The tails in the Helix Nebula NGC 7293
  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. Hydrodynamic calculations reveal velocity structures which 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 faint striations visible in the nebula gas to be plausible. 
J.E. Dyson (Leeds, UK), J.M. Pittard (Leeds, UK), J. Meaburn   (Manchester, UK), S.A.E.G. Falle (Leeds, UK)