Comet Hyakutake - Some Comments (March 12, 1996)

Dust Jets (ESO NTT Image)

In what follows, I provide some brief comments about the recent observations of this comet. I hope they will be useful for the assessment of its current status, just before its close approach to the Earth. It is based on information from La Silla images and spectra, as well as from various other important sources, including the WWW and the IAU Circulars.

Richard M. West (ESO)

Discovery and Orbit

Six weeks have now passed since the discovery by Yuji Hyakutake of the comet that carries his name and is likely to become the brightest since twenty years. He found the new object on January 31, 1996, when it was at a heliocentric distance of just over 2 AU and at magnitude 11. He described the comet as diffuse and with a central condensation with a coma diameter of 2.5 arcmin (IAUC 6299).

Within 24 hours, several confirmatory observations were obtained. They agreed in general about the brightness (possibly half a magnitude fainter than Hyakutake's estimate) and the appearance. Many astrometric positions were measured within another 48 hours (IAUC 6300 and 6303); they allowed Brian Marsden to calculate a preliminary parabolic orbit (IAUC 6303). It immediately showed the particularly interesting characteristics of the comet's motion: a close approach to the Earth in late March 1996 and perihelion passage in early May 1996. A very cautious magnitude prediction indicated naked-eye visibility over a period of many weeks, from mid-March to after perihelion.

Based on nearly 300 positions, mostly obtained by well-equipped amateurs, the orbit was further refined in early March (IAUC 6329), now firmly fixing the closest approach at geocentric distance 0.102 AU on March 25.3 and the perihelion passage at May 1.4. Moreover, the orbital solution now indicated with some certainty a deviation from an originally parabolic one and the orbital period could be roughly estimated at 10,000 - 20,000 years. Despite the uncertainty, this means that the comet does not come directly from the Oort Cloud, i.e. it is not 'new' , but must have been near the Sun before and thus have experienced heating of its outermost layers.

The comet was discovered in the morning sky, at about 85 deg elongation from the Sun. The magnitude was so bright that some surprise has been expressed why it was not picked up before, in particular with professional wide-field telescopes (as was the case for several earlier, bright comets with near-parabolic orbits). For instance, one year before the discovery, on January 31, 1995, the comet would have been at 95 deg elongation and -35 deg declination, geo- and helio- centric distances of 6.1 AU and 6.3 AU, respectively, and, not the least, at a predicted magnitude of about 17.5. It would at that time have been a relatively easy object for a professional telescope and would have remained so during several months thereafter. However, one reason that it was not found is probably the currently reduced, total activity of large Schmidt telescopes located in the south as compared to earlier years, leading to less intensive coverage at faint magnitudes than before.

The Observations

Observations have been made with many different telescopes during the past weeks. The resulting data range from visual descriptions and magnitude estimates, to medium-dispersion spectroscopy with large ground-based telescopes in the optical and radio regions, as well as UV observations with the IUE Observatory in orbit around the Earth. Observations with the Hubble Space Telescope have not been made yet, but are planned when the comet will be close to the Earth in late March and early April.

Brightness

The brightness increase has been very well documented by visual and CCD observations. The former, made by experienced amateur astronomers in many countries, show an increase from about 11.5 (February 1) to 4.5-5.0 (March 10-11; International Comet Quarterly). Based on this material [and using the common formula: observed mag = absolute mag. + 5 log (geoc. dist. AU) + 2.5 n log (helioc. dist. AU)], Charles Morris has deduced an absolute (intrinsic) magnitude of about 5. This brightness value ranks Comet Hyakutake among the top 30 percent of all observed comets with periods longer than 200 years (somewhat over 500 in all). In terms of visual brightness (which mostly refers to the amount of sunlight reflected from the dust in the coma), there is therefore no doubt that it is among the intrinsically `brighter', long-period comets.

For the proportionality factor n between the logarithmic heliocentric distance and the observed visual brightness, Morris deduces a value of about n = 5. This is somewhat higher than the usually adopted value of 4, which reflects the normal development of a comet with a period in excess of 200 years and which is not of the 'new' type, fresh from the Oort Cloud. If this high value were to hold, Comet Hyakutake may in fact brighten faster than most other comets of this type, as it moves closer to the Sun. Past experience shows, however, that n may be variable and the brightness development may not continue at this rate.

Because of the restricted size of the observed field, CCD measurements are normally limited to the inner part of the coma and most of the avaialble CCD magnitudes are fainter than the visual ones. No detailed information has yet been published about the coma light profiles, as recorded on CCD frames, which may serve to judge the outflow of material.

Morphology

The coma

When first observed, the comet displayed a diffuse coma, measuring a few arcmin across. As soon as the first CCD and photographic images became available, it could be seen that this coma was 'tear'-shaped in the direction away from the Sun. This effect is well explained by the pressure of the sunlight on the released dust grains that tends to push them away from the coma. It is strongest on the small grains which are therefore first swept away; the larger grains will stay longer in the neighborhood of the cometary nucleus. This may induce colour differences between the inner and outer coma, but none have been reported yet.

Some asymmetrical structure was observed already in early February, indicating that the evaporation from the nucleus happens mostly on that side which is turned towards the Sun (as expected). This is particularly obvious on exposures with larger telescopes, which show the distribution of the dust in the immediate neighborhood of the nucleus.

Nevertheless, no obvious 'jets', similar to those which have been repeatedly observed in Comet Hale-Bopp in 1995, have yet been seen in Hyakutake. They may develop at any moment, though, as the surface of the nucleus continues to be heated.

Although the exact measure is highly instrument-dependent, the overall size of the coma has now grown to more than 20 arcmin. At the present distance of about 0.5 AU, this corresponds to almost 500,000 km. If the physical size (as expected) remains more or less unchanged during the next weeks, the comet head may therefore measure 3-4 lunar diameters across (1.5 - 2 deg) during the closest approach.

The tails

The first signs of tails were observed in mid-February, at a heliocentric distance of about 1.75 AU. At that moment, a faint ion tail was first observed, extending some 10 arcmin from the inner coma, and most probably shining in the light of ionized water (H2O+). A short, stubby dust tail was also seen. The `onset' of well-developed tails is normally observed at a heliocentric distance of about 1.5 AU; i.e. Comet Hyakutake is rather normal in this sense.

Later observations have shown a rather faint H2O+ ion tail, up to several degrees long and with the typical, complex structure which is caused by the interaction between the charged particles in the tail and the solar wind. `Disconnection events' of the ion tail, due to the comet's crossing of magnetic boundaries in interplanetary space, were observed on at least two occasions (February 28 and March 10), possibly on more.

So far, also the dust tail has not become very conspicuous. This is not abnormal for a comet of Hyakutake's type, but if the dust production increases normally, the development of a better visible dust tail ought to start soon. However, most dust is released around and after perihelion, and not at this time before the perihelion passage.

Production Rates and Composition

As the surface is heated, the cometary nucleus (the `dirty snowball') is expected to release increasing amounts of dust and gas. The corresponding production rates of dust and various gaseous species in the cometary coma of Hyakutake have been studied in different ways.

Observations of the optical spectrum, beginning on February 8, have until now shown a quite normal development, with strong emission lines of CN and C2, and weaker emission by C3 and NH2. Lines of atomic oxygen (O) have also been seen, but no obvious lines of ionic species are yet visible (IAUC 6306 and 6332). This agrees with the so far observed, comparative faintness of the ion tails.

The latest optical spectrum obtained at ESO (March 8) shows a continued, normal development with more lines and bands becoming visible, as the heliocentric distance continues to decrease.

On February 7, observations with the NASA Infrared Telescope Facility at Mauna Kea (IAUC 6336) showed that the colour of the sunlight reflected from the grains in the comet coma was somewhat redder than that of the Sun, suggesting that relatively few icy grains were present.

Two days later, on February 9, quantitative photometric measurements at the Lowell observatory by means of narrow-band filtres showed a quite vigorous production of gaseous species: log Q(OH) = 28.70 (molecules/sec), log Q(CN) = 26.16, log (C2) = 26.15 (IAUC 6311), i.e. somewhat smaller (30 percent in the case of OH) than what was observed for Comet Halley at the same distance.

On February 11, when Comet Hyakutake was still 1.85 AU from the Sun, HCN emission was detected by the JCM telescope at Mauna Kea (IAUC 6318). The production rate was around log Q(HCN) = 26 or about the same as measured from Comet Halley at a heliocentric distance of 1.65 AU. The velocity of the outflow was high, about 550 m/sec, relative to the cometary nucleus.

With the same telescope, CO molecules were detected on March 1 and 2 (IAUC 6335); the production was about log Q(CO) = 28.0 and again similar to Halley.

Thus the measured gas production rates by these instruments indicate that Comet Hyakutake at the time of the observations was about as active as Comet Halley at the corresponding heliocentric distances. This also holds for most of the ratios of the production of different molecules, e.g. Q(CO)/Q(OH) which is about 0.2 (CO/H2O was about 0.25 in Halley).

The gas and dust production was also measured during several observing rounds with the IUE observatory (IAUC 6333). In particular, measurements on February 19, 22 and 27 did not show any significant increase in the production of OH and CS molecules, nor of the dust production over this interval. In fact, the dust became more reddish, signifying that comparitively few icy grains were released.

Observations at Lowell Observatory with narrow optical filtres appear to arrive at the same result for the OH and CN production in the interval February 9 - 25, whereas the C2 production increased by 50 percent and the dust production with 80 percent.

The Nucleus

So far, the nucleus has not been directly observed and it is therefore not possible to judge its size. However, the above similarity between Comet Hyakutake and Comet Halley (what concerns the level of activity), indicate, as a first and rough guess, that their nuclei are of about the same size, i.e. that Hyakutake's nucleus has a mean diameter of approx. 10 km. Nevertheless, it may well be that the active area on Hyakutake's nucleus is significantly different from that on Halley's (about 10 percent of the total surface area); if so, this would of course invalidate this estimate.

Summary

Brightness

In summary, while there may be some concern about the apparent lack of water and dust production, as observed with IUE in late February, and perhaps also because of the still relatively faint tails, it does appear that the comet continues to develop more or less normally. The gas production is found to be similar to that of Comet Halley at about the same distance, in any case showing that Comet Hyakutake must be a major comet.

Until now, the visual brightness has developed nominally, perhaps even somewhat faster than expected. However, the crucial parameter in this connection which will determine the further development is the production of dust from the nucleus (the brightness is mostly caused by sunlight reflected in the dust grains). Water is the most common molecule in the nucleus (about 80 percent in Halley) and the comet is now so close to the Sun that water ice on its surface evaporates freely. Dust is mostly released by this process (the water vapour pushes the dust grains outwards, away from the surface of the nucleus) and the dust production is therefore directly connected to the water production. For any predictions about the comet's future brightness, it will therefore be especially important to continue to monitor the water production rate.

Fortunately, there is now no doubt that whatever happens in this direction, the comet cannot fail to become a most interesting object when it passes the Earth in less than two week's time. Even if the dust production were to remain at the present level (and this is rather unlikely), there is already enough dust in the coma to produce an impressive sight. It is a safe bet to predict that the total magnitude around March 25 will be at least 1, and the chances that it will become even brighter are not so bad.

Thus, the prediction that this will be the brightest comet since the year 1556 to come this close to the Earth may still hold. Nevertheless, it must of course be remarked that the comet will be very large in the sky, so the surface brightness will only be moderate.

Tails

It is much more difficult to make predictions about the tail. At this moment, the tail development is still somewhat modest and it is not excluded, there will only be a short tail of some degrees visible to the unaided eye. It will also be foreshortened by the viewing geometry, as the comet approaches the Earth. However, if Comet Hyakutake develops normally from now on, the ion tail ought to become much more visible during the next few weeks. Thus, there is still good hope that, when seen in a dark sky, also the tail(s) will provide a fine view.

The tail geometry calculations indicate that the Earth will not pass through the comet's tail. During the closest approach, the tail will swing from West to East and three days later, on March 28, the Earth will pass through the comet's orbital plane. At this time the tail(s) will be seen directly from the side and therefore be very narrow. Thereafter, the tail will `open up' and become more impressive, as the comet continues to move inward towards the Sun.

The Need for Rapid and Accurate Astrometry

The comet's extremely rapid motion during the days of closest Earth approach, up to about 3/4 degree/hour, will cause serious problems for the pointing and guiding of large telescopes. In particular, those which rely on blind pointing, e.g. submillimetre and radio-telescopes, will need very accurate predictions of the positions in order to safely acquire the comet during this period.

Astrometric observers are therefore urged to continue to provide accurate positions which will allow to chart the comet's course in the sky with arcsec-accuracy, also at the time of closest approach. The requirements are very particular: not only must the timing be of the order of 1 time-second (the comet moves nearly 1 arcsec in one time-second!), but in order to remain useful, the positions must also be made available to the computers of orbits and from there to the observers within the shortest possible time.

It may be desirable (and even necessary) to set up a special, temporary service (network) in order to achieve this goal.