2 Hale- Bopp ESO Press Releases Richard M. West, ESO __________________________________________________________________ ESO Press Releases Comet Hale-Bopp Update (August 2, 1996) This is a summary of recent developments around this comet; the previouswas 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. C/1995 O1 (Hale-Bopp) has been observed at AU). This distance then increases and reaches a many professional and amaterur observatories maximum of 455 million km (3.053 AU) in late during the past months. Various reports have October 1996 after which it will begin to appeared which all indicate that this decrease until it reaches its smallest value on extraordinary comet continues to develop in a March 22-23, 1997, at 197 million km (1.315 way that permits us to hope that it will indeed AU). become a beautiful and unusual sight early next Comet Hale-Bopp will thus remain quite far year. The following information does not pretend away from the Earth, over 13 times more distant to be complete, but rather to concentrate on some than bright Comet Hyakutake that came within of the critical issues in this connection. 15 million km only, when it passed the Earth in late March 1996. Orbit and ephemeris Comet Hale-Bopp is now located in the Improved orbital elements and the southern constellation of Serpens Cauda (The corresponding ephemerides have recently been Serpent's Tail), well inside the bright band of the computed by Syuichi Nakano (Japan) and Don Milky Way. It moves steadily northwards and Yeomans (USA), based onmore than 1000 will pass the celestial equator on December 6. It accurate positions available up to July 1996. The will reach its northernmost position (+45 deg derived orbital elements agree very well and declination) a few days before the perihelion already now allow to predict the comet's passage. positions early next year to an accuracy of the During the period when it is brightest, it will order of 10 arcsec. There is little doubt that be best observable from the northern hemisphere. further observations will reduce this uncertainty In February and most of March 1997 the best to about just a few arcsec in due time. The orbital time will be in the morning twilight and later in calculations now definitively confirm that the the evening twilight. image found on a photographic plate obtained in April 1993, does indeed belong to this comet. Current appearance and activity The deduced orbital period (at perihelion) is Recent reports, many of which are published around 2500 years. However, this period changes on the List of Recent Comet Brightness with time because of the gravitational attraction Estimates by the International Comet Quarterly, of the large planets and it was apparently longer indicate that Comet Hale-Bopp reached naked- in the past. At this moment, it is not yet possible eye brightness (visual magnitude 6) in late June to calculate with reasonable accuracy the time of 1996. This, however, is only true for perfectly the previous apparation, but further positional dark sites; in most places, a small binocular observations will gradually improve this. It is not telescope will still be needed to see the comet. likely, though, that it will be possible to identify The size of the coma is now visually historical observations of this comet from the estimated at about 20 arcminutes and some earlier return. observers have reported a short dust tail of up to The perihelion passage takes place at about 0.5 - 1 deg, possibly even 1.5 deg. This tail will 3:22 UT in the morning of Tuesday, April 1, undoubtedly develop further during the coming 1997. The perihel distance will be about 136.8 months. From the geometry, it is almost certain million km (0.914 AU) and the comet will be that there will be interesting tail displays in early placed on the opposite side of the Sun, as seen 1997. A set of predictions by Klaus Beisser from the Earth, cf. the view of the orbit. (Nuremberg) will be placed at this Website In 1996, the comet will make the closest shortly. approach to the Earth on August 3, at which time Interesting CCD pictures have been published the distance will be about 409 million km (2.733 recently on the Web, showing the gradual Jurasternwarte 1998, H. Jost Hale- Bopp 2 - 2 __________________________________________________________________ ESO Press Releases development of the comet, e.g. at the Crni Vrh It would be important to know the size of the Observatory in Slovenia. Exposures with higher nucleus of Comet Hale-Bopp to support these spatial resolution have also been obtained in the estimates. Unfortunately, no further second half of July, for instance at the Calar Alto measurements, direct or indirect, have been Observatory (Spain) and with the Mons telescope reported, since an upper limit of about 40 km of the IAC on the Canary Islands, which show up was found from exposures with the Hubble to six jets in the inner coma. Space Telescope. Most astronomers believe that Thus there is little doubt that the dramatic it will turn out to be somewhat larger than that of outflow of dust from Comet Hale-Bopp's nucleus Comet Halley (largest dimension 15 km), but we continues. Moreover, an analysis of images cannot be sure yet. In the same connection, it has obtained with the ESO Schmidt telescope in not yet been possible to measure the rotation August 1995 ESO Press Photos 24a-c/95 show period of the nucleus. During the second half of that some of the dust must have emanated from 1995, outburst were seen to occur at a mean time the nucleus more than three years earlier. interval of about 19.5 days, but it is not certain Together with the April 1993 image which was that this really represents the rotation period. obtained when Comet Hale-Bopp was still nearly The further development of Comet Hale-Bopp 2,000 million km (13 AU) from the Sun, this will be followed with great interest. We shall proves beyond doubt that the comet was active probably have to wait until early 1997, before we already at a very large heliocentric distance. will know how bright this comet will become at The emission of gaseous species has also perihelion. been vigorous. CO molecules were detected already at a heliocentric distance of 7 AU and the Molecules in Comet Hale-Bopp current production rate at 4 AU is about 10e29 Due to its unsual brightness at large mol/s. From measurements of the OH emission, heliocentric distance, it has already been possible the water production has been found to be of the to observe Comet Hale-Bopp with some of the same order. This is very high when compared to best sub-millimetre telescopes in the world and other comets at the same distance. several molecules have been found during the past months. As was the case for Comet How bright will Comet Hale-Bopp become Hyakutake, some of these molecules are 'parents', in 1997? i.e. molecules which are emitted from the This is the most urgent question, now asked nucleus and which later break down into by all astronomers who prepareobservations for 'daughter' molecules in the coma. next year. As is usual in such cases, no one can Until the advent of infrared and sub- give an answer that is completely trustworthy. millimetre techniques, it was not possible to Nevertheless, the currently observed activity and observe the 'parent' molecules directly. Now, its continued and steady increase, combined with however, their numbers and distribution, as well the fact that this comet must have been near the as the motions, can be measured and already Sun several times before, makes it rather unlikely yield interesting clues about the composition of that it will deceive its many fans. the cometary nucleus. A current, conservative brightness prediction Information about these observations have is that Comet Hale-Bopp will reach a total visual recently been published on several IAU magnitude of at least -1 at perihelion. In fact, by Circulars. Another place to look for the latest combining about 300 amateur observations up to news on this front is the Comet Hale-Bopp (1995 the end of July 1996, Alan Fitzsimmons finds a O1) Observers' Bulletin Board and Archives at perihelion magnitude of -2.6. This calculation the University of Maryland (USA). In addition to also indicates that the absolute magnitude, i.e. the above mentioned molecules, CO and H2O, the brightness the comet would have, it it were which were detected earlier at larger heliocentric seen at exactly 1 AU geocentric and 1 AU distances, the following have now been found: heliocentric distance, is extremely bright, H2S (IAUC 6408; May 27) and CS, H2CO, HCN actually the second brightest cometary absolute and CH3OH (IAUC 6421; June 19). A dramatic magnitude since the year 1450, after Comet increase was observed in the H2S production Sarabat of 1729. Nevertheless, Charles Morris between May and June. (So now we know that finds a less steep brightening, although itstill Comet Hale-Bopp does not smell good!) predicts a negative magnitude at perihelion. Jurasternwarte 1998, H. Jost Hale- Bopp 2 - 3 __________________________________________________________________ ESO Press Releases 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). Jurasternwarte 1998, H. Jost Hale- Bopp 2 - 4 __________________________________________________________________ ESO Press Releases Comet Hale-Bopp Update (April 01, 1997) This is a summary of recent developments around this comet; the previous was published on the ESO Web on comet-hale-bopp-summary-mar25-97.