Dust Activity in Comet Hale-Bopp

Abstract

Comet Hale-Bopp has shown an extremely elevated level of dust activity during its apparition. We examine the dust activity of the comet, particularly the dust jets which were observed so frequently between discovery and perihelion + 5 months. Other results from observations carried out during the observing campaign have shown that the dust emission was unusually strong, as witnessed by the considerable jet activity. It is also seen that the dust emission started at an unusually large heliocentric distance and that the dust to gas ratio was particularly large. This final factor is probably one of the key issues in understanding the activity of the comet close to perihelion. Periodicity in the jet activity, which appears to be related to the precession cycle of the comet, is seen clearly in 1995 and, more ambiguously, during 1996. A full understanding of the jet activity in 1995 requires a combination of nucleus topology, the changing solar aspect of the nucleus and precession. However, the stability of the jets seen in 1996 shows that the rotational state must be only minimally excited. A key issue which must be addressed is how typical comet Hale-Bopp's dust and dust activity are in relation to other comets? Whilst the gas activity is found to be absolutely typical (although much greater than for most comets), superheat values suggest that comet Hale-Bopp's dust was highly atypical in some senses, but only when the comet was close to perihelion. These abnormalities should not prevent comet Hale-Bopp from becoming a baseline reference for comparison with other comets.