Comet Hale-Bopp (April 30, 1997)

This is a summary of recent developments around this comet; the previous was published on the ESO Web on April 23, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

It is expected that future Updates will appear at less frequent intervals.

Richard M. West (ESO) 
Munich, April 30, 1997 


Introduction

This Update is published one month after the perihelion passage, at a time when this exceptional Comet is rapidly receding from the Earth, its brightness is diminishing and the visible tail lengths are becoming shorter. The main observational programmes are now over, but it is not excluded that the inspection of the large data sets that have been gathered at the world's observatories will result in the announcement of interesting news during the coming weeks.

Many of the news items presented below have already been displayed during the preceding week as Latest News at the ESO Comet Hale-Bopp Homepage. They are here re-edited and combined with other items of the same general category with the aim of providing a better overview of the current situation.

The information in this Update is presented under the following headings (click to jump):

1. Current state

2. Comet Hale-Bopp now visible from the South

3. Sodium in the tails

4. Motions in the ion tail expected

5. The shape of the dust tail

6. New orbital elements

7. Rotation of the nucleus

8. Spectroscopic observations

9. Molecular update

10. More images

11. Miscellaneous

1. Current state

Comet Hale-Bopp has just passed the 0-magnitude limit and is now estimated at +0.1 + -0.2 by visual observers. The visible tail lengths are getting shorter.

On April 25, Mark Kidger (IAC, Tenerife, Spain) has published an overview about the brightness development of Comet Hale-Bopp. In this, he points out the records now held by this comet, as compared to others, observed during this and earlier centuries.

Comet Hale-Bopp continues to move rapidly away from us; at midnight (UT) between April 30 and May 1, it is located in the constellation of Taurus (The Bull), some 10 o above of the bright star Aldebaran (The Bull's Eye) and to the left of the Pleiades , a well-known star cluster. It will then be about 265 million km (1.77 AU) from the Earth and 158 million km (1.06 AU)from the Sun. The rate of motion in the sky is now just over 1 o per day. Observations are now becoming more difficult, as the Comet is only 32 o from the Sun in the sky. Still, reports continue to be received, both from amateur and professional astronomers.

For instance, on April 27, John Bortle (Stormville, NY) describes Comet Hale-Bopp as follows: ion tail very faint but apparently up to 8 deg long... strongly-curved dust tail very faint after first few degrees but traced for 20 deg.....the tiny, non-stellar nucleus is very intense (surface brightness dozens of times greater that of immediately surrounding coma); from the western side of nucleus originates a fairly obvious spiral arc/envelope, subtending about 180 deg in p.a.; a much weaker envelope is exterior to the first and spans not more than 90 deg in p.a.; ion tail side of comet's central axis much brighter than corresponding dust side -- just the opposite of the situation in March and early April; no trace of the old `great dust streamer' in tail .

In the evening of April 25, Mark Kidger at the Teide Observatory on Tenerife (Spain) reported that he took integrations with the Carlos Sanchez telescope (TCS) and the CAIN infrared instrument in the CO band and K-continuum (wavelength near 2 microns). These frames were spectacular with the shells clearly visible in the raw CO frames. The next evening, simultaneous observations were made in the visible and infrared with the IAC-80 telescope and the TCS.

Nevertheless, professional observations must soon come to an end, as the visibility of the Comet in the evening sky now rapidly deteriorates.

2. Comet Hale-Bopp now visible from the South

Reports from observers in the Southern Hemisphere are now being received in increasing numbers. For instance, on April 24, Jose Rodriguez Freitas and his astronomy friends in Montevideo in Uruguay (latitude -35 o ) were finally able to see the comet (after a period of bad weather), just 10 o above the horizon, at 18:45 local time. Celina Raquel Cudiciotti, Damian Wacker and Adrian Paulo Arquiola of the Observatorio Astronomico Municipal de Funes (Argentina) have also reported a sighting with 10x50 bioculars, lasting until local time 22h35m, when it was only 1 o above the horizon!

Hale-Bopp has been seen by astronomers at the ESO La Silla observatory (latitude -29.5 o ) since April 23.

You will find more about the observing conditions in the regular News Bulletin , published every Friday by Sky and Telescope.

3. Sodium in the tails

Detailed information about the newly discovered sodium tail was brought in the April 23 Update. An image of this tail is available with the Press Release from the Isaac Newton Group. Since it would appear that it may not always be easy to get through to that site, a small version of the discovery image (JPEG; 29k) has been placed here. It shows the sodium tail on April 16, 1997 as a very straight narrow feature, extending from the head of the comet to the upper left. Since a few days, there is also a double image (JPEG; 176k) which allows a direct comparison of this sodium tail (left image) and the ion and dust tails (right image).

Further observations of the sodium tail have been announced by the NASA Team responsible for the POLAR satellite. Paul Feldman(Johns Hopkins University, Baltimore) and Michael Mumma (Goddard Space Flight Center) respectively state that: The detection of the neutral sodium tail in Hale-Bopp is a great discovery and the discovery provides a more complete picture of the physics in the comet .

The NASA communication continues: The new kind of tail is seen in images obtained with a special filter, designed to accept light from sodium atoms while blocking most other light, in POLAR's Visible Imaging System (VIS). The camera was developed under Professor Louis Frank at the University of Iowa, in Iowa City. The appearance of the comet in images obtained through the sodium filter is different from that seen through other filters in the camera, according to Frank's associate, Dr. John Sigwarth. Sigwarth reports that the new tail was about 500,000 miles (800,000 kilometers) wide as imaged by POLAR on March 31, when Comet Hale-Bopp was near its closest approach to the Sun. It was at least 7,500,000 miles (12 million kilometers) long, and probably longer, extending beyond the edge of the VIS camera field of view.

In contrast, when discovered at the Observatorio del Roque de los Muchachos, La Palma, Canary Islands, Spain by the European Hale-Bopp Team on April 16, the neutral sodium tail width had narrowed to about 410,000 miles (660,000 kilometers). (The new kind of comet tail was first found by the European astronomers and subsequently confirmed on images obtained about three weeks earlier and in process of analysis by the POLAR scientists.) Careful analysis of differences in observing methods and conditions will be necessary before scientists draw definitive conclusions from comparison of the ground based and POLAR satellite data.

POLAR's comet tail detection was made during the interval of March 27 to April 2, when Comet Hale-Bopp was too close to the direction of the Sun as seen from Earth to be observed with many conventional and orbiting telescopes. At that time, ground controllers at the Goddard, commanded POLAR to point its complement of three camera systems off its usual target, the Earth, to record the unique spectacle of Hale-Bopp's emissions in a variety of wavelengths (colors of light) of special scientific interest. (Normally, the POLAR cameras focus on the Earth's northern and southern auroral zones, while other onboard instruments measure particles, fields, and wave phenomena in the Earth's magnetosphere.) Because two of the POLAR cameras are designed to observe faint visible- and ultraviolet-light emissions of the Earth's atmosphere in close proximity to the bright day side of the Earth, they are also capable of looking closer to the Sun in the sky than most conventional instruments.

An image that accompanies the NASA press release can be obtained from the World Wide Web at URL: http://www-istp.gsfc.nasa.gov/istp/halebopp/visna_new.gif.

A provisional interpretation of the La Palma sodium tail data, as published by the European Hale-Bopp Team on two recent IAU Circulars, has appeared on IAU Circular 6636 (April 25). Here, Zdenek Sekanina (Jet Propulsion Laboratory) reports, among others: Available information on the sodium tail (IAUC 6631, 6634), combined with the photoionization lifetime of 0.56 day for sodium atoms at r = 0.97 AU from the sun.... shows that sodium atoms could not be released directly from the nucleus or from dust grains of any size (visible or invisible). The first option is eliminated because of the short destruction lifetime, the second - in addition - because the accelerations on all particulates in comets are beta_dust < 3, more than one order of magnitude lower than required. Thus, a sodium tail of this kind would in the best case trace the leading boundary of the dust tail. The most probable parents consistent with observation are sodium-bearing molecular species with relatively long dissociation lifetimes, which could have been initially released from the comet in dust particles but separated soon after their ejection..... .

Three days later, Alan Fitzsimmons (Queen's University, Belfast) and Gabriele Cremonese (Astronomical Observatory, Padua) from the European Hale-Bopp Team report on further observations of the tails on IAU Circular 6638 (April 28): The 4.2-m William Herschel Telescope (+ Utrecht Echelle Spectrograph) at Observatorio del Roque de los Muchachos was used on Apr. 23.9 and 24.9 UT to obtain spectra with a 4.5 x 1.1 arcsec slit at various positions on the dust tail up to 2 deg from the nucleus. Broad sodium emission lines were seen at all locations, with a blue edge corresponding to the geocentric velocity of the comet and a width of typically about 50 km/s. These observations imply that dust grains are a major source of sodium in this comet .

Thus, less than two weeks after the exciting discovery of the `third cometary tail', much new information has already become available about the distribution of neutral sodium around Comet Hale-Bopp. In particular, it has now been established that neutral sodium atoms are released both in the coma (by as yet unidentified particles) and also from dust grains far out in the dust tail.

All of this provides important new insights into the physical and chemical processes in a comet. The involved astronomer teams are now working hard to prepare the first scientific reports and papers and we may thus expect more details to become available within the next few months.

4. Motions in the ion tail expected

In a recent Press Release, NASA and ESA scientists say that between 28 April and 15 May, Comet Hale-Bopp and especially its blue ion tail will be most susceptible to influences from the Sun's solar wind and magnetic field. Its ion tail is expected to begin showing fluctuations, kinks, and perhaps moving structures. Also, it is possible that a disconnection event might occur, when the ion tail breaks off from the comet's head and reforms a few hours later.....

The region of interplanetary magnetic field reversal is shaped like a wavy disk which rotates with the Sun once in 25 days. The complex solar wind is now confined to about 25 degrees from this wavy disk. Also, the tilt of the wavy disk is itself less than 15 degrees..... the region of complex wind will be sinking to the south as Hale-Bopp is approaching 15 degrees, so that this comet's ion tail will not likely be disturbed until it gets down to less than 10 degrees, soon after April 28. The first half of May should be the optimal time of disruptions in Hale-Bopp's ion tail....

The comet tail disruption watch world-wide web home page is located at http://www-istp.gsfc.nasa.gov/istp/halebopp/

5. The shape of the dust tail

On April 28, an email was received from Peter Schlatter (Wohlen near Berne, Switzerland) about a specific feature in the dust tail. He characterises himself as an `amateur astronomer with an education in physics' and writes, among others: On the attached picture [JPEG; 240k] , a false color rendering of Hale-Bopp is shown, each color representing a brightness interval. The least luminous parts of the tail are colored in red. An interesting feature is indicated with an arrow. It looks as if some tail particles are accelerated away from the comet's main axes (the faint, i.e. red, part of the tail is suddenly getting broader). But a real acceleration perpendicular to the main axes is very improbable (where does the force come from?). It rather looks like if a steam of faint particles is emerging from the main dust tail at an angle of about 20 deg.

If this would be the case, various questions would have to be answered, for instance: 1) why is the faint tail missing upstream of the arrowhead (or why is it emerging at the observed point); 2) what is the composition of the faint tail (particle density, particle size), and is it different from the main dust tail; 3) is this feature a consequence of the rotation of the nucleus; and 4) is there some sort of mass spectrometer action taking place due to the rotation of the nucleus and the various forces acting on the tail particles? I would appreciate it very much if you could give me some clues to an explanation of this effect.

He also attaches a rough sketch showing a crude, possible explanation for the observed kink at the indicated position of the tail, and he gives some technical data of the picture: 12 March 1997, 03:30 UT; Gurnigel (near Berne); 2 min exposure on Fuji G 800 PLUS with a Canon FD 200 mm 1:4.0.

The observed feature can most certainly be explained by the usual model of dust emission from the comet's nucleus that includes the time and amount of ejected dust and the distribution of grain sizes, as well as the effects of the motion induced by the pressure on these grains by the sunlight and the geometrical projection. However, the shapes of the resulting dust structures can only be calculated by means of advanced computer codes. I would hope to persuade one of the specialists in this field to do such a computation in an attempt to reproduce the observed shape, especially since this may serve as a very useful illustration of the corresponding research area. More about this later!

6. New orbital elements

New orbital elements have become available from the IAU Central Bureau for Astronomical Telegrams and Minor Planet Center Computer Service. You may also reach the site here.

7. Rotation of the nucleus

Mark Kidger (IAC, Tenerife, Spain) informs that a new comet movie (animation) has been prepared from recent observations with the Mons 51-cm telescope at the Teide observatory. A detailed explanation will help you to understand the way it was made and what it means. It shows the rotation of the jets very well (although it must be remembered that we are seeing how the position angle changes in 24 hours as each frame is the average of a one-hour run each night). This movie is available at the European Hale-Bopp Team page athttp://www.iac.es/comet/jet6mov/apr97.mpg (MPEG; 366 Kb). Due to the sometimes slow connection to the Canary Islands, this movie sequence has also been placed at the ESO Hale-Bopp site. Click here to enjoy it.

8. Spectroscopic observations

On IAU Circular 6638 (April 29), astronomers from the European Hale-Bopp team report the detection with the 4.2-m William Herschel Telescope at the Roche de los Muchachos observatory and the UES spectrograph of neutral potassium (K I; 769.9 nm) emission associated with the comet in spectra obtained on the nucleus and at a distance of 10 arcsec on Apr. 20.9.

9. Molecular update

Here is an updated list of molecules the detection of which - to the best of my knowledge - has so far been announced in IAU Circulars or in research papers:

H 2 O, HDO, OH, H 2 O + , H 3 O + 
CO, CO 2 , CO + , HCO + 
H 2 S, SO, SO 2 , H 2 CS, OCS, CS 
CH 3 OH, H 2 CO, HCOOH 
HCN, CH 3 CN, HNC, HC 3 N, HNCO, CN, NH 3 , NH 2 , NH 2 CHO, NH 
CH 4 , C 2 H 2 , C 2 H 6 , C 3 , C 2 
He, Na, K 
O + 

and the following isotopes:

H 13 CN, HC 15 N 
C 34 S 

Excellent high-dispersion spectra were obtained by the members of the European Hale-Bopp Team with the 4.2-m William Herschel and 2.5-m Isaac Newton telescopes at the Roque de los Muchachos observatory during the past weeks. They cover virtually all spectral regions from the atmospheric limit to near-infrared and show a wealth of spectral features. It is quite likely that a detailed analysis - in any case a very time consuming task - will reveal emissions from atoms and molecules, not yet in this list.

10. More images

New, beautiful images have become available at Bob Yen's site.

Fine images obtained in Wimbledon near London (UK) are placed at Jimi King's Hale-Bopp page. You may also wish to try GianLuigi Coccia's site , based in Avellino, a town near Naples in southern Italy.

New images of Comet Hale-Bopp have also become available at the Picture Gallery of the CNPq/LNA/Observatorio do Pico dos Dias(Brasopolis, Brazil).

And here is a link to Hale-Bopp pictures obtained by the astronauts on-board the Space Shuttle during the recent STS-83 mission.

11. Miscellaneous

Last month, a brief account about the study of comets (before 1950) was placed on the ESO Hale-Bopp site. It was adapted from a review of cometary astronomy, published in 1993. In response to many requests, a continuation, A Brief History of Comets II (1950 - 1993) , is now also available.

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