Collimation of cometary dust jets and filaments

Abstract

It has been suggested that cometary dust jets, as for example imaged by the Halley Multicolour Camera (HMC), possibly originate from vents, or crater-like surface features. Dust flow emitted from such indentations is collimated if compared to emission from a flat surface. Dust liberated from the bottom of a cometary "crater" emerges at the surface level (top of the crater) with a finite velocity. As a consequence these dust particles have a larger outward (radial) momentum than particles leaving the surrounding surface with zero initial velocity. The resultant collimation of dust trajectories (reduced opening angle of the dust jet) is calculated as a function of crater depth and physical parameters of the dust grains applying axisymmetric gas dynamic code. Fine structures observed by HMC in cometary dust jets can be modelled by emission from active regions with inactive centres. The decrease of pressure above the non-sublimating surface leads to a converging gas flow that concentrates larger particles in a radial filament.