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 7, 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. Less optimal viewing conditions

2. Comet Hale-Bopp soon visible from the south

3. The brightness

4. Changes in the ion tail

5. The dust tail

6. New orbital elements and a coincidence

7. X-rays observed from Hale-Bopp

8. The rotaion of the nucleus

9. Spectroscopic observations

10. Molecular update

11. Amateur spectroscopists

12. More images

13. Hale-Bopp on the Web

14. News about other comets

15. Miscellaneous information

1. Less optimal viewing conditions

The viewing conditions are now rapidly deteriorating, as the Moon moves higher in the evening sky, increasingly illuminating the background around the comet. This condition will last until a few days after the next full moon on April 22. Then it will again become possible to enjoy the comet in a dark sky, but for a shorter time than before, as it will be seen progressively closer to the Sun in the sky.

2. Comet Hale-Bopp soon visible from the south

As the comet continues its motion towards the south, now at a rate of about half a degree per day, observers on southern continents may finally begin to get a glimpse of the beautiful sight that has fascinated their northern colleagues during the past months. Preparations are now underway in Australia, South Africa and South America to `receive' Comet Hale-Bopp in the appropriate way.

For instance, Anthony C. Beresford (Astronomical Society of South Australia - ASSA, Adelaide) draws attention to the ASSA Website which contains a viewing guide with an animated diagram of the horizon path for Hale-Bopp for 35 South latitude. The time shown (18:15 Adelaide time) corresponds to 18:02 Local Mean time. It fits the observing conditions for Sydney, Adelaide, Auckland and Capetown (South Africa), as well as in Chile at the latitude of Santiago (beware that the mountains there may obstruct the view!). He adds that if one applies the standard astronomical criteria, also espoused by ASSA member Bill Bradfield, here in Adelaide, we won't see it against a darkish sky till May 24. However, I think the viewing will be better in early May when the comet will be brighter.

Note also the very useful information contained in the News Bulletin from Sky and Telescope that is updated every Friday.

Additional information about Comet Hale-Bopp, including the coverage of the associated media event, is available at the CNN interactive - a stargazer's guide site.

3. The brightness

The mean of 13 visual estimates of the brightness of Hale-Bopp obtained on April 6 - 8 was magnitude -0.6 +- 0.2. On April 10 - 12, the mean was slightly fainter at -0.5. This is still somewhat brighter than that predicted from a simple extrapolation of the pre-perihelion observations. This is a typical effect, observed in many comets, including Comet Halley in 1986. The reason why many comets are relatively brighter after perihelion is connected to the effects of cumulative heating - at the same distance from the Sun, the overall temperature of the surface of the nucleus is higher after perihelion than before. Thus the gas and dust production is higher after perihelion, resulting in an asymmetrical lightcurve.

On April 9, the experienced amateur astronomer John Bortle (Stormville, NY, USA) noted that: comet clearly not as brilliant as three nights ago; ion tail traced 6 deg... with naked eye, 10 deg with 7x50 Binocular; dust tail immense, very broad near its terminus, stretching 21 deg with the naked eye... the nucleus looks like a star... three envelopes, all weaker and more diffuse than previously... only just a trace of the great dust streamer in the comet's tail tonight, and the tail void has essentially filled in completely...

4. Changes in the ion tail

The length of the ion tail is currently estimated as 10 o - 15 o. No reports have yet been received here about observations of the expected effects on this tail, caused by the recent, major outburst on the Sun. This event which was extensively covered by the press, resulted in a temporary increase in the number of particles ejected, i.e. in a higher intensity of the solar wind . This will normally lead to rapid changes in the ion tail and it is therefore likely that a study of wide-field images obtained during the past days will show the related effects.

Sergio A. Ilovaisky (Observatoire de Haute Provence, France) calls attention to the recent images on their OHP Hale-Bopp Web page , in particular three impressive ones taken by Eric W. Elst with the Schmidt telescope on April 2-6 and showing the dramatic changes in the ion tail at that time in great detail (texts in French and English).

5. The dust tail

A truly amazing dust tail length of 42 o was reported by C. Miranda and C. S. Morris on April 9.2 UT, from observations at Whitaker Peak (California, USA). They write that the comet brightness [is] less than last night's obs., but the dust tail is definitely longer; spent a lot of obs. time examining and tracing the dust-tail length; the dust tail `edge' skimmed the top of Epsilon Cas and passed under Gamma Cep and traced to Iota Cep (CM).... for whatever reason, last night the longer tail was very obvious; Carmelita Miranda's description, given above, details what we both independently saw; a couple of comments are appropriate -- the width of the dust tail at its end was at least 10 deg; as I know the skeptics will jump on this (they always do), I performed a test; I blocked out both the comet and the Milky Way, and the end of the tail was still obvious (I don't think my eyes were being fooled!)... (CSM) .

A study of the striae in Comet Hale-Bopp's dust tail, based on a set of wide-angle images obtained at regular time intervals with a number of different telescopes, has been initiated by Zdenek Sekanina (JPL) and his collaborators. It is expected that it will throw more light on the still not fully understood processes that are responsible for this peculiar phenomenon, which was first seen in the tail of the bright comet in 1976.

6. New orbital elements and a coincidence

New orbital elements by Don Yeomans (JPL, Pasadena) and corresponding ephemerides have become available. The new orbital elements are based on 2158 observations up to March 29, 1997. The pre- and post-perihelion orbital periods remain virtually unchanged at 4206 and 2380 years, respectively. There are three additional sets of 3-hour ephemerides at this site, `geocentric' as well as for observers from the La Silla (Chile) and La Palma (Spain) observatories.

On April 13, at 12 UT, Comet Hale-Bopp will be 1.489 AU (222.8 million km) from the Earth, corresponding to a light travelling time of 743 seconds (12 minutes 23 seconds). It will be 0.941 AU (140.8 million km) distant from the Sun and the orbital speed is about 43 km/sec.

In the evening of April 12, I received this email from Al Rubin (arubin@omail.eee.org): Hi. I'm an amateur skywatcher who follows the progress of comets by plotting the positions of comets on a sky chart. I've done this with four of them so far. I've noticed an interesting coincidence that I believe others would appreciate but I don't quite know how to reach them. I'm e-mailing you to solicit any suggestions that you may have for disseminating the information, which is that the position of Hale-Bopp, on April 9, 1997, was in almost exactly the same position that Hyakutake was on April 9, 1996! I just thought it was interesting. Thanks!

Al is (almost) right - what a strange coincidence! Here are the positions (J2000; geocentric; 0 UT) of the two comets during a small interval around these dates:

If these positions are plotted in a graph, it will be seen that the time of the `closest passage', i.e. the moment when the two comets, with a time interval of 1 calendar year, were closest to each other in the sky, was at approx. April 10.5 UT. The corresponding angular distance in the sky is only about half a degree! However, this is because of the perspective; the orbits in space do not cross each other. While Hyakutake was only 81 million km from the Earth on this date in 1996, Hale-Bopp was at a distance of 217 million km in 1997.

7. X-rays observed from Hale-Bopp

As was the case for Comet Hyakutake last year, X-rays have also been observed from Hale-Bopp. On IAU Circular 6614 (April 7), Alan Owens (ESA Space Science Department, Nordwijk, The Netherlands) and collaborators announce - on behalf of the BeppoSAX team - the detection of soft X-rays from the direction of this comet by the LECS instrument onboard the BeppoSAX spacecraft: The observations took place between 1996 Sept. 10.16 and 11.18 UT. After correcting for the comet's motion, a 10-sigma enhancement was found within 3 arcmin of the expected location of the nucleus at much greater than 99.99-percent confidence.... The extracted LECS spectrum is well fit by a thermal bremsstrahlung model of kT = 0.36 keV -- consistent with that observed in other comets (e.g., Lisse et al. 1996, Science 274, 205). The total 0.10-2.0-keV luminosity is 6 x 10 16 erg/s, comparable with the observed luminosities of comets C/1996 B2 [Hyakutake] and C/1990 N1 [Tsuchiya-Kiuchi]. There is no evidence for fluorescent carbon or oxygen emission with 95-percent-confidence upper flux limits of 1 x 10 -4 photons cm -2 s -1 and 3 x 10 -4 photons cm -2 s -1 for narrow line emission at 282 and 523 eV, respectively. This implies that if lines are present, their total luminosity must be < 10 percent of the continuum luminosity. The last remark is of special interest in connection with the discussion of the possible mechanism(s) that cause the X-ray emission from comets.

8. The rotation of the nucleus

Information about the recent coma structure and the rotation of the nucleus has become available on IAU Circular 6620 (April 9) by Laurent Jorda (Max-Planck-Institut fuer Aeronomie, Lindau, Germany) and colleagues: The apparent rotation of the nucleus was clockwise on images obtained before Feb. 1997 (Kidger et al. 1996, Ap.J. 461, L119) and became distinctly anticlockwise during March, showing that the north pole is now directed toward the earth. The spiral jets appeared straight on visible and infrared images taken at Pic du Midi Observatory on Mar. 1 +/- 2 days, showing that the line of sight crossed the equatorial plane of the nucleus at that date. This implies that the source of the bright jet observed since January is located in the northern hemisphere, contrary to what we announced in a preliminary analysis (IAUC 6560). This jet, observed in the hemisphere directed toward the sun, now has the appearance of concentric arcs with increasing curvature....

9. Spectroscopic observations

Yet another molecule has been detected. On IAU Circular 6614 (April 7), D. Mehringer (Caltech Submillimeter Observatory - CSO) and colleagues announce the identification of formamide (NH 2 CHO) in Hale-Bopp: On Apr. 5 UT, we detected the lines of NH 2 CHO at 254.877 and 227.606 GHz with the CSO and IRAM 30-m telescopes, respectively..... A preliminary estimate of the NH 2 CHO production rate is 3 to 5 x 10 27 molecules/s.... .

Moreover, deuterated water ( HDO ; also known as `heavy water') has now also been securely detected and a first measure of the important D/H-ratio in Hale-Bopp has finally become available. On IAU Circular 6615 (April 7), R. Meier (University of Hawaii) and collaborators report observations with the James Clerk Maxwell Telescope at Mauna Kea on Apr. 4.8 UT: The integrated line intensity of the 101-000 HDO line at 464.925 GHz was (0.64 +/- 0.11) K km/s on the antenna temperature scale..... Assuming a rotational temperature of 80 K, we derive a preliminary HDO production rate of 4 x 10 26 molecules/s. This makes the HDO/H 2 O ratio about 2 x 10 -4 , assuming a water-production rate Q(H 2 O) = 2 x 10 30 molecules/s (IAUC 6589). The corresponding D/H-ratio of about 10 -4 is consistent with other values for hydrogen in condensed matter in the outer solar system and in seawater on earth, and is well above the values reported for H 2 in the interstellar medium or in outer- planet atmospheres. Thus Hale-Bopp, appears to be a `normal' comet in this respect with an origin similar to that of other comets, notably Halley and Hyakutake.

Rita Schulz of the Comet Hale-Bopp European Team reports the first successful spectroscopic observations with the 2.5-m Nordic Optical Telescope (NOT) and the SOFIN instrument on La Palma at very high spectral resolution (R = 160,000) in the visual region. One of the goals is to study the carbon isotopic ratio.

10. 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:

This list may not be complete. Would anybody who has additional information kindly contact the author via the email address given at the bottom of this page? Thank you in advance.

11. Amateur spectroscopists

Two amateur astronomers, Nick Glumac and Joseph Sivo have obtained a spectrum (preview version; GIF; 16k) of Hale-Bopp in the range 350 nm to 520 nm, under light polluted urban skies near New York City, on April 4, 1997 between 8:00 and 9:00 PM Eastern Time. They used a 10-inch Meade LX-200 telescope, an Ocean Optics S200 Fiber Optic Spectrometer (a miniature spectrometer with a linear 2048 CCD pixel array), a novel designed eyepiece-holder/fiber optic coupler, a 15 watt photomultiplier tube (PMT), and a laptop 486 computer. With this system, less than 1 nm resolution was achieved. More details are available at the JPL Hale-Bopp site that now stocks over 2800 images.

Bob Fosbury ( ST-ECF ) calls attention to his special webpage about photographing spectra with a transmission grating mounted in the filter adapter of a camera. It contains much useful information and includes a montage with spectra of the Sun, some stars and Comet Hale-Bopp (April 7).

12. More images

Images of Comet Hale-Bopp keep coming by the hundreds. No other comet has ever been so intensively observed! The JPL site now stocks over 3000 images of all types, ranging from snapshots with amateur cameras to advanced, semi-professional renderings of the finest details in the coma and tail systems.

There are several new images at the Hale-Bopp Photo Gallery at the IAC website.

Bengt Ask (Svaloev, Sweden) has established a Hale-Bopp website with impressive pictures. There is also a fine animation of the Partial Solar Eclipse on October 12, 1996, on another page at this site. And while speaking about Hale-Bopp and eclipses, you may also wish to check out Dave Bridge's homepage with comet images and an animation of the March 23-24 Partial Lunar Eclipse.

Do have a look at the latest wide-field image by Eckhard Slawik (April 7, 1997) , now available here.

13. Hale-Bopp on the Web

Ray Long (ESA; ISO Webmaster) calls attention to the availability of more ISO images at http://isowww.estec.esa.nl/science/galleries/sso/halebopp.html. Note also the link to ISO-related information at the Other Sites' page.

14. News about other comets

Joe Montani (Lunar and Planetary Laboratory, Tucson, Arizona, USA) has reported (IAU Circular 6622; April 10) the discovery of a comet (provisional designation C/1997 G1 ). The comet is located in the northern constellation of Gemini and is quite faint (magnitude 19). When further observations had become available (IAU Circular 6624; April 12), the comet was named after the discoverer ( Comet Montani ). A first, very uncertain orbital calculation by Brian Marsden (IAU Central Bureau for Astronomical Telegrams) and based on only 13 positions, indicates that this comet is quite far from the Earth (about 4.45 AU, or 666 million km), with perihelion time in September 1997 and perhaps of short period. A better orbit may become available within the next few years when more measurements have been made.

Measurements of another comet of current interest, 46P/Wirtanen , the target for the Rosetta mission , show it at magnitude 9.6 - 10.0, that is about 10,000 times fainter than Hale-Bopp.

15. Miscellaneous information

Several questions have been received about the possibility to observe meteors during the next days. In fact, a minor meteor shower, the Virginids (i.e., which are seen radiating from the direction of this constellation) has its maximum on April 12. However, this stream - which is actually the superposition of several minor streams - has rather low count rates and comparatively few, bright meteors are seen. The chances are better during the period (April 19 - 25) of the Lyrids (radiating from the direction of the northern constellation of the Lyra ). Their rate peaks at about 10 - 15 meteors per hour on April 22-23. Unfortunately, this year the time of the maximum coincides with full moon so the observing conditions will not be good. This meteor stream is associated with Comet Thatcher which was observed in 1861 and has a period of just over 400 years.

It has been announced by the International Earth Rotation Service that a positive leap second will be introduced such that the sequence of UTC second markers will be: 1997 June 30d23h59m59s, 30d23h59m60s, July 1d00h00m00s. This adjustment of civil time is necessary from time to time because of changes in the Earth's rotation rate, mostly a very slow and progressive slow-down. It is important for all high-precision time-related programmes and activities, e.g. global positioning, astrometric measurements and orbital calculations. More information about leap seconds and time scales is available in various places on the Web, e.g. at the UK National Physical Laboratory.