ING OBSERVATIONS OF COMET HALE-BOPP

Comet Hale-Bopp was discovered at a heliocentric distance of 7.2 AU in July 1995. What was significant about this discovery was both the large distance at which it was discovered, and that it was already at an integrated magnitude of ~10.5. To put this into context, at the same distance from the Sun Comet Halley was at V=22.8. This difference was mostly due to the fact that Hale-Bopp had generated an atmosphere, or coma, around itself, while Halley had not. At such large distances the optical coma of a comet is dominated by scatered sunlight from dust grains. These are released from the comet nucleus (generally 1–20 km in diameter) through sublimation of surface ices, at this distance primarily volatiles such as CO. Therefore the presence of so much dust implied an extremely active nucleus, with either a large fraction of its surface undergoing outgassing, or perhaps just a very large nucleus.

Subsequent spectrophotometry with the WHT a month after discovery revealed the presence of the CN molecular band, formed from the HCN being released from the nucleus and then being photo-dissociated via solar UV photons. Monte-Carlo modelling of these data revealed an outgassing rate for the parent HCN molecule of 6e25 mol./second. This confirmed the high activity of the nucleus, as Halley had an outgassing rate a factor of 10 lower when it was at 4.5 AU from the Sun. This meant that the discovery of Comet Hale-Bopp at an unusually large heliocentric distance provided an unprecedented opportunity to follow its evolution from beyond Jupiter into the inner Solar System. To take advantage of this, spectroscopic follow-up was carried out using variously the WHT with ISIS and the INT with the IDS. A spectrum of the comet was obtained on 3 September 1996. Even though the comet was still 3.2 AU from the Sun, where most comets show little activity, Hale-Bopp had a spectrum tremendously rich in molecular species.

While the gradual brightening of the comet is clear, any short-term variability in the dust production, and hence outgassing, rate is difficult to obtain from these observations. Therefore in August 1996 CCD imaging of Hale-Bopp was obtained with the JKT over 13 nights, with the primary goal being an investigation into the short-term (hours–days) variability of the comet. By fitting the comet images with a modelled isophote distribution and subtracted it to reveal more clearly the underlying structure, a similar process to that used in the study of shell galaxies, it is possible to study the morphology of the coma. On 27 August 1996 comet Hale-Bopp was imaged with an R-band filter in seeing of 0.6 arcseconds using the JKT. Six well defined jets were seen emanating from the nucleus. These were due to the outgassing from the nucleus being confined to several localised hotspots, where the insulating mantle was thin or non-existent thereby allowing heating of the nuclear ices.