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 December 23, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Hale-Bopp develops normally

Although the comet is now very close to the Sun as seen in the sky, no more than 29 degrees at this moment, it is being observed by dedicated amateur astronomers almost every night. Most recent observations come from countries in the North where the nights are long and the viewing conditions are better. Indeed, one of the observers, B.H. Granslo near Oslo in Norway, now observes it both in the evening and in the morning, at the beginning and the end of the 16-hour nights there, as often as the weather permits!

The latest brightness estimates date from January 6 and show the comet at almost magnitude 3. It thus appears to develop normally.

The tail is not well visible at this moment, because it is pointing away from the Earth (the comet is on the other side of the Sun and the tail, as will be known, points away from the Sun).

For an excellent recent overview, please consult the Sky & Telescope article by John Bortle.

Incidentally, today Comet Hale-Bopp crosses the orbit of Mars (1.66 AU) during its inward motion.

A recent image

One of the most recent images appears to be the one from Soenderborg Amtsgymnasium in Denmark, obtained on January 4.

Some remarks about the coma structure

We have received some comments by email about the images which have been published at the ESO site. Unfortunately, there was a mistake in one of the accompanying texts, about the way the indicated position angles were measured. Here are related comments, for which I am very thankful, by Hermann Boehnhardt of the Munich University:

About orientation and position angle in the images of November 2, 1996 ( ESO Press Photos 41a-d/96 ): the orientation (North up, East left) is correct. However, contrary to what is claimed in the text the listed position angles PA were measured North over West, i.e. clockwise from the top on these images. (This has now been corrected in the figure caption! RW).

About the orientation of the image of August 18, 1996 ( ESO Press Photo 37/96 ): during this exposure, the DFOSC instrument was rotated such that North is almost in the upper left corner of the frame as seen from the nucleus (brightest spot in the coma of Hale-Bopp), again East lies 90 deg counterclockwise from the North direction. Unfortunately, this information on the image orientation is not mentioned in the figure caption. Therefore, it is somewhat difficult to identify the similarities and diversities in the jet structures as seen in August and November 1996 images. But hopefully, the above explanation should help to verify our conclusion that the position angles of the jets remained almost constant while their relative brightness changed with time.

Why is this so?

The stationary appearance of the jet structures is indeed puzzling (not only to amateurs!). However, based upon the so-called fan scenario, proposed by Zdenek Sekanina of JPL for the 1995 coma structures in this comet, one can argue for a very similar explanation of the stable jet features in 1996.

Let me try to outline briefly the fan scenario as applied to the 1996 jet coma (assuming ideal conditions):

A near-polar, active region constantly illuminated by the Sun produces a so-called emission cone of gas and dust in the coma while the nucleus rotates around its axis. The cone axis is identical with the direction of the rotation axis of the nucleus, the cone opening angle is defined by the `nucleo-graphic' (or `nuclear') latitude of the emitting spot. If seen almost in sideview, such a 3-dimensional emission cone appears as a fan with two relatively sharp border lines (limitations), the fan opening angle is still very close to the opening angle of the real cone. If the position angle of the rotation axis does not change with time and the illumination and viewing geometry varies only very slowly with time (as for comet Hale-Bopp in 1996), one can imagine a fan structure which changes only very slowly with time (i.e. over months).

On the other side, such an emission cone or fan does not easily allow an accurate assessment of the rotation period, since the emission patterns of the cone/fan are very much reproduced (and thus smeared out) during each rotation cycle of the nucleus and almost nothing is seen from the rotation motion of the active area in the fan projection of the emission cone.

Now, in the case of Hale-Bopp, we may face a somewhat unique situation of having 4 active regions at different nuclear latitudes producing 4 emission cones with in total 8 jet-like border lines as limitations of the respective projection fans. However, as said in the beginning, this may only be a possible explanation and of course needs verification (or rejection) by observations and a detailed modelling.

Questions etc.

I have received many questions by email from visitors to this Website. Thank you very much! However, many have forgotten to indicate their return email address and for this reason I cannot answer them. Please note that your email address is not transmitted automatically, if you use the mailform facility at the bottom of the Hale-Bopp Webpages here!

A common question has been whether Comet Hale-Bopp will collide with the Earth. No, it will not - the minimum distance is almost 200 million kilometers, that is more than the distance between the Earth and the Sun!

Also, I am unaware of any companion to this comet, as surmised by some of those who wrote to me. At least until now, it has not broken and no secondary nucleus has been observed. It would be fine, however, if some splitting were to take place later, since this would undoubtedly increase the overall brightness because of increased dust production from the `fresh' surfaces.

Addition on January 9 : It may of course be that some of these emails refer to the recent upheaval around a `mystery object' near Comet Hale-Bopp. For those who have not yet heard about the solution to this, I strongly recommend that you take a look at the article by Stuart Goldman , just published by Sky & Telescope.