COLLIMATION OF COMETARY DUST JETS AND FILAMENTS

It has been suggested that cometary dust jets, as for example imaged b y the Halley Multicolour Camera (HMC), possibly originate from vents, or craterlike surface features. Dust flow emitted from such indentatio ns is collimated if compared to emission from a flat surface. Dust lib erated from the bottom of a cometary ''crater'' emerges at the surface level (top of the crater) with a finite velocity. As a consequence th ese dust particles have a larger outward (radial) momentum than partic les leaving the surrounding surface with zero initial velocity. The re sultant collimation of dust trajectories (reduced opening angle of the dust jet) is calculated as a function of crater depth and physical pa rameters of the dust grains applying axisymmetric gas dynamic code. Fi ne structures observed by HMC in cometary dust jets can be modelled by emission from active regions with inactive centres. The decrease of p ressure above the non-sublimating surface leads to a converging gas fl ow that concentrates larger particles in a radial filament.