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 November 28, 1996. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages.

Although Comet Hale-Bopp is now very close to the Sun in the sky, observations continue in some places. They show that the comet continues to be very active. Several jets are visible on most images.

At the same time, visual observers have noted a relative brightening, following a period of slow increase. The latest brightness predictions now place it Comet Hale-Bopp somewhere between -1 and 0 magnitude at perihelion. This is good news for all prospective observers, but we must still await the development during the next month, before we can be sure.

Selected images

We show here some typical images of Comet Hale-Bopp, obtained after the beginning of November 1996.

A series of high-resolution images were obtained on November 2 by Hermann Boehnhardt, working at the Danish 1.5-meter telescope at La Silla Observatory. They show the comet in the light emitted by CN-molecules and in sunlight reflected off the dust particles in the coma. The caption gives interesting information about the different positions of the observed jet-like features.

Mogens Winther, a physics teacher at the Amtsgymnasiet Soenderborg (Denmark), and his students continue to obtain excellent images of the comet, as often as the weather permits. We show here three images from November 08 , December 13 and December 20. The field measures 1200 arcsec in the vertical direction. Full information is available at the Amtsgymnasiet Soenderborg Astronomy Website.

Another recent, fine image was obtained on December 16 by Marcus Arzt of the Swabian Observatory, Stuttgart (Germany).

And here is the latest image from December 3 in a long series, obtained by Hermann Mikuz and colleagues at the Crni Vrh observatory in Slovenia. Full information is available at their Hale-Bopp Website.

Infrared images were obtained in early December with the TIMMI camera at the ESO 3.6-metre telescope at La Silla by ESO-astronomer Ulli Kaeufl. They are not yet ready for display, but we expect to bring them in early January.

Brightness development

Comet Hale-Bopp is now clearly brighter than 4th magnitude, as reported by several visual observers. Due to its extremely low position in the western sky, these estimates are difficult. The coma diameter is correspondingly uncertain. However, there is no doubt that the comet, as expected, is now beginning to brighten more rapidly.

There are now quite a few websites with information about the brightness development of the comet, both in the form of lightcurves and discussions about the current trend. Note in particular those compiled by Charles Morris , Herbert Raab , Alan Fitzsimmons and Mark Kidger.

These sources now agree that the comet has recently accelerated in terms of brightness after a period of slowing down to the great dismay of its many friends. The predictions now center around magnitude -0.5 at perihelion and most evalations assume that the peak magnitude will in any case be negative.

Spectroscopic observations

A visual spectrum covering the 3500 (left) - 9300 (right) A region, was obtained on September 11, 1996, by Holger Pedersen (Copenhagen Observatory) with the DFOSC instrument at the Danish 1.5-metre telescope at La Silla. The strong horizontal band is mostly reflected light from the dust particles in the inner coma, but there are also many emission lines from gaseous species, reaching far out to both sides. One of the most conspicous features is the CN-band to the left. Other bands towards the centre originate from C 2 and C 3 molecules. The length of the slit was about 10.5 arcmin and the spectrum was recorded during a 10-minute exposure on a LORAL CCD chip. There is most probably overlap between the 1st and 2nd spectral order above 7000 A. Some stellar spectra are also seen as fainter, horizontal lines.

Writing in IAU Circular 6515 (December 6), a group of astronomers report the detection of HNC[4-3] emission at 362.6 GHz using the James Clerk Maxwell Telescope on December 1. The HNC/HCN production ratio is estimated as 0.07. This is similar to the value measured earlier in Comet Hyakutake (IAUC 6353). Since the two comets were observed at quite different heliocentric distances (2.14 and 1.22 AU), the results suggest that the observed HNC is a parent molecule present in the nuclear ices and not a product of photochemistry in the coma. The HNC/HCN ratio is consistent also with an origin for these species in an interstellar cloud.

On IAU Circular 6519 (December 9), another group reports observations in early August with the Anglo-Australian Telescope and the mid-infrared camera NIMPOL. They find that already at this time, the 8.5-12.5-micron colour temperature was 187 +/- 10 K, that is well above the 147 K expected from a blackbody in radiative equilibrium at 3.57 AU from the Sun. They also detected the silicate feature near 11 micron data and they found that over a period of 6 days, the comet faded by at least 15 percent.