MPI/ESO 2.2-m + EFOSC II (May 14, 1996)

It is an updated version (orbit and ephemeris) of the report issued on December 22, 1995.

Richard M. West (ESO)

General

Newly determined orbital elements show that the comet is now moving in an orbit with a period of about 4200 years. The current passage is therefore not the first one and the comet must have been subjected to heating before.

This comet is characterised by an unusually bright coma at the current distance of 6 AU (900 million km) from the Sun. The coma consists almost exclusively of dust, but emission from CN and CO molecules has also been detected.

Millimetric (radio) telescopes are able to perform observations during the conjunction period. Recent measurements of emission from the CO molecule show that the cometary activity, as characterised by the gas flux, is continuing at about the same level as before, although it is highly variable.

Visual estimates, obtained by experienced amateurs in November 1995, continue to indicate a total magnitude of about 10, i.e. there has been a slight brightening since discovery, approximately as expected from the changing helio- and geocentric distance. No additional, significant intrinsic fading or brightening is therefore evident at this moment.

There appears to have been five outbursts in August-October, beginning with a brightening of the nucleus and followed by the appearence of a dust jet structure which expands and fades. The mean interval between the outbursts is about 19 days, but there are significant variations.

A provisional reduction of observations, obtained with the Hubble Space Telescope (HST) in October at the moment of relative quiescence, points towards a nuclear diameter of about 40 km; this value, however, is quite uncertain. It is unlikely, however, that the diameter is above 70 km.

It is provisionally concluded that comet Hale-Bopp is likely to become comparatively bright, perhaps close to magnitude 0 near perihelion in April 1997. At the same time, it appears that there is now less evidence that it will reach the exceptional brightness of -2 or even more, as earlier suggested in some places. Nevertheless, this prediction of course does not take unexpected events into account, e.g. nuclear splitting, which may lead to an important increase of the brightness.

Observations of gaseous species

CN was first detected in late August in optical spectra obtained with the 4.2-m William Herschel Telescope on La Palma (IAUC 6252). The corresponding production rate was approximately Q(CN) = 6 10e25 mol s-1.

OH emission was searched for with the IUE satellite observatory in late August (IAUC 6244), but no signal was detected. This places a not very constraining upper limit on possible water production of about 10e29 mol s-1.

A first inspection of ultraviolet spectra obtained with the HST in October did not show any gaseous species. The corresponding upper limits have not yet been established.

None of these measurements therefore indicate an exceptional gas production at this heliocentric distance.

Observations of dust

It has now been confirmed that a diffuse image found on a UK Schmidt plate obtained in April 1993 (IAUC 6198, 6202) is indeed of this comet (IAUC 6287). This shows that the dust production must have started already when the comet was at heliocentric distance 13 AU or possibly even before. This appears very unusual, and only a few comets have ever shown activity at this large distance (e.g. P/Halley at 14 AU). However, all such cases occurred after the perihelion passage; in fact, only comet Hale-Bopp has been observed at this large distance before perihelion.

The dust production is characterised by semi-regular outbursts, followed by the emergence of impressive jet structures. They slowly expand while fading into the background. Many of these are displayed on the WWW Hale-Bopp Homepages at the respective observatories. Particularly extensive series are available from the Teide Observatory Hale-Bopp Homepage.

The known outbursts have taken place on the following dates (best estimates): August 16, September 9, September 24, October 14 and October 31. The corresponding time intervals are 24, 15, 20 and 17 days, respectively. The mean interval is 19+-4 days. The nuclear brightening at an outburst is about 2-3 mags in the R-band. More details are available in a recent report by Mark Kidger.

The evolution of the observed jet structures has been analysed, in particular by Zdenek Sekanina (IAUC 6248), who concludes that most originate in one and the same source on the surface of the nucleus. Still, the possibility that more than one vent is active cannot be excluded. Ejection velocities are of the order of 30 - 50 m s-1.

The size of the nucleus

An indirect estimate, based on the measured CO-production, gives a diameter of about 10 - 15 km, or at least, no need to invoke a larger size (IAUC 6234).

High-resolution images were obtained with the HST in September and October, at the time of an outburst and in a quiescent phase, respectively. The October images have been reduced and provisionally interpretated by Harold Weaver. By extrapolating the coma brightness profile towards the centre, a light contribution from the nucleus is detected, despite the fact that the nominal resolution of even the HST at this heliocentric distance is 440 km. The measurement appears to place an upper limit on the size of the nucleus of about 70 km; the most likely value is considered to be around 40 km. More details may be found in the extensive report about these observations.

It is therefore probable that the nucleus of comet Hale-Bopp is at the most a few times larger than that of comet Halley (largest diameter 15 km; mean diameter 10 km), but it cannot be excluded that it is only of about the same size. In that case, the vent(s) must either cover an unusually large part of the surface, or it (they) must be extremely active in order to produce the observed amount of dust.

Orbit and ephemeris

Another ephemeris has been published by Yeomans et al. (UMD).

Upcoming observations