Observations of Comet Hyakutake

Figure 1: Comet Hyakutake above Hale Pohaku. Photo courtesy of Bill Dent.

Comets are interesting particularly because they are thought to be relatively pristine samples of the material from which the Sun and planets formed. Also, Hyakutake (and Hale-Bopp, as it happens) has a very long period, now estimated at perhaps around 30,000 years, suggesting that the material is less processed by repeated perihelion passages than, say, Comet Halley. Normally comets are rather faint targets for radio telescopes. But Hyakutake, by virtue of its passage close to the Earth gave an unprecedented opportunity to study the composition and development of the molecular and dust components of the cometary coma with excellent sensitivity and spatial resolution. In fact, the first detection of molecules in the coma (the J=4-3 rotational transition of HCN) was made already on February 10 at the JCMT. Subsequently, CH3OH was first seen on February 27 and CO J=3-2 was found on March 1, both at the JCMT. Like most other telescopes, the observing schedule of the JCMT for the following months was already well in place when Hyakutake was discovered, and because of this an international consortium was put together to make target-of-opportunity observations, shoehorned into the existing schedule of the JCMT. The consortium involved people (listed at the end of this article) from the Joint Astronomy Centre, the University of Hawaii at Manoa, and the Observatoire de Paris at Meudon.

The observing program at the JCMT focused on aspects well suited to the sub-mm capabilities of the telescope, and in areas where the data obtained could complement those from other facilities. Largely for these reasons we found ourselves observing mostly with the heterodyne receiver B3i (300 - 380 GHz), with additional observations making use of the continuum bolometer system UKT14. In the latter case, this was the last major program carried out with this venerable instrument. The observations we obtained at the JCMT fall into three groups:

a) Monitoring of the development of key molecular transitions in the coma. These were CO J=3-2 at 345.8 GHz, HCN J=4-3 at 354.5 GHz and a pair of transitions of CH3OH J(k,k') = 2(1,2) - 2(0,1) and J(k,k") = 4(1,2) - 4(0,1) at 304.2 and 307.2 GHz. The last two transitions offered a means to determine the kinetic temperature of the coma gas from their intensity ratio. Conveniently, because of the particular configuration of the receiver, it was possible to observe these two lines simultaneously. For the same reason, it was also possible to observe the CS J=7-6 and H2CO J(k,k") = 5(1,5) - 4(1,4) transitions in combination with CO and HCN, respectively.

Figure 2: HCN spectra obtained as a function of time. The bottom spectrum was taken on February 10th with more recent spectra going up the figure. See text for details.

b) Searches for other species. In addition to CS and H2CO, which were obtained "for free" as mentioned above, we made efforts to detect HNC, H3O+, CH3CN and other, higher excitation, transitions of H2CO and CH3OH in particular.

c) Continuum observations. We were able to obtain both continuum spectra of the emission from the dust in the coma at wavelengths from 2 mm to 350 microns, as well as simple maps at 800 microns wavelength of its distribution at a time just before close approach. For comparison we obtained crude maps of the distribution of gas in the coma, using the HCN 4-3 transition.

A number of first results of this work have been published in a series of IAU Circulars, as was appropriate at the time for rapid dissemination of the information. The weightier syntheses and interpretation is now proceeding. However a couple of examples of the data obtained will illustrate the quality of the material with which we are working.

Figure 2 shows a series of HCN 4-3 spectra obtained throughout the lifetime of this program (February through June 1996; in early July Hyakutake was already far to the South and receding rapidly from Earth).

Figure 3: Map of the HCN J=4-3 emission taken just after the comet's closest approach to the Earth.

In this figure all the spectra are shown on a common velocity scale, with 0 km/sec representing the geocentric velocity of the comet at the time. All seven representative spectra are shown on a common antenna temperature scale, offset from one another by varying amounts for clarity. The earliest HCN spectrum, that of the initial discovery on February 10, is shown at the bottom. At this time, Hyakutake was about 1.85 a.u. from the Sun, approaching the Earth at a distance of 1.46 a.u. Thereafter the HCN line strength grew rapidly, the line width increased and a distinctive double-peaked structure quickly developed, as shown in the third spectrum, obtained shortly after closest Earth approach, when the comet was about 0.94 a.u. from the Sun. As Hyakutake approached the Sun, the HCN 4-3 emission became weaker, as a result of the combined effects of increasing excitation temperature and distance. The second spectrum from the top was taken a few days after perihelion (May 1); it is arguable whether anything has been detected, and at this time the orbital elements were rather poorly known. After perihelion passage, as shown in the topmost spectrum, when the comet was about 0.73 and 1.15 a.u. from the Sun and the earth respectively, the line profile once again became quite visible and maintained a double-peaked structure.

A simple preliminary map of the HCN J=4-3 emission from Hyakutake is shown in Figure 3. These data were obtained just after closest Earth approach. They show that the coma was noticeably more extended than the beamsize of 14.5 arcsec.

The spectral lines observed toward Hyakutake are the strongest ever observed in a comet, and as a result we were able, even in the limited time slots available to us, to obtain spectacular results for other molecular transitions. We detected strong H2CO and CS, the latter for the first time in the radio/mm/sub-mm regime, and also CH3OH. Perhaps most significantly, we also detected HNC toward Hyakutake on March 16. This was the first time HNC had been detected in a comet, and the result was subsequently confirmed at the CSO when the comet was much closer, and the transition considerably brighter. The abundance ratio of HNC with respect to HCN is typical of that of interstellar clouds with temperatures of around 40 K; the significance of this result is discussed in a paper submitted to Nature (Irvine et al; 1996).

In Figure 4, a continuum spectrum of Hyakutake obtained in the days before closest Earth approach is shown. This is the first high quality measurement of the continuum spectrum of any comet. The emissivity index determined from the slope of the spectrum is 0.8. Just as in the disks of pre-main sequence stars, the submillimeter continuum is due to thermal radiation from solid grains. In the case of Hyakutake, the measured emission is from about 106 tonnes of dust located within the central 1300 km of the coma. These dust grains have sizes ranging up to centimeters, and were formed in the proto-sun's accretion disk prior to the accumulation of the cometary nucleus. The properties of the Hyakutake spectrum, and of the grains responsible for it, may thus convey invaluable information about solar- system processes identical to those occuring in the disks of T-Tauri stars.

Figure 4: Continuum spectrum of the comet.

The passage of Hyakutake through the inner solar system was a dramatic curtain-raiser to the forthcoming show cautiously expected of Hale-Bopp. With the bright line and continuum emission observed towards Hyakutake, we now have a better idea of what to look for in Hale-Bopp in the sub-mm region. However, comets are notoriously fickle where expectations are concerned. Hale-Bopp, unlike Hyakutake, will not come very close to the Earth. On the other hand, it seems to be a much more active object, and this may compensate. One respect in which Hale-Bopp is better for mm/sub-mm astronomy results from the fact that the perihelion distance for it is only a little less than 1 a.u.; thus we can expect that excitation temperatures for Hale-Bopp, unlike Hyakutake, will remain in the region favourable to the production of strong lines in our wavelength band.

Throughout this campaign we were supported by the tireless efforts of Brian Marsden, Dave Tholen and Don Yeomans in the production of osculating elements and ephemerides for C/1996 B2. Their role was crucial in ensuring that the JCMT was pointed toward the correct position. This was not at all an easy task when one considers that at closest approach the comet was moving at an angular rate well in excess of 10 degrees per day. The situation was further compounded near perihelion, when non- gravitational forces conspired to modify the orbital elements at a time when the comet was essentially impossible to see.

Henry Matthews, JAC

Other members of the Hyakutake consortium:

Joint Astronomy Centre, Hilo, Hawaii:

John Davies & Bill Dent

University of Hawaii at Manoa, Honolulu:

Dave Jewitt, Toby Owen & Matt Senay

Observatoire de Paris, Meudon, France:

Nicolas Biver, Dominique Bockelee- Morvan, Jacques Crovisier, Daniel Gautier & Heike Rauer