March 1996 (IAU 6361; March 28) and C2 and C3 emission was seen in April 1996 (IAUC 6400; May 17).