The tails in the Helix Nebula NGC 7293

Aims. We have examined a stream-source model for the production of the cometary tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately supersonic stream of ionized gas overruns a source of ionized gas. We have compared the velocity structures calculated with the available observational data. We have also investigated the suggestion that faint striations visible in the nebular gas are the decaying tails of now destroyed cometary globules. Methods. We have selected relevant results from extensive hydrodynamic calculations made with the COBRA code. Results. The velocities calculated are in good agreement with the observational data on tail velocities and are consistent with observations of the nebular structure. The results also are indicative of a stellar atmosphere origin for the cometary globules. Tail remnants persist for timescales long enough for their identification with striations to be plausible.