Comet Hale-Bopp (May 16, 1997)

This is a summary of recent developments around this comet; the previous was published on the ESO Web on April 30, 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, May 16, 1997 

Introduction

This Update is published at a time when Comet Hale-Bopp is moving very close to the Sun in the sky and is no longer visible in a dark sky. Optical observations have now become very difficult, indeed almost impossible, also with small amateur instruments. However, interesting news frequently appear as data from the earlier observations are reduced and radio observations still continue in some places.

Should particularly important news become available, it will be placed as Latest News at the ESO Comet Hale-Bopp Homepage. Otherwise, it is planned that, until further notice, future Updates will appear about once every two weeks.

1. Current state

During its southward motion, Comet Hale-Bopp crossed the Earth's orbital plane (Ecliptica) on May 5. This happened at a distance of approximately 15 million km from the Earth's orbit, at the sector where our planet is located in January.

At midnight (UT) between Friday (May 16) and Saturday (May 17), it will be situated at a distance of 2.04 AU (305 million km) from the Earth and 1.22 AU (182 million km) from the Sun. It will be located in the constellation of Taurus, and it will be moving in a direction towards Betelgeuse, the bright star in Orion's shoulder, at a rate of about 0.75^o/day. The tail system points in the general direction of West and continues to swing southwards at a rate of about 1.6^o/day. The comet is moving away from the Earth and the Sun at speeds of 28 km/sec and 19 km/sec, respectively, while the orbital speed has decreased to 40 km/sec.

The mean of ten magnitude estimates by visual observers from May 10-12, 1997, is +0.9 +- 0.4. Several observers have commented on the more rapidly decreasing brightness. The coma diameter is down to about 10 arcmin (but the moonlight interferes with such measurements), and the estimated length of the dust tail is now 1^o - 2^o.

2. The visibility of Hale-Bopp and other comets

One week ago, Mark Kidger (IAC, La Laguna, Spain) issued another report about the comet's development. He writes, among others: The best part of the big show is now over and the comet is very definitely fainter than magnitude 0 and dipping deeper and deeper into twilight.

As near as can be estimated from the light curve the magnitude finally faded below 0 on April 26th, although there were still some estimations being made at negative magnitude in early May. This gives a final total period as a negative magnitude object of around 50 days (7 weeks), slightly less than we might have hoped, but still really exceptional.

Hale-Bopp is made more exceptional by the fact that most of the time when it was brightest it was visible in a dark sky after the end of twilight. Only a very few comets over the last few centuries have ever been magnitude zero or brighter in a dark sky and I can find no really clear-cut case where the observations (not the theoretical light curve) seem to imply that a comet was significantly brighter than Hale-Bopp in a dark sky.... The best estimate is that maximum was about 2-3 days before perihelion, in other words, around March 29th-30th.....The fade in brightness post-perihelion is rather slower than the pre-perihelion rise....

In the same connection, Jacques Sauval (Observatoire Royal de Belgique, Brussels) has prepared a list that summarizes the Longest Visibility of Ancient Comets. It also contains information about the 13 `absolutely brightest' comets observed in historical times. As will be seen, Comet Hale-Bopp ranges very high on both lists!

Several visitors to this site have asked about the visibility of the comet during the next months. As it moves closer to the Sun in the sky and becomes fainter, observations with the unaided eye become increasingly difficult; this is particularly true for observers in the North, i.e. the `summer' hemisphere. The angular distance to the Sun is now about 26^o, it will continue to decrease until early June when it passes a minimum around 21.5^o. Thereafter, the distance again increases, reaching 30^o in mid-July. At that time, the celestial declination will be negative and observations are only possible from the southern hemisphere.

The website of the Astronomical Society of South Australia continues to be an excellent place to get information about the comet's visibility from the south.

Likewise, the regular News Bulletin, published every Friday by Sky and Telescope, may continue to carry relevant information for Hale-Bopp observers for some time still.

3. Sodium in the tails

On May 13, 1997, a Press Release was issued by the Boston University, informing that some of their astronomers have released a composite photograph showing three distinct tails extending from comet Hale-Bopp. These images can be found on the World Wide Web in colour at http://vega.bu.edu/cometc.gif and in Black/White at http://vega.bu.edu/cometbw.gif.

The Release further states that this photo confirms a discovery made in April by European astronomers working at a Canary Islands observatory: the team found that the position of Hale-Bopp's sodium gas tail was different from the ion and dust tails associated with most comets.

"Our observations had been made in March at the McDonald Observatory in Fort Davis, Texas," says Jody Wilson, post-doctoral research associate, Boston University's Center for Space Physics (CSP). "We were in the process of analyzing our data in early April when reports came from the Canary Islands about the new tail. It then took just a day's work to get our results."

The telescopes used to capture such tail structures are very modest in size, according to Jeffrey Baumgardner, senior research associate, CSP. "The goal is to photograph a large portion of the sky, not the fine detail that would come from a large telescope," he says, noting that the team's instrument's main lens is only four inches in diameter. "The technological sophistication lies in the detector system at the end of the telescope. The light signal from sodium gas is very faint and not visible to the naked eye, but easily detected with our camera," adds Baumgardner....

"The point is that the position of this sodium tail and its pattern of brightness away from the nucleus are very different from the normal comet tails," says Michael Mendillo, professor of astronomy, Boston University. "This implies that the source of sodium is not necessarily on the nucleus, but extended from it....."

I think that it is interesting to note that on these images, the Na-tail is situated between the ion- and dust-tails, i.e. not to the left of the ion tail, as was the case at the time of the discovery in mid-April by the European group. It is not known at this moment, whether this may simply be a projection effect or whether these new images rather show the emission from Na-atoms released from dust particles in the dust tail, a phenomenon also reported by the European group on IAU Circular 6638 (April 28), cf. April 30 Update.

Six low-dispersion spectra of Comet Hale-Bopp were recorded with a small telescope located at the European Southern Observatory Headquarters in the vicinity of Munich during the period 9 March to 24 April, 1997. The observers (and makers of this facility) were Gerardo Avila, Jesus Rodriguez, Carlos Guirao (amateurs from ESO) and Juan M. Alcala (Max-Planck Institute for Extraterrestrial Physics) with a home-made spectrograph, attached to an amateur telescope with fibre optics. Sodium emission was observed in April. A full account of this interesting work is presented here, together with selected spectral tracings.

Observations in April by Bob Yen in the Mojave Desert (cf. his website at http://www.comet-track.com/hb/hb.html ) appear to show the straight, new sodium tail. Two of these images are available here (please respect Bob Yen's copyright!). The first [GIF; 182k] was obtained on April 15 (note that the date indicated on the image is incorrect), and the second image [GIF; 168k] is from April 18, 1997. A closer scrutiny, including an accurate measurement of the position angles of the various tail features seen on these unfiltered pictures, may be necessary in order to be sure that it is indeed the new tail. If this is confirmed, it would be the first time that a tail of this new type has been photographed with amateur equipment.

4. Plasma observations

Till Credner (Max-Planck-Institute for Aeronomy, Katlenburg-Lindau, Germany) has sent some information about his and Klaus Jockers' recent plasma observations at the Terskol Observatory in Caucasus. He has prepared one OH⁺ observation together with some more information and has placed it on the web image gallery, which he still maintains at the University Bonn. In order to see the brightest and faintest structures simultaneously, this image has been processed with an unsharp mask and logarithmically scaled.

More information about these observations and the Terskol Observatory, the high-altitude field station of the Main Astronomical Observatory (Kiev, Ukraine).

Recently, observers have been told to watch out for rapid developments in Comet Hale-Bopp's ion tail, as it continues to move towards lower solar latitudes, see for instance the position indicated on the graphical representation for April 16 by NASA, published by Sky and Telescope. It is also available as an animated GIF-file [132k] that shows how the comet moves down towards the Sun's equatorial plane (orange line), crossing it on May 3rd. North is up in this side-view schematic diagram.

It is reported that Steve Larson (University of Arizona) believes to have observed a `disconnection' event in the ion tail on May 6.

At the same time, new information has become available about spacecraft watching for disruptions of Comet Hale-Bopp's tail.

Just a few days ago, I received an email from John Lindquist - lindquis@bact.wisc.edu (Department of Bacteriology, University of Wisconsin-Madison, USA) who writes: ....My own humble efforts at comet photography are presently at http://www.bact.wisc.edu/lindquistjohn/jlmp1 as a part of my work-related website. Regarding the tail fluctuations expected for Hale-Bopp, I think I may have seen something relevant late Friday, May 9. Just after sunset, while scanning the sky to the `lower right' of the crescent moon with binoculars, I saw a bright object for about a second or two - like a small luminous backslash (\). I didn't see any definite indication of the comet till dark, after a number of stars became visible. Then, while photographing it with my 70-210mm telephoto lens around 9PM (CDT), the tail appeared to flare up for about a second or two. Perhaps it was a local atmospheric disturbance, but I was wondering if such dramatic observations are expected or have been reported. I don't know if I got any of the latter observation on film, as I won't get my photos back till Monday noon. Anyway, I thought I'd just check in with this amateur observation.

It would be interesting if anybody has performed observations at about the same time and may be able to confirm this observation. If so, please contact John Lindquist at the above email address and let me know also.

5. The dust

Olivier Lardiere (Haute-Provence Observatory, France) writes that J.C. Merlin, S. Garro and I, have recently observed the comet Hale-Bopp with the 0.8m-telescope at that observatory. On May 8, we have noticed some rapid changes in the inner coma. Now, the coma structure looks like the February structure. Dust shells become less visible, whereas new radial jets are emerging around the nucleus.

Moreover, on May 8, 1997, a huge and bright concentration of dust was rapidly ejected from the nucleus toward the anti-solar direction. One day after the ejection, the dust spreads on over 50,000 km from the nucleus. This dust ejection does not appear at each nucleus rotation, so it's certainly a unique phenomenon. It's certainly our last Hale-Bopp observations!

You may see these observations on their Hale-Bopp page at URL: http://www.obs-hp.fr/~lardiere/e_halebopp.htm.

What appears to be the same phenomenon is also reported by Mark Kidger (IAC, La Laguna, Spain). He writes, among others, that Pablo Santos has just been showing me images from the Mons Telescope [at the Teide observatory on Tenerife] from May 8th and 9th. There appears to be a bright shell of material to the north of the comet seen in the exposures on May 8.91, probably expelled from the nucleus around May 8.8. This structure is seen as a faint and quite distant shell on May 9.91, but with the same distinctive concave shape....Nothing unusual is seen in the field on either the 7th or 10th.

Referring to the strange feature seen in the dust tail by Peter Schlatter (Wohlen near Berne, Switzerland), the well-known expert on cometary dust, Marco Fulle (Astronomical Observatory, Trieste, Italy), has kindly provided this explanatory comment:

Every dust tail can be considered as the superposition of many synchronic tubes of dust. A synchronic tube is defined by the dust ejection time from the inner coma, and its width is given by the ejection velocity times the elapsed time between ejection and observation. It may happen, for a given time dependence of the dust ejection velocities, that the width of older synchronic tubes is smaller than the width of younger synchronic tubes. The axis of each synchronic tube is approximately a radial line pointing away from the nucleus. The older the synchronic tube, the more distant from the antisun direction is the corresponding axis. Thus it may happen that, an older tube appears to `emerge' from a younger one, and the edges of the two tubes build up a kink , as the one observed by Peter Schlatter, at the intersection of the two edges.

On IAU Circular 6645 (May 7, 1997), R. W. Russell and collaborators at The Aerospace Corporation, report a clear detection of features at 11.8 and 11.25 microns in the high signal-to-noise BASS spectra of comet C/1995 O1 in 1996 October and November, similar to features seen in the spectra of comet 1P/Halley. Spectral-synthesis modeling results in the identification of both features with emission by crystalline olivine grains [eds. bold], adding strength to their earlier detection.... In contrast to the earlier work, this model uses laboratory emissivity data on celestial dust-analogue samples as inputs. The presence of all of the expected structure in the region 8-13 microns strengthens the identification with a crystalline component of olivine-like composition in this comet.

6. Orbital elements

You will find very useful information about the latest orbital elements at the website of the Central Bureau for Astronomical Telegrams.

7. X-rays from comets

Ever since the discovery of X-rays in Comet Hyakutake last year by the orbital observatory ROSAT , cf. the overview of cometary X-ray observations , scientists have tried to understand the mechanisms which cause this radiation.

In a recent report , published at the JPL Hale-Bopp site, a group of US scientists discuss an apparently promising scenario. It is based on a supercomputer simulation of Comet Hyakutake's interaction with the solar wind, performed at the NASA Goddard Space Flight Center, Greenbelt, Maryland, USA. The results, just published in the journal Science, match and explain March 27, 1996, observations of Comet Hyakutake by the ROSAT satellite. Here follow some excerpts from this announcement:

"Cometary X-rays present a potentially powerful new tool to monitor comet activity far from Earth, as well as the composition and flux of the solar wind," said co-author Dr. Tamas Gombosi of the University of Michigan, Ann Arbor. "By capturing these X-rays' detailed energy spectrum, it might be possible to monitor the propagation and evolution of spectacular solar wind phenomena, such as the coronal mass ejections seen this January and April."

About one percent of the solar wind, which flows from the Sun out past Pluto, is composed of minor ions: atoms (such as oxygen, carbon and neon) that have been nearly stripped of their electrons and thus have a high positive charge. Dr. Thomas Cravens of the University of Kansas theorizes that these minor ions steal electrons from neutral atoms and molecules of cometary origin. The electrons are first seized in excited states, traveling in the ions' outer orbitals. As the electrons fall to lower orbitals, Cravens' theory asserts that X-rays are emitted, in addition to other forms of radiation.....

Within this pattern, some electron orbital transitions emit distinct wavelengths of X-rays that can be measured. The computer simulation shows that the overall X-ray spectrum for Comet Hyakutake depends mainly on the solar wind composition, and not on the comet. Because of this independence, researchers can determine the relative size of the comet's atmosphere from the proximity of the brightest X-rays to the icy nucleus.

"In Hyakutake, the brightest X-ray region was 18,700 miles (30,000 kilometers) ahead of the comet, on the Sun side," said University of Michigan co-author Dr. Michael Combi. "If the comet has enough of an atmosphere, the solar wind minor ions recombine with electrons far from the nucleus. If the comet were producing less atmospheric gas, the place of maximum emission would be closer to the nucleus," Combi said.

The report is illustrated by a figure, showing the plasma flow field in the coma of Comet Hyakutake on 27 March 1996, as computed by the 3D magnetohydrodynamic model. A small pre-view version [GIF; 21k] is available here.

This theory will be tested on Comet Hale-Bopp, which is scheduled to be observed by Japan's ASCA X-ray satellite this September. Moreover, a regular proposal to observe comet Hale-Bopp with ROSAT after the perihelion passage has been accepted.

8. Spectroscopic observations

Emission from two more molecules in Comet Hale-Bopp has been detected. On May 5, 1997, R. Meier and colleagues announced observations of DCN (cyanic acid in which the hydrogen atom is replaced by the isotope deuterium) in comet C/1995 O1 with the James Clerk Maxwell Telescope at Mauna Kea on Apr. 27.9 UT (IAU Circular 6641): The 10-sigma detection of the DCN 5-4 transition at 362.046 GHz was taken together with a quasi-simultaneous measurement of the HCN 4-3 line.... Our preliminary analysis leads to an approximate DCN/HCN ratio on the order of 10^-3. At the time of the observations, the comet was passing in front of the Taurus dark clouds, and we have been careful to exclude the possibility of contamination by background sources. The detection of DCN is the first detection of cometary deuterium in a molecule other than water. The availability of a D/H ratio in both water (IAUC 6615) and a carbon-nitrogen-bearing molecule may provide fundamental information on the origin and evolution of comets.

Two days later, on May 7, 1997, P. Colom and collaborators reported on IAU Circular 6645 that on Apr. 5 UT, we detected the blend of J(21-20) lines of CH₃OCHO (methyl formate) in C/1995 O1 at 227.562 GHz with the IRAM 30-m telescope.... A preliminary estimate of the CH₃OCHO production rate is 6 x 10²⁷ molecules/s, which corresponds to an abundance relative to CH₃OH of about 3 percent.

9. Molecular update

With the new detections included, the updated list of observed molecules in Comet Hale-Bopp now looks as follows:

H₂O, HDO, OH, H₂O⁺, H₃O⁺
CO, CO₂, CO⁺, HCO⁺
H₂S, SO, SO₂, H₂CS, OCS, CS
CH₃OH, H₂CO, HCOOH, CH₃OCHO
HCN, DCN, CH₃CN, HNC, HC₃N, HNCO, CN, NH₃, NH₂, NH₂CHO, NH
CH₄, C₂H₂, C₂H₆, C₃, C₂
He, Na, K
O⁺

and the following isotopes:

H¹³CN, HC¹⁵N 
C³⁴S

Is the list complete? If not, kindly let me know by email to rwest@eso.org.

10. More images

The JPL Hale-Bopp site now encompasses over 4300 images.

Do take a look at Philippe Demoulin's home page in Belgium! Here is one of his fine photos [GIF; 38k].

Mark Vornhusen (Osnabrueck, Germany) has informed about his interesting Hale-Bopp Galerie and Stefan Beck has prepared a very informative site with Astrophotography with small telescopes. He mentions that he works with a professional photolab using the latest scanner, printer and laser techniques. He and his group produce prints they will soon present to the public by means of a travelling exhibition, for instance in banks, public buildings, etc. A most laudable initiative!

Felipe Mac-Auliffe who works at the Swedish-ESO Submillimetre Telescope (SEST) is also an amateur astronomer of great dedication. Some days ago, he obtained a lot of pictures of the Hale-Bopp comet from La Silla. In these pictures, he attempts to use terrestrial references like the ESO-Domes in order to show the real scale of the comet. He writes: There are some nice shots and, who knows, you might be interested to take a look of them. I've created GIF versions and will put them on my website wihout delay.

11. More Links

Here are some interesting links:

Andrew Yee calls attention to The Lunar and Planetary Institute (LPI) that recently placed 3-D images of solar system objects on their website. All images are displayed in anaglyph format, which means you need a pair of red-blue or red-green stereo glasses for viewing. Some images are displayed in parallel (side-by-side) format and you don't need any 3-D glasses to view them; your eyes simply fuse the stereo pairs into 3-D views.

There have been several further requests for information about meteors and their connection to comets. Try to look at the site of the International Meteor Organization (IMO) which was founded in 1988 and now has more than 250 members. It was created in response to an ever growing need for international cooperation of meteor amateur work.

Those readers who want a broader coverage of astronomy at the high-school to undergraduate university level, will find the Astronomy Unbound site in the U.K. a very useful and instructive one.

Georgi P. Gladyshev (Moscow, Russia) has prepared a webpage with references to a series of his own papers that discuss, among others, a possible mechanism for the formation of spatially-periodic structures of comet atmospheres.

12. More comets

No less than three new comets have been found during the first half of May 1997, and one more has been recovered. You will find full details at the corresponding webpages (the Headlines) at the Central Bureau for Astronomical Telegrams. Here are the individual links to the new ones:

• C/1997 H2 (SOHO). New discovery. IAUC 6650, 1997 May 9.
• C/1997 J2 (Meunier-Dupouy). New discovery. IAUC 6648, 1997 May 8.
• C/1997 J1 (Mueller). New discovery. IAUC 6642, 1997 May 5.
• P/1997 H1 (McNaught-Hughes). Recovery of P/1991 S1 (= 1991y = 1991 IX). IAUC 6640, 1997 Apr. 30.

On IAU Circular 6653 (May 5, 1997) the discovery of four Sungrazing Comets was announced on images obtained with the SOHO coronagraphs, with the following approximate discovery positions:

     1996 UT           R.A. (2000) Decl.
                  Aug. 20.933       9 34.9      +10 17       C/1996 Q2
                       30.073      10 14.2      + 7 49       C/1996 Q3
                  Sept.22.265      11 28.5      - 1 32       C/1996 S3
                  Dec. 22.490      18 12.9      -26 21       C/1996 Y1
             
             

Orbital computations by the Brian G. Marsden suggest that all these SOHO comets are of the socalled Kreutz sungrazer type, with perihelion times at Aug. 22.13, Aug. 30.87, Sept. 23.58 and Dec. 23.26, 1996, respectively.

On the same IAU Circular, Suyichi Nakano (Sumoto, Japan) reports that low-altitude CCD observations of Comet C/1996 J1 (Evans-Drinkwater) by J. Kobayashi on May 5.8 UT thatshow a bright condensation (`nucleus A') of m1 = 9.8 and a fainter condensation (`nucleus B') of m1 = 12.9, separated from the former by about 81 arcsec.... The comet's splitting was confirmed by A. Sugie... on May 9.8....

Moreover, new information about this comet which was observed at ESO at the time of its discovery last year, has become available on IAU Circular 6662 of May 15, 1997. Here,Zdenek Sekanina (Jet Propulsion Laboratory) writes that a standard analysis....of the reported separation of the two nuclei (IAUC 6653) confirms that nucleus B is the primary component and suggests tentatively the second half of July 1996 as the most probable time of splitting... The estimated uncertainty is several months, and the comet might have broken up even before discovery, although there seems no hope for detecting the secondary nucleus on the available 1996 images, which range from May 10 to July 18. Predicted separation distances and position angles for companion A relative to primary B: 1997 May 12, 83 arcsec, 253 deg; June 1, 88 arcsec, 258 deg; June 21, 94 arcsec, 262 deg; July 11, 101 arcsec, 266 deg.

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