Interpretation of the inner coma observations of comet P/Halley by the Halley Multicolour Camera.

Abstract

The brightness dependence with distance of the inner coma of comet P/Halley, as observed by the Halley Multicolour Camera on board Giotto, shows 3 distinct regions. From about 400 - 2000 km from the nucleus, deviations from a z^-1 law are evident where z is the distance from the nucleus centre. Fountain model calculations indicate that solar radiation pressure acting on old dust particles can only produce a minor percentage of the deviation. Models of the brightness of gas emission at these distances fit the observational data well. From about 100 - 600 km from the nucleus, the brightness closely follows a z^-1 law. From 10 to about 150 km from the nucleus centre, strong deviations from z^-1 are visible. A detailed discussion of several mechanisms which can produce deviations from z^-1 is presented with particular reference to the observations. It is concluded that 26% of the observed intensity at 120 km was due to additional particles created more than 9 km from the nucleus centre. Modelling the observed behaviour using spherically symmetric production and an exponential decay gives a scale length for fragmentation of 15 - 20 km and splitting at rates greater than 2.7 daughters per parent. The inferred fragmentation implies that the dust size distribution observed 600 km from the nucleus gives a distribution of smaller sizes than at the immediate source.