Comet Hyakutake - Update (April 10, 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 April 4 . 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 10, 1996; 14:30 UT

Ephemeris and observing conditions at perihelion

Comet Hyakutake has now passed the orbit of Venus during its inward motion towards perihelion on May 1. A new ephemeris has recently become available from Don Yeomans (JPL). It is based on 636 positional measurements and indicates that the perihelion passage will take place at about UT 09:27 in the morning of that day and at a distance of 34.44 million kilometres from the Sun. The predicted magnitude is -1, i.e. as bright as the brightest star (Sirius) and several times brighter than it was at the closest approach to the Earth in late March 1996.

Unfortunately, at that moment, the angular distance between the comet and the Sun (as seen in the sky) is only about 7 degrees. Thus, it will in principle not be possible to observe the comet at the exact time of the perihelion passage. However, this distance will increase during the following days and on May 10, it will reach 15 degrees, i.e. similar to the distance at which Comet Halley was recovered in mid-February 1986 after its perihelion passage on February 9 that year.

After perihelion, Comet Hyakutake will be best observable from the Southern Hemisphere again.

During the past few nights (April 8 - 9), Comet Hyakutake has been reported by visual observers at magnitude 2.0 - 2.5 and with a dust tail of about 10 degrees. A few observers claim to have seen a much longer ion tail also, but only at completely dark sites. The comet continues to be an impressive view for Northern Hemisphere observers, low in the north-western sky, during the early evening hours, before the waning Moon rises.

Ultraviolet spectra with the HST

The first report about the recent observations with the Hubble Space Telescope has now been published (IAUC 6374). It concerns ultraviolet spectra obtained on April 1 and covering the wavelength region between 1300 A (just above the strong Lyman-alpha line from hydrogen) to 3280 A (and therefore including the strong OH-line at 3060 A). In addition to many lines from carbon monoxyde (CO), the S2 molecule is clearly detected with an abundance of about 0.0001 of that of water. This molecule was first seen in another comet that made a close approach to the Earth in 1983 (IRAS-Araki-Alcock) and strong S2-lines were also detected in the UV 'splash' spectra obtained with the HST at the time of Comet Shoemaker-Levy 9's collision with Jupiter in July 1994.

The HST team, led by Harold Weaver (Space Telescope Science Institute, Baltimore) has also found several, as yet unidentified emission in the ultraviolet spectrum of Comet Hyakutake.

Polarimetry

The first polarimetric observations of Comet Hyakutake have just been reported by a team of Indian astronomers, with a 1.2-metre telescope at Gurushikhar. These measurements indicate a polarization of about 6 percent of the light received from the dust coma. Similar measurements at other comets, e.g. Comet Halley in 1986, show a typical change of this value with the phase angle (Sun - Comet - Earth); from the way this change takes place, important information about the size and (in the optimal case) the composition of the dust grains in the coma, can be obtained.

Variations in dust and gas production

Further measurements have been made at the Lowell observatory (Arizona) which show short-term variations in the gas and dust production rates during the period March 23 - 30 (IAUC 6372). A search for periodicities did not (yet) yield one outstanding period which might be interpreted as the rotation period of the nucleus. Nevertheless, one possibility is about 6.25 hours, close to the period deduced by the French group at Pic du Midi by means of the jets observed earlier. Although multipla of this number cannot be ruled out, there is no doubt that a rotation period of this order is the currently best estimate.

Of particular interest in this connection is also the continued observation of the effects of fragmentation. For instance, another condensation, tailward of the nucleus, was observed on a CCD image, obtained from Pic du Midi on April 7. Moreover, images made in the light of CN molecules with a 1.05-metre telescope at the Misato observatory (Japan) on March 26, ressemble the C2 images from Pic du Midi. It is assumed that the very complex structures seen in the near tail region result from evaporation from small fragments.

Statement by Yuji Hyakutake

The discoverer of this comet, Yuji Hyukutake, has made an informative statement to the Press about his life as an amateur astronomer.