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Comet Hale-Bopp (February 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 January 28, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.
Richard M. West (ESO)
Munich, February 7, 1997; 18:00 UT
There have been many new developments around Comet Hale-Bopp during the past days and this Update - the 10th since the first of this series was issued in December 1995 - contains much information of interest to observers and other friends of cometary astronomy.
Hale-Bopp brightens further
Observations of Comet Hale-Bopp are now being received in steeply rising numbers and it is obvious that the media interest is also growing at a high rate. During the past few days and now with no interfering moonlight, it has for the first time be possible to enjoy it in all its splendor against the backdrop of a perfectly dark sky. I was myself fortunate to have such an opportunity yesterday morning (February 6), when it was visible above Altair in the Eastern sky above Munich. Like other naked-eye observers I estimated the magnitude as somewhat brighter than magnitude 2.
The latest report available on the Web at this moment is an observation by Dan Green (Editor of the International Comet Quarterly (ICQ) and working at the IAU Central Bureau for Astronomical Telegrams ) at the Smithsonian Astrophysical Observatory (Cambridge near Boston, USA); this morning he estimated a magnitude of 1.6.
A new, very useful site for news about the comet's lightcurve has just been set up by Mark Kidger (IAC, Canary Islands, Spain). Three light curves are given: one with Spanish data (not up to date), one with all magnitudes that he has available since last March (updated daily, in principle) and one showing just the last 100 days before perihelion.
The coma diameter has been reported as almost 1 degree, but this depends on the technique used - it will of course appear largest of the deepest (longest) exposures. A prominent tail is developing; on some reports on the Web, it is estimated at up to 5-7 degrees. Claims of 20 degrees would appear to be somewhat optimistic at this moment, but there is good hope that tail lengths of this order and perhaps even longer will be easily visible at a later stage.
Some images of the tail structure clearly show the presence of a strong ion tail, the result of the interaction between the solar wind and the charged particles (ions) in the cometary coma. On at least one of these pictures, an impressive image [JPEG, 60k] obtained on February 4, 1997 at the Crni Vrh Observatory in Slovenia, a strong kink , or perhaps a disconnection event , is visible. Such tail features appear when the comet crosses magnetic boundaries in interplanetary space, resulting in a disruption of the plasma in the tail.
There can now be no doubt that Hale-Bopp is on the verge to become a Great Comet . See below for more information about this term.
Very complex coma structure
As is the case for the tail, the past days have also seen the development of a complex structure in the coma. While dust jets have been visible since a few weeks after the discovery, these structures have now reached an almost unprecedented degree of complexity.
The first indications of this appear to have come from a group of American astronomers who reported the appearance of changing structure in jets of Comet Hale-Bopp from CCD images obtained with two telescopes (the 1.2-m Whipple Observatory telescope and the 0.36-m telescope at the Goodricke-Pigott Observatory) on February 1 and 3 (IAU Circular 6555; February 5, 1997): Bright linear jets are seen at p.a. 25, 166, and 208 deg and fainter ones at 91 and 306 deg. An additional bright but curved jet is seen close to and south of the central condensation on both days with different orientations... (the) southern jets contain at least five periodic brightness enhancements 8 arcseconds apart extending along each jet. Each enhancement in both of these jets has curved tail-like extensions fading towards the approximate anti-solar direction.
Perhaps the most impressive example of recent coma (i.e. near-nucleus) observations with a large telescope is a computer-processed version of a CCD image [JPEG,42k] , obtained at the Pic du Midi Observatory (France) through an optical filtre that isolates the reflected sunlight from dust in the coma. This exceedingly intricate structure has not yet been satifactorily explained, but it indicates an apparently `pulsed' emission of dust from the nucleus. This may well be connected to the rotation of this body (because more dust is released when an `active region' on the surface of the nucleus is in sunlight), but this interpretation is not yet securely established. More information about these exciting observations and a full explanation with other images from Pic du Midi is available in a special file at the JPL Hale-Bopp site.
New, detailed coma images have also just become available from the Wendelstein Observatory (Bavaria, Germany) ( UT 5:18 February 7, 1997 [GIF,41k] and UT 5:20 February 7, 1997 [GIF,40k] ). Hermann Boehnhardt (University of Munich, Germany) provides the following information about them: These two R filter exposures of Comet Hale-Bopp were taken with the MONICA camera at the the 80cm Mt. Wendelstein Observatory of the University of Munich, Germany, and processed by Laplace filtering with box width 15 pixels. North is up and east to the left. The field of view is about 260 x 260 arcsec. A strong and slightly curved jet with variable morphology appears at position angle (PA) 25 deg, a much fainter jet points towards PA = 320 deg. In the south-western quadrant, a rather complicate wavy pattern with 7 - 8 shell-like wavefronts and embedded jets and knots may indicate the daily motion of the dust outflow from one (or maybe several) rotating active region(s) on the surface of the nucleus. The projected distance between the wavefronts in this fan-shape geometry is about 15000 km. Another much fainter wave pattern "flows" into the south-east quadrant and may even partially overlap with the former one. Observer: C. Ries; Processing: H. Barwig, H. Boehnhardt, A. Fiedler and W. Mitsch.
Note also the recent image obtained with the William Herschel Telescope at La Palma and processed at the Instituto de Astrofisica de Canarias ( [GIF,128k] ).
A very fine drawing [JPEG, 120k] , was made in the morning of February 4 by H. Fukushima. It shows intricate structure in the coma; see also the Website of the National Astronomical Observatory (Japan).
Also on IAU Circular 6555, a group of European astronomers report the detection of Comet Hale-Bopp in the continuum at 250 GHz with the IRAM 30-m radio telescope on Pico Veleta (Spain)..... Multiple scans through the comet give no indication for a significant beam broadening. Under the assumption of a nuclear diameter of order 30 km, more than 90 percent of the observed signal must come from the (ice) particle halo .
More about the ortho-to-para ratio (OPR) for H 2 CO
Maria Womack and colleagues have commented on the remarks by D. Bockelee-Morvan and colleagues in the Meudon groupabout their observations reported in IAU Circular 6542; see also the Update (January 08, 1997). They write that we acknowledge... that the ratio can be determined more accurately when numerous lines of ortho- and para- states are observed. However, we do not agree that it is impossible to measure the ortho-to-para ratio (OPR) for H 2 CO using only two lines. Further, although a gas temperature of 45K is indeed a good choice, this changes the OPR by a very small amount-- less than 10 percent relative to the results quoted in IAUC 6542. We thank the Meudon group for their interest in our IAUC communication .
With the increasing certainty that Comet Hale-Bopp will become a truly Great Comet, meetings about this exceptional object are being announced in several places.
A major professional meeting, the First International Conference on Comet Hale-Bopp has just been announced by the Comet Hale-Bopp European Team. It will take place in February 1998 on the Canary Islands.
In just a few days' time, the 3rd Meeting of (not just) European Planetary and Cometary Observers will take place in Violau (Bavaria, Germany).
More Hale-Bopp Homepages
Here are two homepages you may like to visit for interesting information about Comet Hale-Bopp: BAA Comet section home page
and Universidad de Sonora at Hermosillo (Mexico).
As Comet Hale-Bopp moves towards its perihelion, its velocity slowly increases with the increasing gravitational pull of the Sun. However, how does it move, relative to the Earth? This is one of the questions, I have recently received by email from visitors to this site.
The information is already available on the Web, in the area where observers' ephemerides are posted. These give the comet's position in the sky at a given time, and if you have a look at the most recent ephemeris by Don Yeomans , you will also find in one column (marked Deldot ; this refers to the first derivative of the distance, Delta , between the comet and the Earth) the so-called radial velocity , that is the speed (in km/sec) with which the comet is approaching (negative) or receding (positive) from the Earth. The current value of course depends on the comet's position and velocity in its orbit, as well as the position and motion of the Earth in its orbit around the Sun.
At this moment, this velocity is quite negative, about -35 km/sec, meaning that Comet Hale-Bopp is approaching the Earth with that speed. If you look further down the table, you will find that it diminishes steadily until March 23 when it becomes 0 km/sec. Thereafter the comet starts to move away from us with increasing speed, reaching about +30 km/sec in early May.
Great Comets in history
Comet Hale-Bopp has sometimes been referred to as the Great Comet of 1997 (albeit so far most often with a question mark!), implying that it will be an unusually bright and impressive object. But what is the definition of a Great Comet really?
There is no rigorous definition in use by professional astronomers for this, but in a 1989 article ( Cometary Apparitions: Great Comets in History for the `Reference Encyclopedia of Astronomy and Astrophysics'), Don Yeomans of the Jet Propulsion Laboratory (Pasadena, USA) has discussed this question and prepared a list of what he considers to be Great Comets in the recorded history of astronomy.
He considers that in order to be included in this rare class, a comet should preferably achieve magnitude 0 when seen in a dark sky and it must in any case possess an impressive tail. These criteria may exclude some comets which were very bright when they were extremely close to the Sun and were therefore never seen in a completely dark sky. Nevertheless, Don Yeomans lists a total of 82 Great Comets from 373 BC to 1976 AD, corresponding to approximately 4 per century. More information about the individual comets is available in his now famous book about comets ( `Comets' , John Wiley and Sons, 1991).
Including the fine comet Hyakutake that appeared last year, the are now seven and, with Comet Hale-Bopp, most probably eight Great Comets in the 20th century. It seems that we are living at the right time!
Great Comets in the 20th century
|Year ||Name ||Max. Brightness in a dark sky |
|1910 ||Great January Comet ||1-2 |
|1910 ||Halley ||0-1 |
|1927 ||Skjellerup-Maristany ||1 |
|1965 ||Ikeya-Seki ||2 |
|1970 ||Bennett ||0-1 |
|1976 ||West ||0 |
|1996 ||Hyakutake ||0 |
|1997 ||Hale-Bopp ||? |
The bright comet of 1811 which has shown similarities to Comet Hale-Bopp reached magnitude 0 on 20 October of that year and is also included in Don Yeomans' list.
There is now good hope that Comet Hale-Bopp will reach magnitude 0 or brighter and its visibility is such that it will definitely be seen in a dark sky around the time of maximum brightness. Thus, it would be very strange if it would not qualify as a Great Comet .
Post-perihelion observations of Comet Hale-Bopp
As stated earlier, Comet Hale-Bopp's orbit will change somewhat during the present passage in the sense that the orbital period will decrease from approximately 4200 years to just under 2400 years. This effect is due to the gravitational attraction by the planets and is typical for long-period comets.
This also means that in some years, when the Comet is sufficiently far out in the solar system that the attraction from the individual planets plays a minor role, its orbit will have become an extremely elongated ellipse with a semi-major axis of 180 Astronomical Units (AU) or approximately 27,000 million km. The aphelion , the point in the orbit farthest from the Sun, will be reached around the year 3200. At that time, Comet Hale-Bopp will be no less than 360 AU (54,000 million km) from the Sun, or 9 times farther out than the mean distance of planet Pluto.
The Comet will move outwards in this orbit with steadily decreasing velocity. Just over two decades from now, in the beginning of the year 2020, it will be 43 AU from the Sun and, assuming a diameter of approximately 20 km, the apparent magnitude of the nucleus will be around 29-30 (2,500 million times fainter than what may be perceived with the unaided eye).
It is interesting to note that the observation of an object so faint will in principle present no unsurmountable problem for observers at the new generation of large telescopes. For instance, one of the 8.2-m unit telescopes of the ESO Very Large Telescope (VLT) would be able to secure an image of Comet Hale-Bopp's nucleus at this distance during an exposure lasting less than 1 hour. The main problem of such an observation is that there will be many other astronomical objects at these faint magnitudes (stars, galaxies) which are recorded in the same frame. In fact, the nucleus may only be identifiable by means of a careful comparison of two frames taken some hours apart, because of its motion in the meantime.
Comet Hale-Bopp's orbit is nearly perpendicular to the main plane of the Earth's orbit (the Ecliptic). When, after the year 2020, this comet continues outwards through the outer regions of the solar system, it will be located far below this plane and in the sky it will be placed near the southern celestial pole in the constellation of Octans at around R.A. = 21 hours and Decl. = -82 deg (2000 coordinates). This corresponds to galactic latitude -31 deg, ensuring that future observations will not be greatly disturbed by galactic stars which are not so numerous at this latitude.
However, when, in the year 2050, Comet Hale-Bopp has reached a heliocentric distance of 75 AU, and the nuclear magnitude will be only around 32, the observers will most certainly have serious problems because of the interference (overlapping) of a very large number of images of faint and distant galaxies. Back to ESO Hale-Bopp Homepage