Comet Hyakutake - Update (April 1, 1996)

Dust Jets (ESO NTT Image)

This is another Update about recent observations of this comet; the latest was published on the ESO Web on March 28 . It is based on information received directly by email and also from IAU Circulars and on other Hyakutake WWW pages.

Richard M. West (ESO)
April 1, 1996; 13:00 UT

Following its closest approach to the Earth, Comet Hyakutake is now getting somewhat fainter - last night most observers put it at total magnitude 1.5 - 2.0. Moreoever, the seeing conditions are worse because the Moon does not allow to view it in a dark sky during the next nights, except at the time of a total moon eclipse that will take place on April 3 - 4 . Observing conditions should then be fine from many sites on the northern hemisphere, including Europe, during about 1.5 hours of totality. The table gives the relevant timings in Universal Time and Central European Summer Time (CEST = UT + 2 hours), quoted from the Nautical Almanac 1996:

Moon eclipse, April 3 - 4, 1996

During the eclipse, the comet will be situated at RA = 03:08.3; Decl. = +47:39 (eq. 2000), i.e. in the constellation Perseus. It will be west of the star Alpha Persei (Mirfak or Algenib) and of about the same total magnitude. The location is southeast of Cassiopeia's 'W' and within the Milky Way band. There is no doubt that this rare viewing opportunity will result in fine sky-photos becoming available and showing both the comet and the Milky Way stars and nebulae in this area.

For interested persons in the Munich area: The Munich Public Observatory (Volkssternwarte) is arranging a special session during the night of the eclipse, beginning at 23:00 and ending at 04:00 in the morning.

Fragments, jets and shells

Some discussion has recently taken place on the net among observers who have detected fragments (condensations) near the main nucleus. At one moment, a report about a possible splitting of the main nucleus was also received, but since such an effect was not observed elsewhere, and in particular not on the high-angular resolution HST images obtained at about the same time, it was most probably not real.

Outward-moving condensations have been observed in many places and it has been possible to follow some of them for several days. Based on the positions (relative to the main nucleus), Zdenek Sekanina (JPL), an expert on these matters, finds that one of these condensations is typical of a 'minor cometary companion', as observed in several other comets (IAUC 6360). This particular fragment apparently split from the main nucleus on March 21.0 and the predicted lifetime is about 10 - 20 days. This refers to the time it is visible - the corresponding cessation of activity is believed to result from complete evaporation of the fragment.

A further analysis (IAUC 6364) has now been made of the Wendelstein images , first reported on this Homepage. The shell structures are apparently identical with the remnants of old jets from previous revolutions of the nucleus. Up to three generations of signatures from on-going and previous jet activities are seen and they are most likely produced by dust. The projected shell expansion velocity is estimated to be of the order of 250 m/s.

In this conncetion, it is also of interest to note the infrared measurements reported by the University of Minnesota observers (IAUC 6365) and which indicate an unsually high temperature of the dust in the coma. On March 23, the measurements showed variations on short time-scales.

Size and rotation of the nucleus

More information has become available about the March 24-25 radar observations, reported earlier with the Goldstone antenna. Steve Ostro estimates that the nucleus is significantly smaller than thought until now; the radar echoes point towards a diameter of 1 - 3 kilometres , rather than 10 km. This indicates that the nucleus must have an unusually high activity per surface area.

If this smaller size is indeed correct, it would not be the first time that a cometary nucleus has turned out to be smaller than estimated. Recent observations made at La Silla and Mauna Kea on Hawaii of receding, very distant comets (where the activity has completely ceased and only the 'naked nucleus' is visible) have shown most of the nuclei to be only a few kilometres across.

This would also explain why the Hubble Space Telescope may not have been able to view the nucleus of Comet Hyakutake as an extended object, despite the pixel size of these images which measures only about 7.5 km. Still, the ongoing analysis of this material may well bring some corroborating evidence to the radar masurements.

The rotation of the nucleus appears to be much faster than that of Comet Halley, but it is not known whether this has anything to do with its smaller size. From the location of jets, observed to emanate from the active areas on the nucleus on Pic du Midi and ESO NTT images, Laurent Jorda and Jean Lecacheux now estimate that the rotation period is only 6.1 hours (as opposed to at least 2 days in the case of Halley). They also suggest that the nucleus now displays its southern hemisphere to observers on the Earth and that the Sun lies near the nucleus' equatorial plane.

Spectroscopy

The latest addition to the increasing list of detected molecules is CH3CN (IAUC 6364), observed with the IRAM interferometer on the high-altitude Plateau de Bure in France. The detection is quite secure, since two lines were observed. The cometary abundance of this molecule is 20,000 times smaller than that of water.

Deuterium (heavy hydrogen) has also been found in Comet Hyakutake (IAUC 6362). The observation, which was made with the Caltech Submillimeter Observatory on Mar 23 and 24, led to the detection of deuterated water (HDO) at a ratio of 1:3600 to normal water (H2O). It is interesting that this ratio is very nearly the same as that in the Earth's oceans; for Comet Halley, this ratio was twice as high.

Orbit

The lastest orbit from Brian Marsden reaffirms the earlier computations, but slightly changes the deduced orbital periods (IAUC 6359). The earlier revolution period is found to be about 8000 years and the future is now 14,000 years. The latter figure can be much refined after the perihelion passage.