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 September 13, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

This summary is issued at a time when the elongation (angular distance, as seen in the sky) of the comet from the Sun is getting quite small and observations in most spectral regions are becoming very difficult. At this moment, it is located in the constellation of Serpens Cauda , just one degree south of the celestial equator, at a geocentric distance of 2.94 AU (440 million kilometres) and 2.17 AU (325 million km) from the Sun.

It is visible with the unaided eye at about magnitude 4.5, very low in the western sky, just after sunset. When the sky becomes sufficiently dark, the comet is so close to the horizon that with few exceptions, large optical telescopes are no longer able to point towards it. This will not change before mid-January 1997 when the elongation will again become larger than about 30 o. In the meantime, professional observations will largely be restricted to the infrared and radio spectral regions.

It is the impression that, while many observations have recently been made in various places, rather few of these have been quickly reduced and published. Many observers seem to be collecting long series of measurements which will later serve to document the development of this unusual object. The present report will therefore be accordingly short.

More, recent observations from ESO La Silla will beceome available within a week or two.

Brightness development

Much interest is directed towards the current brightness predictions. There have been various messages during the past months about an apparent slow-down of the brightness development and fears have been expressed that Comet Hale-Bopp, after all, may not become a very bright object, as earlier predicted.

All cometary forecasts are based on at least two important types of observation. First, a careful study of the change of brightness, based on photometric measurements, with the eye by visual observers or, more objectively, with light detectors like CCDs.

The second aims towards a classification of the comet. Is it new or old ; does it show similarities with earlier comets; at which heliocentric distance does a particular type of activity start, etc.?

By comparing the development of earlier comets of the same `physical' type with the observed development of the present comet, predictions become possible, although it is well known that some comets have behaved erratically (outbursts, sudden luminosity drops).

The brightness development of Comet Hale-Bopp has indeed shown some slowing-down, as compared to the earlier, somewhat optimistic prognoses. This is clearly visible on some of the regularly updated lightcurves, now accessible on the Web. Two examples are those compiled by Charles Morris and Herbert Raab (see under `Helligkeitsinformationen'). Both of these appear to indicate that the present development, if maintained, will lead to a maximal brightness around 0 magnitude. This is also supported by the most recent visual estimates by experienced observers that cluster near magnitude 4.5, although some of these observations may have suffered by the bright moonlight. In any case, very few comets have ever reached such a bright magnitude at the current heliocentric and geocentric distances.

There is also no doubt that Comet Hale-Bopp, in terms of outgassing (see below) and dust expulsion, continues to be extremely active. The complex jet structure, mentioned in the earlier summaries in this series, are still visible. They bear witness to several active areas on the nucleus - a detailed analysis is now being undertaken by the experts and will hopefully soon lead to a `mapping' of the surface in terms of activity. This steady state, now lasting many months, is a further positive sign that the comet may keep up the activity during the remaining four months until its perihelion and will not disappoint the observers and the public.

Spectroscopic observations

As mentioned above, only a few observations have been published recently.

On November 26, an international group of astronomers announced the results of submillimetre (IRAM) and radio (Nancay) observations (IAUC 6511) carried out on November 8 - 10. This included measurements of the production rates of several important molecules, e.g. CO, 1.3 x 10E29 molecules/s; HCN, 1.1 x 10E27; CH 3 OH, 2.6 x 10E28; H 2 S, 1.3 x 10E28; OH, 1.1 x 10E30. For comparison, the OH-production is about 30 times higher than that of Comet Halley at the same heliocentric distance. The isomer HNC was also observed.

Observations in the infrared spectral region with the Infrared Telescope Facility (IAUC 6500, November 4) show that the comet's central condensation displayed a strong silicate emission feature whose gross shape of the silicate emission closely resembles the trapezoidal feature previously seen in some other comets, including Halley. In this spectral region, the colour temperature was 255 +/- 15 K (-18 C), or about 85 K above the radiative equilibrium temperature (168 K) for a blackbody at heliocentric distance of 2.73 AU.

Other information

An updated ephemeris by Don Yeomans is available.

A new list of stellar appulses has been prepared by Ovidiu Vaduvesco.

Various activities related to Comet Hale-Bopp have occurred during the Astronomy On-Line project, the major WWW-event in astronomy education that has recently passed through its first, very hectic Hot Week . In particular, the associated programme also contains a Collaborative Project that is aimed at observations of Comet Hale-Bopp.