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 April 13, 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.

A brief introduction to this Update

Some 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. 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. Last chance for Northern Observers

2. Comet Hale-Bopp observed from the south

3. New type of tail discovered!

4. The innermost part of the ion tail

5. Dust and meteors?

6. More X-rays from Hale-Bopp

7. Rotation of the nucleus

8. Spectroscopic observations

9. Molecular update

10. More images

11. Hale-Bopp on the Web

12. Meetings

13. News about other comets

1. Last chance for Northern Observers

The strong moonlight during the past week has made it virtually impossible to make reasonable estimates of the tail lengths. However, the phase of the moon is now past full and after a few more days, the observing conditions in the evening will again be better.

Nevertheless, the comet is now rapidly moving closer to the Sun in the sky and the period until the next new moon will provide the last good opportunity for northern observers to see the comet while it is still near its maximum brightness. This follows after an unprecedented period of excellent visibility of nearly two months.

From a magnitude of about -0.5 to -0.3 about ten days ago, the latest brightness estimates of Comet Hale-Bopp indicate a decrease to near 0 (the mean of 12 observations from April 19 - 23 is -0.1 +- 0.2). For the latest lightcurve, please consult the page with diagram , maintained by Mark Kidger at the Instituto Astrofisicico de Canarias (Spain). Charles Morris has also updated his page with information about the comet's lightcurve and provides a new prediction about the future development.

Since the descending branch of this curve is now quite well defined, it is possible to determine that the comet was at its brightest very close to perihelion (April 1) and that it reached a magnitude just below -1.0.

Observers of Comet Hale-Bopp will have noted that as it now recedes from us, it is beginning to move more slowly in the sky. Because of the increasingly northern position of the Sun in the sky, the interval of good visibility in the evening is gradually shortening for northern observers. At the same time, the tails continue to swing counterclockwise, now at a rate of about 1.5 o /day.

At midnight (UT) between April 23 and 24, the comet will be 1.651 AU (247 million km) from the Earth, from which it is receding at about 29 km/sec. The heliocentric distance is 1.001 AU (150 million km), i.e. it is almost exactly as far from the Sun as is the Earth. It now moves outwards at about 42 km/sec along its very elongated orbit.

2. Comet Hale-Bopp observed from the south

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

Several reports have now been received from observers in the Southern hemisphere who have sighted the comet. One of the first was by Jean-Christophe `Papou' Millot who is Vice-President of the `Association Caledonienne d'Astronomie' (Noumea, New Caledonia; 22 o 16'S, 166 o 27'E). He said that the first observation there was made by Alexandre Millot (his father) on Sunday 13 at 18:23 local time (07:23 UT).

German Morales Ch. (Astronomia Sigma Octante, Cochabamba, Bolivia) saw Hale-Bopp on April 14, from 18:49 until 19:02 (local time). At about the same time, it was also seen by observers from La Paz, the capital of this country, travelling to a site 30 km East of the town of El Alto, located at 3900 m altitude at -16.5 geographical latitude.

3. New type of tail discovered!

On IAU Circular 6631 (April 18), an exciting discovery was announced by Gabriele Cremonese (Astronomical Observatory, Padua, Italy), on behalf of the European Hale-Bopp team. It concerns the detection of an extensive neutral sodium (Na) tail , i.e. a third type of cometary tail (the two first being the dust and the ion tails). He writes: The structure is visible on images obtained by Don Pollaco (Isaac Newton Group) with the wide-field CoCAM instrument (aperture 50 mm, 2000 x 1000 EEV CCD, 26 arcsec/pixel) at the Observatorio del Roque de los Muchachos (La Palma, Spain). The following description is based on two 10-min exposures obtained through a Na D filter on Apr. 16.88 UT under mediocre sky conditions. The filter (lamda_0 = 589.2 nm, FWHM = 2.1 nm) has no leak at the 0.02 percent level over the entire sensivitity range (300 - 1100 nm) of the CCD. The images display two main tails. The first is a very linear feature 6.6 deg long and less than (or equal to) 10 arcmin wide with parallel edges over its entire length and a very sharp south edge at p.a. 52 deg (3 deg north of the projected antisolar direction). The second is a broad diffuse northern tail extending to the edge of the frame, i.e., to more than 6.5 deg. While the second tail coincides with the dust tail as seen in an adjacent continuum filter, the first tail has no such counterpart, and we identify it as a neutral Na tail. In particular, near-simultaneous imaging with an H2O+ filter (lambda_0 = 619.3nm, FWHM = 4.5 nm) displays an ion tail of entirely different and more complex morphology with a main ray at p.a. 46 deg. We believe that this is the first clear separation of a neutral tail of a comet from the dust and ion tails. The existence of this third type of cometary tail was confirmed in additional CoCAM images obtained on Apr. 17.8 UT.

Under the reasonable assumption that it points nearly in the anti-solar direction, the indicated length of the new tail corresponds to about 50 million km. (Later images have shown an even greater extension).

This message was followed up on IAU Circular 6634 (April 21) with new information from Gabriele Cremonese and his colleagues, Heike Rauer (Observatoire de Paris, Meudon) and Alan Fitzsimmons (Queen's University, Belfast), who on behalf of the European Hale-Bopp Team reported further observations of the new tail: Spectra were obtained on Apr. 19.90 UT with the Utrecht Echelle Spectrograph at the 4.2-m William Herschel Telescope at the Observatorio del Roque de los Muchachos, La Palma, with a 4.0 by 1.1 arcsec slit positioned 1.5 deg along the sodium tail at p.a. 56 deg (this position angle being determined by simultaneous imaging with the CoCAM instrument). The location of the slit corresponds to a cometocentric distance of 1.1 x 10 7 km along the tail. A series of spectral windows were observed between 500 nm and 800 nm at a resolution of 7 km/s. In a 15-min exposure, strong Na D lines were detected (their strength relative to the sky lines being approximately 1.3:1), redshifted at a line-of-sight velocity of +105 km/s, which corresponds to a cometocentric velocity of approximately 95 km/s along the tail. The lines are resolved with an intrinsic FWHM of about 11 km/s. These data confirmed that neutral Na is present in the newly-detected third type of cometary tail. Emissions from H 2 O + , NH 2 and C 2 (among other molecules) were not detected at this tail position. The same instrumentation was used on Apr. 20.85 UT to obtain spectra at 10 arcsec from the nucleus and at 0.7 deg (cometocentric distance 5.0 x 10 6 km) along the Na tail. Once again, Na D-line emission associated with the comet was observed, at cometocentric velocities of 0 and 58 km/s, respectively. From the measured velocity distribution and surface brightness along the tail, it would appear that the data are consistent with a model in which neutral sodium atoms are released from still-unidentified sources within the coma and then accelerated in the antisolar direction by simple fluorescence [Editor's enhancement]. At the same time as the Apr. 19.90 spectroscopic data were being secured, CCD imaging was performed by J. Licandro, Instituto de Astrofisica de Canarias, with the 1-m Jacobus Kapteyn Telescope through a narrow-band filter (lambda_0 = 587.7 nm, FWHM = 4.9 nm) and an adjacent continuum filter. A comparison between these images over the 5 arcmin field of view did not reveal any features that could be identified as peculiar to neutral Na.

This second text indicates that a probable cause has now been found for the presence of sodium atoms in the tail, even very far from the cometary coma. Apparently, they are formed in the coma (by a yet unknown process) and are then pushed outwards by the pressure of the solar light, much like the dust particles are. However, the sodium atoms achieve much higher velocities and produce an almost straight tail, virtually in the anti-solar direction.

Knowing the life-times of these atoms (that is, how long it will take before they will become ionised) and also their speeds in the tail, the astronomers were able to calculate (in relative terms) how many atoms there ought to be at a given distance from the coma. The more atoms there are in a given area of the tail, the brighter it will shine. A comparison with the actually observed brightness distribution of the tail appeared to show good agreement and thereby provided important, independent indication that it is indeed this acceleration mechanism that is at work.

More information and an image of the new sodium tail is available as a Press Release from the Isaac Newton Group. Since it appears that it may not always be easy to get through to that site, a small version of the discovery image (JPEG; 29k) has therefore been placed here. It shows the sodium tail in Comet Hale-Bopp taken on the 16th April 1997. The tail appears as a very straight narrow feature extending from the head of the comet to the upper left.

The discovery of a third type of cometary tail did not go unnoticed by the media, see for instance the report publiched by CNN at http://cnn.com/TECH/9704/19/comet.tail.ap/index.html.

In addition to the astronomers mentioned in the quoted IAU Circulars, Hermann Boehnhardt (University of Munich, Germany) and Gian Paolo Tozzi (University of Florence, Italy) (and myself) belonged to the team that observed Hale-Bopp at La Palma during the past week, with a total of 5 telescopes. Together with simultaneous observations at the Teide observatory at Izana, near Teide on Tenerife, these observations were carried out within the so-called International Time for major research projects at Large Scale Facilities, a programme supported by the European Union. It goes without saying that those past days and nights were extremely hectic and also very exciting for all of us! Only a few days and nights passed from the appearance on the computer screen of the first CoCAM image with a `possible sodium tail' to the present situation where the new phenomenon is thoroughly confirmed and is probably also quite well understood. This was a wonderful period for all of us, with intense discussions, doubt and subsequent verification, planning of additional observations, data reduction as well as theoretical considerations, supported by frequent email exchanges with several of our team colleagues in other places. In other words, one of those (rare) euphoric experiences all scientists long for!

4. The innermost part of the ion tail

Among the many images that have been placed at the JPL Hale-Bopp site (there are now over 3400!), there is one particularly impressive and also very instructive from Pic du Midi (France) , illustrating the advantages of digital image subtraction. In this particular case, a red image of the comet's head was subtracted from a blue one, `removing' the dust in this region from view. The resulting image accordingly shows the innermost structures of the gas emissions much better.

5. Dust and meteors?

On IAU Circular 6625 April 14), Yu. N. Gnedin and T. M. Natsvlishvili of the Central Astronomical Observatory (Pulkovo near St. Petersburg, Russia) and their colleagues at the Special Astrophysical Observatory in Nothern Caucasus, report preliminary results of polarimetric observations during Mar. 28 - Apr. 1: The amount of polarization is similar in the coma, the dust tail, and the ion tail, varying generally from 5-10 percent in U, 10-19 in B, and 7-17 in both R and I. The V polarization is generally less than 1 percent in the coma, increasing to 10-20 percent in both the dust and ion tails. The high degree of polarization and associated strong wavelength dependence....give strong evidence for molecular-band polarization. The existence of CN, CO + , and H 2 O + species are considered most probable.

Polarisation measurements like these are very useful for the study of the dust grains and may give clues to their composition and the size.

There have been quite a few questions by email about the possibility that a meteor shower may perhaps be caused by dust particles released from Hale-Bopp? Iwan P. Williams of the Queen Mary and Westfield College (University of London), an expert in these matters, has sent the following considerations: I do not think that we will have a spectacular Hale-Boppids display for the following reasoning. A meteor stream is formed from dust grains ejected from the parent comet at speeds far less than the orbital speed. This means that all the meteoroids follow an orbit that is very similar to the orbit of the comet. Meteoroids do not move much sideways. Even if they are initially projected towards the Earth's orbit, this velocity is not maintained, conservation of angualar momentum ruling the motion. Thus to see meteoroids ejected from this return of Hale-Bopp, planet Earth would have to pass very close to the orbit of this comet, or more precisely, the orbits of the meteoroids. The miss distance is actually more than 0.1 AU (15 million km), an enormous distance. Meteoroids ejected at the last return have had 4000 years to evolve. Some will undoubtedly move closer to Earth, but the general trend will be of dispersal of the meteoroid concentration. hence again suggesting that a storm will not take place. The nodes of the stream are very close to those of the Quadrantid stream, and, if a major outbreak of Hale-Boppids was on the way, We should have had a foretaste while the comet was on its way in last year.

He adds: Of course,....I would be delighted to be proved wrong and that the Earth witnesses a splendid display of Hale-Boppids close to the next nodal crossing! (i.e. at the point where the comet's orbit crosses the plane of the Earth's orbit, the Ecliptica ).

6. More X-rays from Hale-Bopp

M. J. Mumma and V. A. Krasnopolsky (Goddard Space Flight Center, NASA) and their colleagues report (IAU Circular 6625 April 14) the detection of soft X-rays, He I (58.4 nm), and O II (53.8 nm) during 1996 Sept. 14-19 UT (r = 3.07 AU) with the EUVE orbiting observatory.... The central brightness was displaced from the nucleus by 140 000 +/- 60 000 km in the sky plane. A region of extended x-ray emission is also seen, and this is anticorrelated with the dust jets imaged simultaneously in the visible. He I 58.4-nm and O II 53.8-nm lines were detected with S/N = 4. The absence of Ne lines shows a depletion of Ne in the comet by more than 25 times relative to the solar abundance.

The observation of line emission is very interesting and will undoubtedly help to elucidate the mechanisms responsible for the X-ray emission from comets. The low abundance of Neon is puzzling.

7. Rotation of the nucleus

More information has been published about the rotation of the nucleus. It now appears that the main period af just aover 11 hours varies slightly with time, with a modulation of about 18 - 19 days.

On IAU Circular 6635 (April 22), Mark Kidger (Instituto de Astrofisica de Canarias - IAC, Tenerife Spain)) reports on behalf of the European Hale-Bopp team that near-infrared observing runs of up to 11 hr were undertaken for comet C/1995 O1 with [infrared] narrow-band Br-gamma, CO, and K-continuum filters at the 1.5-m Carlos Sanchez Telescope (+ CAIN infrared camara with NICMOS array) on a number of days around perihelion. Analysis of the data taken by B. Goetz (University of Munich) and J. Licandro (IAC) supports the period of rotation of the nucleus suggested by Lecacheux et al. (IAUC 6560) and since refined by Jorda et al. (IAUC 6583) and by Sarmecancic et al. (IAUC 6600), giving a value of 11.20 +/- 0.04 hr for 1997 Mar. 31 - Apr. 2. These data, together with previously published results, suggest that the period of the reported modulation of the rotation period may be as short as 18 days. Further long runs of data are being analyzed, and observations are continuing in an attempt to define this modulation more exactly.

In this connection, it is also of some interest to look at the image made on April 7th at the Purgathofer-Observatory through the 1-m RC-Telescope (f/8.8) and a CCD-Camera with a resolution of 768 x 512 and 9 micron pixels. We superimposed 30 exposures and then made a contrast enhancement with 3 different filtering methods, each showing different structures in the coma. A smaller version (JPEG, 30k) is available at the ESO site. It was obtained by Rudolf Pressberger , Andreas Kreutzer and Robert Schaefer . They add the following comments: From other observers is reported, that the various arcs are ejected by the same active spot (IAUC 6583, IAUC 6587) and the amateur astronomer Bill Matthews thinks, that the shells are in real a spiral looked from the side. F. Manzini and his colleagues (Instituto di Fisica dello Spazio Inter-planetario del CNR di Frascati, Italy) remark that both bursts, which they observed on Sep. 11, 1996 and Feb. 9, 1997, evolved from a single jet, and that this speaks against the hypothesis by Zdenek Sekanina about the origin of multiple jets on Hale-Bopp. However, when looking at our picture we think that two spirals can be seen, one large outer one that creates the large 'shell structure', and a smaller one that is reponsible for the smaller one. This can be achieved by two jets with different radial velocities. Thus, we think that this supports the model of Sekanina, which uses multiple jets.

8. Spectroscopic observations

On IAU Circular Circular 6625 (April 14), D. Mehringer and collaborators report the first ground-based spectral identification of H 3 O + in a comet with the CSO: On Apr. 6 and 10 UT, we detected the 3(2)-2(2) line of H 3 O + at 364.797 GHz in Comet C/1995 O1.....The line is approximately centered on the comet's rest velocity, and its full width at half maximum is about 3 km/s..... .

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:

and the following isotopes:

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 telescope at the Roque de los Muchachos observatory during the past week. 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

The number of images available at the JPL site is steadily increasing and is now approaching the 4000 mark!

In the April 13 Update , mention was made of the fact that Comets Hale-Bopp and Hyakutake were in the same part of the sky one year apart. Here are the adresses of two photos at the JPL site that illustrate this:

http://www.jpl.nasa.gov/comet/dru59.html and
http://www.jpl.nasa.gov/comet/hutch24.html

Auroras (Northern Lights) were recently observed in some northern countries. Jens Revsbech (aged 14) obtained a nice image with auroral curtains and Comet Hale-Bopp in the evening of on April 11 when observing from Nydam Mose, a swampy area near the town of Soenderborg in Southern Jutland (Denmark).

One of the finest animations on the Web is the one by Terry Platt , who spent most of night between March 28 and 29 to obtain short exposures at 45 minute intervals during one full rotation of the nucleus. This was possible since the comet was circumpolar from the Mr. Platt's position and could therefore be observed all through the night. The expanding features give a most vivid rendering of the activitiy near the nucleus, very much like what is observed with larger, professional telescopes.

11. Hale-Bopp on the Web

An interesting page with images of Comets Hale-Bopp and Hyakutake has been set up by Terry Acomb , a student of geology at the University of Cincinnati (USA).

If you want to learn about an most interesting educative project, Comet Hale-Bopp for Kids , you iwhs to check out this site: http://www.lafsd.k12.ca.us/hv/Hale-Bopp !

12. Hale-Bopp Meetings

The IAU Executive Committee has added a Special Session (SPS) on Comet Hale-Bopp at the time of the Kyoto General Assembly - the timing is August 23 (Sat), 14:00-17:30. For further information, pls contact Martha Hanner (msh@scn2.jpl.nasa.gov).

More news will soon become available about the International Hale-Bopp meeting , organised on February 2 - 5, 1998, on Tenerife (Canary Islands, Spain).

13. News about other comets

The last Hale-Bopp Update (April 13) carried news about the discovery of a comet by Joe Montani . Just one day later, IAU Circular 6626 (April 14) carried the message that Joe Montani (Lunar and Planetary Laboratory) has just found another comet with the Spacewatch 0.9-m telescope at Kitt Peak (designation C/1995 G2 ). The coma subtends 7 - 8 arcseconds and a tail is plainly visible in the Apr. 12 CCD scans. The magnitude is about 18.

The discovery of two comets by the same observer at an interval of only three days (April 9 and April 12, 1997) is very unusual but it is not the shortest known in history. Brian Marsden (IAU Bureau for Astronomical Telegrams, Center for Astrophysics, Cambridge near Boston, USA) informs that Metcalf discovered comets two days apart (one of them his Brorsen-Metcalf rediscovery) in August 1919, and Pons discovered comets two days apart (one of them 2P/Encke) in November 1818. Mori was credited with the independent discoveries of two comets within 90 minutes of each other on 1975 Oct. 5.... [although the circumstances are not entirely clear] .