Comet Hyakutake Update - March 22, 1996

Comet Hyakutake Update (March 22, 1996)

This is an update to my earlier summaries about observations of this comet, published on the ESO Web on March 12 and 21. They are based on results of recent La Silla observations, as well as on information available on IAU Circulars and on other Hyakutake WWW pages.

Important new information has become available today about 1) more molecules detected; 2) planned HST observations, and 3) ROSAT observations to be performed.

Richard M. West (ESO)

The orbit

Recent orbital calculations confirm that this comet does not come (directly) from the Oort Cloud. A calculation by Don Yeomans, based on 347 reported positions (January 1 - March 18, 1996) gives a period of about 18,400 years. Brian Marsden, from 371 positions (January 1 - March 18, 1996) finds an 'original' period of about 8600 years and a 'future' of about 17000 years (MPEC 1996-F03). The first refers to the orbit before the current entry into the inner solar system and the second to the orbit that will result after the changes (the so-called 'perturbations') induced by the gravitational influence of the planets during the present passage.

The appearance

Visual reports in the morning on March 22 put the comet's total magnitude at about 2, possibly slightly brighter, i.e. almost exactly as predicted one week ago (I can confirm from own experience that it is beautiful naked-eye sight!). If the comet continues to develop in the same way, i.e. if no unusual events occur (very different dust and gas production rates, nuclear splitting), it may be expected to reach a maximum between visual magnitude 1 and 0 at the time of the closest approach on March 25.3.

The head is now reported to be larger than 1 degree, when viewed from a dark site. The maximum size will probably be about 2 degrees (4 lunar diameters).

Ion tails up to 20 degrees long have been reported by naked-eye observers. The dust tail is still much fainter, although the recent increase in dust production, as testified by the new activity near the nucleus, indicates that this tail will from now on become more important. Predictions are extremely difficult, and estimates range from some 30 deg to perhaps twice this size for the straight ion tail and much less for the curved dust tail.

Production rates

The latest gas production rates measured at the Lowell Observatory (IAUC 6344) indicate some variations when compared to Comet Halley at the same heliocentric distance. While the OH-production (i.e. H2O sublimation) finally seems to have taken off (it is now indirectly estimated to be above 10e29 mol/sec; IAUC 6345), i.e. it is now significnatly higher, while the CN-production is about a factor of 3 lower than that of Halley. Both rates are also rising somewhat slower than was the case for Halley. The dust production, which is the main indicator for the future brightness development, is very nearly the same as that of Halley.

Yet another molecule, OCS, has now been detected by means of the NRAO 12-m telescope at Kitt Peak, Arizona, USA (IAUC 6344).

The additional detection with the same telescope of H2CO (formaldehyde) and CH3OH (methyl alcohol), and also again of HCN and CO, was reported on March 21 (IAUC 6345).

The nucleus

High angular resolution observations obtained during two nights (March 18-19 and 19-20)at ESO (see this Homepage) and the Pic-du-Midi Observatory in France (reported by Laurent Jorda and collaborators on IAUC 6344) clearly indicate rapid changes in the inner coma structure on a time scale of minutes. There seems to be at least two curved jets with a length exceeding 2000 km (15 arcsec) and which rotate clockwise. A comparison of these series of images allows a preliminary estimate of the period of rotation of the nucleus. It ranges from about 6 - 10 hours; the French group suggests 6.6 hours as the most likely value.

There is no doubt that the dust production by vents on the surface of the nucleus has now really begun. The onset of this phenomenon was expected for some time and it provides yet another confirmation of Comet Hyakutake's status as a major comet.

The ESO observations, carried out during a spell of very good seeing (down to 0.39 arcsec or 56 km projected), apparently do not show a signal from the nucleus itself. It is therefore likely that it is considerably smaller. It is expected that the upcoming Hubble Space Telescope observations (se below) will achieve significantly sharper images, although the comet's extremely high rate of motion must be exceedingly accurately compensated for in order to fully exploit this telescope's inherent very high angular resolution.

Upcoming observations

A great multitude of observations will be made from observatories in the Northern Hemisphere. Many of these will start within the next few days. No complete listing is available at the present moment, though.

HST observations will be made on four occasions, from March 26 to April 2. They will involve both direct imaging in the UV and visual parts of the spectrum, as well as UV spectra. While the former will provide the sharpest images possible with existing telescopes, the UV spectra are particularly suited to study the atomic and molecular emissions from the coma and tail. The comet's unusual brightness and small distance will provide a unique opportunity for very detailed studies. More details are now available on a HST Press Advisory (March 21).

At NRAO's Very Large Array radio facility in New Mexico (USA), measurements will be made of the thermal (heat) emission from the nucleus. Moreover, a radar experiment will be carried out during which a radar beam from the Goldstone facility in California will be bounced off the cm-size dust surrounding the cometary nucleus (possibly - hopefully - also from the surface of the nucleus, although it is not yet sure whether the dust coma will permit the radar beam to penetrate to it) and the echo will be received by the VLA.

Infrared observations are planned with all major facilities on the Northern Hemisphere.

Observations with the ROSAT X-Ray satellite observatory will be performed on March 26 - 28. The total integration time will be nearly 20,000 seconds.

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