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Comet Hale-Bopp (March 7, 1997)
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 February 26, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.
Addition on March 11: Please note that during the hectic phase in March - April 1997, additional, very recent information may be displayed on the front page of the ESO Comet Hale-Bopp Homepage.
Richard M. West (ESO)
Munich, March 7, 1997; 16:00 UT
Hale-Bopp approaches magnitude 0
Magnitude estimates by visual observers from this morning are just below 0, i.e. we may now expect that the Comet will pass this special `threshold' during the next days. In fact, the first observer to report a negative magnitude is A. Baransky (Kiev, Ukraine) who estimated magnitude -0.1 during a naked-eye session in the early morning of March 5.
Most predictions of the maximum magnitude are now between -0.5 and -1.0. For instance, Mark Kidger (IAC, Tenerife, Spain), provides a thorough contribution to the discussion about the prospects , concluding that Comet Hale-Bopp will probably reach magnitude -1.0.
He also notes that the absolute magnitude of the comet is now steady at approximately -0.8, the third brightest absolute magnitude recorded since the year 1400. Comet Sarabat of 1729 (absolute magnitude -3.0) and the Comet of 1577 (-1.8) were intrinsically much brighter, while Comet De Cheseaux of 1747 (not the 6 tailed comet) had an absolute magnitude of -0.5. These three comets and Hale-Bopp are the only ones known to have had a negative absolute magnitude. (The absolute magnitude of a comet is the magnitude with which is would be seen it it were located at a distance of exactly 1 AU (149.6 million km) from both the Sun and the Earth. It is thus a measure of the comet's `intrinsic' brightness.)
Another experienced cometary astronomer, Charles S. Morris (USA), arrives at a similar prediction. From a study of the development , most recently updated on March 5, he indicates a magnitude of -0.8 at maximum.
This may be compared with the brightest star in the sky, the brilliant Sirius in the constellation Canis Major (The Greater Dog), whose (visual) magnitude is -1.5.
These and other studies are based on the very reasonable assumption that the comet will continue to develop during the next three weeks in the same way as it has been observed to do during the past month. Nevertheless, as had been repeatedly said, surprises cannot be excluded, for instance that a splitting will occur, thereby resulting in an unexpected brightness increase (see below).
Length of the tail
The tail length, as quoted by experienced observers at dark sites, is now of the order of 10 o , in some reports even longer. The Comet has indeed become a splendid object and I now believe that there is a good chance that a tail of at least 20 o , perhaps even 25 o (or 30 o !) may develop some weeks from now. If this really happens, it will more than rival the bright comet in 1976!
Where to observe Hale-Bopp?
A good guide on the Web, also for the inexperienced observer, has been written by Alan MacRobert (Sky & Telescope) with the inviting title When, Where, and How to See Comet Hale-Bopp. This also includes well-prepared, easy-to-read sky charts that will help the observer. However, the Comet is now so bright that there should be no problem in finding it, if you just look in roughly the right direction!
In this context, it is interesting to note that, in particular from northern locations, Comet Hale-Bopp is now also becoming well visible in the evening after sunset, no longer just in the early morning sky. This is because of its increasingly northern position in the sky.
With a current declination of +38.5 o , the Comet is now circumpolar at all sites north of approximately 90 o - 38.5 o = +51.5 o geographical latitude, i.e. in large parts of Canada and in the countries in Northern Europe. (The highest declination, +46 o , will be reached around March 25 - and then this latitude will be +44 o ). This means that if you are located north of this limit, the Comet will be above your horizon during 24 hours - it will never set.
At lower geographical latitudes, the Comet will be below the horizon for some hours around midnight, but also here, it may be well visible in the evening sky.
In late March, the viewing conditions will be such that it is best observed in the evening (and then you do not have to get out of your bed very early any more). This will probably be the best period to enjoy it and I strongly recommend everybody to do so, also parents with their children, who will surely be excited by this unusual sight!
Note that this Comet may perhaps become so bright that the head may even be visible in small telescopes during daytime. In order to see it, you must know quite accurately where to point your instrument and you must in any case be extremely careful not to point it towards the Sun when you are searching for the Comet . Looking directly at the Sun through a telescope will cause immediate and permanent damage to your eye(s)!
Solar eclipse on March 9, 1997
A total solar eclipse will take place on March 9, 1997; it will be visible in Central Asia. During this event, the Moon will pass between the Sun and the Earth and cast a shadow that moves rapidly across the surface of our planet. Observers located along the eclipse's path will be able to see the Comet in a dark sky during the phase of totality, when the Moon completely covers the Sun. There is little doubt that this will be a splendid sight.
Information about this eclipse and others that will happen during the next years is available at various Websites, for instance at the Osservatorio di Catania (Italy) and at NASA. On this map from the Catania Observatory , the path of the eclipse on March 9 is indicated as no. 5 .
A rather complete description of the eclipse has been prepared by Sky & Telescope and there may be an opportunity to see live images via Japan.
For information about the motion of the Sun and Moon and how such eclipses happen, I suggest to look at the relevant Astronomy On-Line site for the partial Solar Eclipse on October 12, 1996.
Will the nucleus split?
The possibility of a splitting of the nucleus of Comet Hale-Bopp has been discussed on various occasions. This refers to more or less dramatic events, i.e. from the `breaking-off' of smaller pieces from this nucleus (the dirty snowball at the centre) to a complete `break-up'. In both cases, fresh surfaces would be subjected to the heating effect of the sunlight, resulting in an overall increase of the evaporation of gas and emission of dust particles and therefore an increase in brightness.
What are the prospects for such an event in the case of Hale-Bopp?
In various recent research papers, some of which have been inspired by the breaking-up of the famous comet Shoemaker-Levy 9 near Jupiter in July 1992 (two years before the resulting fragments collided with this planet), Zdenek Sekanina (Jet Propulsion Laboratory, Pasadena, USA) discusses the circumstances of split comets. He considers two types of such events: 1) those which take place in the neighbourhood of another object ( tidal events ) and 2) those which happen when the comet is far away from all other bodies and is therefore not subjected to any particularly strong tidal forces that increase the internal stress in the nucleus ( non-tidal events ).
According to Zdenek Sekanina, there are three known cases of comets that have split when they were very close to the Sun and two others, including Shoemaker-Levy 9, broke up near Jupiter. [Incidentally, Hartmut Frommert (Lunar and Planetary Laboratory, Tucson Az. USA) has just announced a new and more complete List of sites holding information about the SL9 crash. I also call your attention to the corresponding ESO SL9 site.]
The other group, referred to as the non-tidally split comets , comprises just over two dozen objects, beginning with the famous Comet Biela which was observed in 1846 and 1852 and up to the most recent, Comet Schwassmann-Wachmann 3, that split in 1995. The latter event was well documented by observations from ESO, cf. ESO Press Release 01/96 and ESO Press Photo 01/96 [GIF; 73k].
The most recent, very bright comet to split was the one in 1976. Soon after the perihelion passage in late February that year, four individual pieces were clearly seen that slowly drifted apart. It is now believed that it was indeed this event that caused it to achieve its unusual brightness. It is, however, not possible to compare it directly with Hale-Bopp - these two comets differ in at least two important respects. Contrary to Hale-Bopp, the comet in 1976 was apparently visiting the inner solar system for the first time and it came also much closer to the Sun (29.4 million km at its perhelion) than Hale-Bopp will on April 1st (136.7 million km).
The only feature of Hale-Bopp that may possibly increase the chances for a splitting is its incredible level of activity. There is no doubt that the continuing, extremely intensive outflow of dust and gas must create rapid changes near the vents on the surface and around the reservoirs within the nucleus from where this matter comes. That may conceivably lead to internal stresses, perhaps even cracks that may further widen so that smaller or larger pieces eventually `fall off'. Only continued observations will show if this happens or not.
In this connection, note also the information about False Nuclei and Nucleus Splittings by Mark Kidger .
There are many new reports about important spectroscopic results from Comet Hale-Bopp.
This time they mostly concern the observation of specific atomic and molecular species.
On March 4, J. M. Veal and collaborators in the USA and Taiwan report the detection of HCO + using seven antennae of the Berkeley-Illinois-Maryland-Association Array (IAU Circular 6575): The J=1-0 transition at rest frequency 89.189 GHz was detected in emission on Feb. 19, 22, and 28 UT... The HCO + spectral lines are consistently asymmetric with a redward wing.... We believe this to be the first detection of HCO + in a comet.
On the same IAU Circular, T. Kawabata and his colleagues in Japan report observations of sodium lines at the BAO 1-m telescope: High-resolution slit spectra of C/1995 O1 show emission lines of Na D1 and D2 on Feb. 26.9 UT; the observed velocities within 7 arcsec of the comet's nucleus in the north-south slit were -32 and -29 km/s for D1 and D2, respectively, while nearby absorption lines have an average velocity of -48 +/- 3 km/s. It appears that the Na emission has an asymmetric spatial profile and is more extended toward the south (dust-tail side) than the north .
The described asymmetrical distribution of sodium atoms in Hale-Bopp is typical; it is caused by radiation pressure. Since sodium evaporates at a temperature of 877 o C, i.e. much higher than of the comet at the time of the observation that was made at a heliocentric distance of about 162 million km, it is believed that the observed sodium may not come from silicate dust grains, but rather from some (not yet identified) complex molecules that evaporate and are then destroyed by the strong UV-light.
On March 3, D. C. Lis and collaborators reported the radio detection of sulfur monoxide (SO), carbonyl sulfide (OCS), ionized carbon monoxide (CO + ), and the cyanogen radical (CN) at the Caltech Submillimeter Observatory (CSO) on Feb. 20-23 (IAUC 6573).
M. J. Mumma and colleagues have continued their observations at the NASA Infrared Telescope Facility in early March and they report observations on a number of molecules on IAUC 6573. In addition, he has sent an enthousiatic message to professional colleagues on March 5, parts of which are reproduced here: Comet Hale-Bopp is certainly meeting all our expectations; it is providing a scientific bonanza, and is also a wonderful sight! We....managed to obtain a total of 6 hours observing on the comet. The water production rate is running about 1e31 molecules/sec, and the spectral lines are bright. However, the continuum longward of 3 mum has brightened enormously since Jan. 21.5, so the line/continuum brightness ratio is small - typically 0.1 for bright lines.... We believe we are seeing the signature of organic grain emission in the continuum intensity vs. wavelength .
I think that Michael Mumma is here expressing the sentiments of many other professional colleagues - this Comet may indeed be enjoyed in many different ways!
Rotation of the nucleus
Supporting evidence of a 12-hr rotation period has been obtained by F. Mannucci and G.-P. Tozzi (Italy). On IAUC 6574 (March 4), they write that Near-infrared J, H, and K observations with the TIRGO telescope between Feb. 3 and 10 show several shells with a projected velocity of 0.80 arcsec/hr, confirming a rotation period of about 12 hr.... A preliminary analysis shows that the nucleus' spin-axis position is almost perpendicular to the line-of-sight .
During the past week, I have received quite a few emails from observers who inform me about their images. Thank you very much - it is always a pleasure to learn about your work! However, it is not the intention to establish a major image archive at this site, so please accept that I will not display them all!
Following my comments about the virtues of drawings in the recent summary , several observers have sent copies of their drawings. I note for instance the ones obtained in February and March by Luc Arnold (Observatoire de Haute Provence, France) with the 80-cm telescope there and displayed at his Hale-Bopp Website. There is also another one from February 28 [JPEG; 59k] by Russell Sipe (Anaheim, California) showing the strong jet and a series from Francis Graham and his students (Kent State University, East Liverpool, Ohio) [ Feb. 18; JPEG 15k , Feb. 24; JPEG 24k and Feb. 26; JPEG 31k ]. Note especially that the one from February 24 also has a drawing of the comet's spectrum! That this was possible is another obvious proof of the comet's exceptional brightness.
Gary W. Kronk who maintains a very comprehensive Website with lots of interesting information about comets and other astronomical objects, has called my attention to his recent drawings. You may reach them via the main site or directly (at the bottom of the file).
Did you ever see one of the few 1997 images of Comet Hale-Bopp, obtained with a large Schmidt telescope? If not, here is one by Eric W. Elst , a well-known Belgian astronomer who studies asteroids and planets. This double image [GIF, 46k] was taken by him with the 90/62cm Schmidt telescope of Observatoire de Haute-Provence (France) on February 12. The two versions were processed with Photoshop 3.0 for PowerMacintosh, and are shown to illustrate the tail structure close to and far from the head. The ion tail is clearly visible on the right-hand side version. N is at the top and E to the left. The field of view shown is 1.6 x 2.0 degrees.
A group of students from the Technical High School og Amtsgymnasiet in Soenderborg (Denmark) continues to take images at the school observatory whenever the weather permits. After many days and nights of bad weather during the month of February, they managed to get this beautiful image under perfect sky conditions (180 second exposure on Fuji HG800 Press, just after 4 UT in the morning of March 5) by means of a 16-inch SCT Meade telescope. You may also wish to visit their Hale-Bopp site to learn more details and to see more of their images. They were very proud to get their image on the front page at the JPL site on March 6!
Interestingly, one day later another observer from a nearby Danish town, Bernd Schatzmann in AAbenraa, achieved the same honour with a colour photo obtained during a 480 second exposure on Fuji HG800 Press on March 5, 1997, with a 10-inch f4.8 Newton telescope.
Do also have a look at the newest images at the Crni Vrh Observatory in Slovenia (there is a beautiful true-colour one from February 28).
More about the jets
I have received a detailed message from F. Manzini and his colleagues (Istituto di Fisica dello Spazio Interplanetario del CNR di Frascati, Italy) concerning some Hale-Bopp images which they took on September 11 - 13, 1996, with a 33.5 reflector of SAS Observatory (Novara, Milano, Italy), equipped with a CCD camera without filter. They include a sum of 20 CCD pictures from September 13 , each of 15 second exposure and the same image in an isophote version in which it is easier to see a bright spot inside the jet at p.a. = 310 o. More images were obtained on Feb. 9 - 22, 1997; as an example here is a sum of 10 CCD pictures on February 9 , each lasting 5 seconds; it shows the same pattern of multiple shells, as first observed on February 4 on Pic-du-Midi pictures.
They remark that 1) Both bursts (or bright `blobs') we observed (on Sept. 11 and Feb. 9), evolved from a single jet . This seems to us to speak against the Sekanina hypothesis about the origin of multiple Hale-Bopp jets (as reported in IAU Circular 6542 and in your Jan. 8 UPDATE, every double, symmetrical jet should arise from an emission cone of dust and gas produced by near-polar active areas continuously in sunlight). Really, if Sekanina is right, a burst inside a jet must produce a counterpart of it also inside a second, symmetrical jet. But both bursts we observed on both Sep 11, 1996 and Feb. 9, 1997, are located only in a single position! 2) The multiple shell systems are astonishing and difficult to explain, above all because they show different geometrical centers.
You will find more information and images from this Italian group at http://gwtradate.tread.it/tradate/gat/relazion/halebopp.htm.
New orbit and ephemeris
Don Yeomans has issued another set of orbital elements and ephemeris. The orbital periods remain unchanged, 4200 years before the current apparition and 2380 years thereafter. The change is mainly due to the influence of Jupiter during an approach to this planet to within 0.77 AU (115 million km) in April 1996.
About these Updates
The main idea behind these Updates has been to provide all interested parties, also those who are not so close to cometary astronomy, with frequent overviews of important, recent activities around Hale-Bopp. Although I do attempt to keep much of the text relatively simple and straightforward, it is difficult to avoid that some sections end up being more technical than others. I also try to bring here and there some additional `background' information that may serve to illustrate some of the many aspects of this kind of astronomical studies.
For your information: according to the ESO Web statistics available at the time of the release of this Comet Hale-Bopp Update, the one issued on February 7 has so far been accessed approximately 24,000 times and that from February 26 about 13,000 times. Back to ESO Hale-Bopp Homepage