[ ESO ] Comet Hale-Bopp (March 25, 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 March 17, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

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 25, 1997; 20:00 UT



A brief introduction to this Update

The flow of exciting information about Comet Hale-Bopp is forever increasing and it has now become impossible to cover everything in a single report of reasonable length. As this comet is now approaching its perihelion - which will be reached on April 1 at about 03:19 UT in the morning - observations by professional and amateur astronomers have become so numerous that the present account can only include a limited selection of the most significant developments.

Many of the news items presented below have already been displayed during one of the preceding days as Latest News at the ESO Comet Hale-Bopp Homepage. However, 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.

In order to provide easier access to the individual sections, you will find the information in this Update under the following headings (click to jump):

1. Has Hale-Bopp reached its maximum brightness?

2. How to observe the comet

3. The tails

4. The comet's orbit

5. Solar eclipse on March 9, 1997

6. Lunar eclipse on March 23-24, 1997

7. Try Your Skills on Comet Hale-Bopp!

8. The size and mass of the nucleus

9. Rotation period of the nucleus confirmed

10. Did the nucleus split?

11. Structures in the coma

12. More molecules found

13. List of detected molecules

14. A rocket launch

15. More beautiful images

16. New Links

17. Hale-Bopp meeting at IAU General Assembly in Kyoto

The ESO Hale-Bopp web-area is now receiving about 50,000 hits per day and the rate is apparently still rising.

1. Has Hale-Bopp reached its maximum brightness?

Following a rapid brightness increase in mid-March, reaching about -0.6 around March 17, with most estimates between -0.3 and -0.8, some optimistic predictions of the maximum brightness went up to -2. However, more observations during the subsequent days indicated a slow-down of the rate of rise, and three days later, it was still at about -0.6 to -0.8.

At the present time, the magnitude estimates by experienced visual observers continue to be on the faint side of -1. Thus there is no longer any doubt that a levelling-off, as was to be expected, has taken place. Although the strong moonlight (full moon on March 24) has probably played a certain role - the comet looks fainter on a bright sky background - it is therefore not excluded that Hale-Bopp has by now (almost) reached its maximum brightness. But of course, you never know what may happen (this is the usual disclaimer)!

Some of these reports include descriptive information about the general appearance of the Comet. As an example, here is one by one of the most reliable observers, John Bortle (Stormville, NY, USA), filed with the International Comet Quarterly on March 24.38 UT (i.e. at 04:07 local time): Comet appears noticeably fainter than the previous evening, obviously an effect of relative altitude [height above horizon]; w/ 7x50 Binocular, no indication of ion tail at all!; dust tail extends 8.5 deg in position angle 342 deg; 41-cm L (70-114x) shows the three envelopes once again, their distances from the nucleus being approximately 75 arcsec, 55 arcsec, and 40 arcsec (thus little if any change from the previous evening).

Early this morning (March 25.0 UT), another very experienced observer, Dan W. E. Green (Cambridge, MA, USA), estimates magnitude -0.6 with the naked eye.

An unusual problem has arisen: the Comet is now so bright that it has become difficult to find appropriate comparison stars to judge the magnitude. Sirius (magnitude -1.5) is still brighter and may be used in the evening, while Jupiter (magnitude -2.0) is in the morning sky during March and Mars (magnitude -1.2) is also seen in the evening sky.

2. How to observe the comet

From now on, the Comet will be best visible in the evening after sunset as it moves higher in the north-western sky. For detailed information about viewing conditions, check the Sky & Telescope Hale-Bopp On-line Viewing Guide; a Sky Chart for late March with horizon is particularly useful. In any case, the Comet is now so prominent that if you look in the roughly correct direction, you cannot avoid seeing it!

Today, March 25, it has reached its northernmost position in the sky, at a declination just below +46o. Henceforth, it will begin to move southwards in the sky, crossing the celestial equator on June 26. At the same time, the distance in the sky from the Sun will decrease, so that the observing conditions will gradually deteriorate. Still, the observing conditions from the northern hemisphere will continue to be relatively fine, at least through April. Later that month it will also become observable by those southern observers who are now eagerly waiting to get a glimse of this splendid object.

Have you seen the animated finding chart for Comet Hale Bopp by the Soenderborg group? (Added on March 27: The line appears to have been completely overloaded, so we have installed a copy [GIF; 8k] of this file at the ESO site.)

3. The tails

Comet Hale-Bopp continues to display fine ion and dust tails. The first may soon become more `active' and interesting structures are now visible in the latter. The maximum lengths reported are of the order of 20o and 10o, respectively, but these estimates are likely to increase, now the comet will become visible in a dark sky, without interfering moonlight. The physical length of the ion tail now exceeds 100 million km.

On IAU Circular 6598 (March 21), Kurt Birkle (Max Planck Institute for Astronomy, Heidelberg) and his German colleagues report about the presence of at least 12 straight dust striae [which] appear almost identically on exposures with the Calar Alto Schmidt [located in southern Spain] on Mar. 17.185 (see http://caserv.caha.es/CAHA/halebopp.html) and 18.172 UT at a nuclear distance of 1.5 to 3 deg in the dust tail, the projected drift velocity of the pattern being about 15 km/s. The striae are 1 to 3 arcmin wide, and some of them extend over more than 1 deg across the tail. The average position angle, about 335 deg, of the striae indicates that the pattern does not follow the synchronic direction for direct emission from the nucleus. The striae may thus be considered as due to secondary processes influencing the dust grains in the outer coma or in the dust-tail region. The overall appearance resembles the striae patterns seen in comets C/1975 V1 (West) and probably also in comet C/1910 A1 (Great January Comet)....

News have been received about the activities of the Ulysses Comet Watch which is using data from instruments onboard the NASA/ESA Ulysses spacecraft that study the properties of the solar wind, i.e. the fast particles emitted by the Sun. In the last two years, Ulysses has investigated the steady, high-speed winds at high latitudes, and alternating, slow and fast winds near the Sun's equator. Hale-Bopp will soon enter the lower latitude zone, where the disturbed solar wind resides, and where dramatic changes in the comet's plasma tail are expected to occur.

The Ulysses Comet Watch group will provide worldwide observations of the comet as it descends from the polar regions of the Sun. Images from more than 200 observers around the world will be posted on the Ulysses Comet Watch home page (try also this address). Observations will continue to be posted well after the comet makes its closest approach to the Sun on April 1.

The Ulysses group is watching for changes in the comet's blueish plasma tail, which consists of ionized gas being emitted by the comet and picked up by the magnetic field being swept along by the solar wind. Cometary plasma tails change constantly due to their interactions with slow and high-speed solar wind. One of the more dramatic changes that can occur is the abrupt disconnection of the tail, which then drifts away. Often a new tail will form within hours. The comet, by acting as a `solar wind sock,' can be used to map the conditions in different latitudes of the solar wind.

4. The comet's orbit

Comet Hale-Bopp passed it closest point to the Earth on March 22, at a distance of approximately 197 million km. Perihelion is reached on April 1, when the distance to the Sun will be 137 million km. It then moves outwards again, `crossing' the Earth's orbit (at a solar distance of 149.6 million km) on April 23.

Information about the current orbital elements, that is, the parameters that determine the comet's orbit in space, may be found until further notice at the corresponding webpage at the IAU Central Bureau for Astronomical Telegrams (Brian Marsden).

Note also the comet-related classroom exercises, referred to below in Section 7. One of these involves use of some of these orbital elements.

5. Solar eclipse on March 9, 1997

An exhaustive account of the solar eclipse on March 9, based on observations in Siberia, has been placed on the Web by Daniel Fischer (Bonn/Koenigswinter, Germany). [NB! There appears to be problems to access that site with Netscape 3.01]. You will here find many interesting `exotic' links.

See also the report at Sky & Telescope and don't forget to visit the SOHO site! .

6. Lunar eclipse on March 23-24, 1997

A lunar eclipse took place in the morning (UT) of March 24. Reports about this event, including observations of Hale-Bopp, are expected soon, Keep an eye at, e.g., the Sky & Telescope Lunar Eclipse site.

7. Try Your Skills on Comet Hale-Bopp!

A number of comparatively simple comet-related classroom exercises have been prepared by the European Student Project Group of the European Association for Astronomy Education (EAAE) and placed within the Astronomy On-Line framework. They should be quite useful for 14-18 year old students, possibly also younger ones.

Astronomy On-Line was set up by the EAAE in 1996, under the auspices of the Fourth European Week for Scientific and Technological Culture. There was also a Collaborative Hale-Bopp project which was run via the Hale-Bopp Public Dark Sky Network (HBPN) in Austria.

8. The size and mass of the nucleus

In the last Update (March 17), information was provided about measurements which place an upper limit on the size of the nucleus (about 45 km diameter). The experience of many observations of `naked' cometary nuclei, when seen at large distances from the Sun, has shown that they are mostly smaller than expected and when asked, I usually quote 20 km diameter for Hale-Bopp.

Prompted by a call from the Munich Public Observatory (where a corresponding question was received by email), here are some considerations about the activity and mass of this nucleus:

Recently, a water production rate of about 250 tons per second was measured, that is more than 10 times higher than what was found at Comet Halley when it was most active. For Halley, the dust production was about 5 times less than the water production. Assuming the nucleus has the shape of a sphere with a diameter of 20 km (radius = 10 km), how long can Hale-Bopp carry on losing water at this profuse rate before there is nothing left of the nucleus?

The density of cometary nuclei is not well known, but many scientists assume that it is around 0.5 tons/m3, that is about half that of water ice, as we know it. At this density, and if it were made up of such (loose) water ice alone, a 20 km nucleus, with a volume of about 4200 km3, would have a mass of about 2.1 x 1012 tons. As 1 year = 3.16 x 107 seconds, the present water production rate could in theory be sustained during (only) 265 years.

However, the current, very high activity level only lasts a few weeks, and the comet's nucleus will therefore only lose a small fraction of its mass during the present passage. In case of Comet Halley, which has a nucleus that measures about 6 x 6 x 15 km, and has a volume of 550 +- 165 km3, more detailed calculations have shown that it will most probably be able to `survive' at least another 30 such passages, perhaps twice as many. (With a period of revolution of about 76 years, Halley should thus be with us at least another 2000 years.)

By the way, how can we measure the mass of a cometary nucleus? (question received today by email from Ole Melson and his high-school students at Alleroed Gymnasium in Denmark). It is in theory possible to deduce this mass from a complicated calculation, based on the jet-effect observed in some comets. Gas and dust is mostly emitted from that part of the surface of the nucleus where it is `afternoon' and therefore hottest (it is the same effect as on the Earth - the highest daily temperature at any site is normally measured during the early afternoon). For this reason, the nucleus is subjected to a certain force in the opposite direction. This is the so-called non-gravitational force and although it is relatively small, it is persistent and may result in changes in the comet's orbit which can be observed (this effect was first realised by the German astronomer Johann F. Encke in the 1820's during his study of the motion of the comet that now carries his name). If the size and direction of this force is estimated from the observed quantity (and distribution) of the emitted material, the mass of the nucleus can then be deduced from the observed changes in the orbit, albeit with great uncertainty.

9. Rotation period of the nucleus confirmed

J. Sarmecanic (University of California) and colleagues have imaged the Comet with the UCSD mid-infrared camera at the 1.5-m telescope on Mt. Lemmon, Arizona (IAUC 6600, March 21). The inner coma (40 000 km across) was continuously observed for 11.5 hr at four infrared wavebands on March 3-4 and 5-6. The inform that a bright jet, initially directed to the southwest, appears in all filters; over time, it rotates clockwise and reappears in the southwest 11.3 hr later, consistent with previously reported rotation periods of between 11.2 and 11.65 hr (IAUC 6560, 6583).

10. Did the nucleus split?

There have recently been rumours circulating about a possible splitting of the nucleus - in one report published on the Web, a photo from a medium-size telescope is displayed which purportedly shows six pieces. However, at least until this moment, there have been no confirming reports from major professional telescopes which have presumably obtained more detailed images in the meantime. It is therefore rather unlikely that the reported effect is real - it is most probably due to the presence of very complex structures in the coma and the relatively low angular resolution on the published image. Moreover, visual observers with good-quality optical instruments (and there are many of them now!) would undoubtedly have seen any splitting, had it really happened.

On the same subject, I received on March 20 an email from David Bridges who has ...an image [GIF; 30k] .... I believe is showing a fragment being sublimated as it falls behind the comet. It appears it did not take very long to sublimate below the visibility of my camera.... The film I used was Fuji SG+ 400. The grain size is small. I got the opinon of a PhD at the university I attend, that it is not a film defect. Let me know what you think. If you post the picture, please include a link to my page. The URL for my .... page that includes the images is: http://www.pbmo.net/suburb/dbridges/Fragment.html. I hope you find all this interesting....

While I cannot rule out that this photo (obtained on March 6) documents a real splitting event, I am quite sceptical, for several reasons. First, in view of the large number of images being obtained at any time, it would be strange if it had not been recorded by somebody else. Second, a fragment of the indicated size and brightness would not normally disappear within a few minutes, nor hours. Third, from my own experience, this very much looks like a `double' exposure, during which the image of the comet `jumped', possibly due to a guiding problem. On Mr. Bridges' website, trailed stellar images are visible on the photo, but they are probably too faint to have resulted in double images, if this is the correct explanation. Still it might be worthwhile to check that.

Whatever the cause, thanks anyhow for the message!

11. Structures in the coma

More observational results have become available on IAU Circular 6590 (March 17). C. M. Wright and colleagues report that images obtained in a broad-band 8-13-micron filter (N band) with the MPG/ESO 2.2-m telescope (+ mid-infrared camera MANIAC) on 1996 Oct. 28.8, 30.8, 31.8, and Nov. 1.8 UT show a clear fan-shaped structure extending about 30 arsec in length, with a symmetry axis pointing almost due east. The size and orientation appeared stable over all four nights, and there was no obvious jet activity.

Observations of Comet Hale-Bopp were carried out on the morning of March 19 by Mark Kidger and Jose Nicolas Gonzalez-Perez using the Brocam instrument on the 2.56-m Nordic Optical Telescope (NOT) on La Palma (Canary Islands, Spain). Some 30 frames were obtained in total in U, B, V and I filters. The raw frames showing surprising detail with two bright hoods in the sunward direction and many other structures visible. The raw images appear to show some differences between the continuum I-band images and the U-band which is largely dominated by CN emission.

An introductory presentation of infrared observations by the European Space Agency's (ESA) Infrared Space Observatory (ISO) has become available, cf. http://www.esrin.esa.it/htdocs/tidc/info/1997/9708.html for the text and http://www.estec.esa.nl/spdwww/iso/html/hale-bopp.htm for photos (this site has now been linked from here). Detailed results are due for publication at the end of March. They concern the composition of the comet's dust and vapour (gasses), and also rates of escape of vapour, which will help in assessing the loss of material from Comet Hale-Bopp.

Comet Hale-Bopp has been observed with the Steward Observatory 2.3-m telescope by Steve Larson and Carl Hergenrother. This splendid image [JPG; 26k] is a `quick-look version' of the full scale image that is available at http://www.lpl.arizona.edu/bss/HB0315CO.html. It was obtained in the light of CO+ on March 15.5 UT. The banded structure in this high-pass filtered version near the nucleus is due to dust, but the complex, wavy outer structure is caused by CO+ ions being driven back into the tail by the solar wind magnetic field. The ion features change on a timescale of minutes. North is up, east to the left, and the field is about 350,000 km at the comet. More information, also about other comets, may be found at the Comet Site of the Lunar and Planetary Laboratory (Tucson, Arizona).

On IAU Circular 6594 (March 19), Zdenek Sekanina (JPL, Pasadena) writes that excellent dust models have been obtained to match the diurnal evolution of the bright jet monitored by L. Jorda et al. on Feb. 28 (IAUC 6583) and the system of nearly-concentric halos observed by S. M. Larson and C. W. Hergenrother on Feb. 8. It is confirmed that the nucleus is not in a state of pure spin. The two features are ... from different sources on the nucleus... both being activated only between local sunrise and sunset.

12. More molecules found

More information about spectroscopic observations has been published, mostly on the IAU Circulars. Hale-Bopp is indeed providing a bonanza for these investigations and the list of detected molecules continues to lengthen!

A. Lovell and colleagues (IAU Circular 6590; March 17) have obtained maps (field size 4 arcmin x 4 arcmin; resolution 1 arcmin) during Mar. 11-13 with the FCRAO 14-m antenna (+ focal-plane array receiver) of the HCN J=1-0, CS J=2-1, and HCO+ J=1-0 transitions which show that HCN and CS emission peaks at the position of the nucleus, but the HCO+ emission is highly asymmetric (peaking at a point offset 1.5 arcmin from the nucleus in p.a. about 320 deg; anti-sunward direction was about p.a. 340 deg).

On IAU Circular 6591 (March 19), J. E. Wink at Institut de Radio Astronomie Millimetrique (IRAM; Grenoble, France) and collaborators from Observatoire de Paris, Observatoire de Bordeaux (France) and the Joint Astronomy Centre (Hilo, Hawaii, USA) report the detection of SO2 in Hale-Bopp on March 18 at the IRAM interferometer at Plateau de Bure (France), using its five antennae in a single-dish mode: The SO2 11(1,11)-10(0,10) rotational line at 221.965 GHz was detected... Assuming thermal excitation at 80 K, the derived SO2 production rate is about 6 x 1027 molecules/sec.... This is the first detection of SO2 in a comet, one of the sources of SO (Lis et al., IAUC 6573). Comparison of observations of SO ... and H2S... lines, made simultaneously in both single-dish and interferometric modes at IRAM on Mar. 13.3, shows that the bulk of SO does not come directly from the nucleus, in contrast to H2S.

On the the same IAU Circular, G. Narayanan (Steward Observatory, Arizona) and colleagues report observations of HCO+ and CO with the 10-m Heinrich Hertz Telescope on Mar. 5 and 9 and find the following production rates: CO, 1.7 x 1029 molecules/s; HCO+, 7.0 x 1026 molecules/sec.

On IAU Circular 6599 (March 21), A. J. Apponi and colleagues from Steward Observatory and Arizona State University, report the supposedly first detection of methyl cyanide in this comet: On Mar. 20 the K = 0, 1, 2, 3 and 6 components of the J=8-7 transition of CH3CN near 147.1 GHz were observed using the NRAO 12-m telescope...In a search for vinyl cyanide (H2CCHCN) at 151.9 GHz and ethyl cyanide (H3CCH2CN) at 152.3 GHz, no emission was detected to limits of 0.03 K peak-to-peak of main beam brightness temperature. [Following the publication of this Circular, Dominique Bockelee-Morvan (Observatoire de Paris) reminds us that this detection of CH3CN is not a first detection. She points out that this molecule was first detected in comet Hale-Bopp in August 1996 at 3.4 AU from the Sun (IAU Circ 6458).]

J. E. Wink (IRAM, Grenoble, France) and collaborators report on the same IAU Circular the detection of formic acid at the IRAM Plateau de Bure interferometer using its five antennae in a single-dish mode: On Mar. 20.6 UT the HCOOH rotational transitions at 224.977 GHz, 10(5,6)-9(5,5) and 10(5,5)-9(5,4), were detected....The HCOOH lines at 224.929 and 225.238 GHz were also detected. Assuming thermal excitation at 80 K, the derived HCOOH production rate is about 5 x 1027 molecules/s, which is 50 times smaller than that of the related species CH3OH.

13. List of detected molecules

Dominique Bockelee-Morvan has kindly prepared the following list of molecules whose detection has so far been announced in IAU Circulars or in research papers:

H2O, HDO, OH, H2O+
CO, CO2, CO+, HCO+
H2S, SO, SO2, OCS, CS
CH3OH, H2CO, HCOOH
HCN, CH3CN, HNC, HC3N, HNCO, CN, NH2, NH
CH4, C2H2, C2H6, C3, C2
Na

and the following isotopes:

H13CN, HC15N
C34S

14. A rocket launch

A two-stage NASA sounding rocket is supposed to have been launched at White Sands yesterday (March 24) to a height of 200 - 300 km. At the time of publication of this Update, no information had yet been received about the outcome.

There are four on-board experiments. 1) Jim Green (University of Colorado at Boulder): a spectrograph that will take data during 5 - 7 minutes on the chemical composition of the comet by observing it in the ultraviolet portion of the light spectrum, including lines from charged oxygen molecules released by Hale-Bopp, which should provide information on the comet's temperature and interactions with solar radiation. They will also search for traces of noble gases like argon and neon, thought to have been locked up in the comets since the objects first formed early in the solar system, each at a unique threshhold temperature. 2) Paul Feldman (Johns Hopkins Univ.): A Far-UV spectrograph with a 5 arcmin slit and a band pass from 1300-1900A; 3) Alan Stern (Southwest Research Institute): An EUV spectrograph with tunable resolution from 2-32 A, and 4) Walter Harris (University of Wisconsin): A wide field UV polarimetric imager that will be launched to observe the distribution and polarization of CI (1657 A) and reflected solar continuum from dust at 2800 A. The rocket (Wisconsin Imaging Survey Polarimeter-WISP) is an all reflective Schmidt with a waveplate and brewster polarizer that feeds a lumigen coated CCD covering a 5 x 1 degree area of the sky on 1 arcmin pixels.

15. More beautiful images

During the past days, more fine photos by Peter Staettmayer were placed in the area with Local news.

You may also want to have a look at some other recent, very nice images at sites which may be less known, for instance: Bob Yen (travelling in the Mojave desert; photos used by the Newsweek journal), Munich Public Observatory (a very active group), Bengt Ask (Sweden) (very northern location) and Alan Fitzsimmons (Belfast, N. Ireland, UK) (beautiful off-duty photos by a professional astronomer; if the link to Belfast is too slow, have a pre-view here [JPEG, 28k]).

And thanks to Bernd Warmuth at Forum der Technik - Munich for calling attention to the great photos at the Hale-Bopp site of Jarle Aasland (Norway).

16. New Links

Here is some information about links to other Hale-Bopp pages:

The load on the JPL Hale-Bopp site and the first mirror site is now so high that a Jet Propulsion Laboratory (JPL) - mirror site no. 2 has been set up. There are almost 2000 images available here! Ron Baalke has just sent a message about the JPL Comet Hale-Bopp home page receiving 558,000 hits on March 24, setting a new single day record for a JPL Web site. The previous record was 426,000 hits held by the Galileo home page, when the first set of Ganymede images were released in July 1996.

More links to Educational Sites have been placed in the List of Hale-Bopp Websites. Try for instance the Comet Hale-Bopp Weekly Observing Guide of the Planetarium at the Carnegie Science Center in Pittsburgh (USA).

I have also been notified of two other links which you may find interesting to try: the page of Agrupacion Astronomica de Gran Canaria, a comprehensive Spanish-language page with regular updates by Victor R. Ruiz, and a fine example of a high-school page: The Hale-Bopp Comet Page compiled by Bill Drennon (Central Valley Christian High School, Visalia, California, USA) for his students.

17. Hale-Bopp meeting at IAU General Assembly in Kyoto

Preparations for a special Hale-Bopp session at the time of the General Assembly of the International Astronomical Union in Kyoto (Japan) in August 1997 are underway. The Scientific Organising Committee will be chaired by Martha Hanner and Mike A'Hearn. It will include short, invited presentations and posters. It is suggested that interested GA participants with potential contributions make themselves known to them without delay.


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