The creation of the Helix planetary nebula (NGC 7293) by multiple events

A deep continuum-subtracted image of NGC 7293 has been obtained in the light of the Hα+[N II] emission lines. New images of two filamentary halo structures have been obtained, and a possible detection of a collimated outflow made.

Spatially resolved long-slit profiles of the Hα+[N II] lines have been observed across several of these features with the Manchester echelle spectrometer combined with the San Pedro Martir 2.1-m telescope; these are compared with the [NII] 6584Å, [OIII] 5007Å, HeII 6560Åand Hα profiles obtained over the nebular core.

The central HeII emission originates in a ~0.34-pc diameter spherical volume expanding at <=12 km s^-1, which is surrounded by, and partially coincident with, an [OIII] 5007 Åemitting inner shell expanding at 12 km s^-1. The bright helical structure surrounding this inner region is modelled as a bipolar nebula with lobe expansions of 25 km s^-1 whose axis is tilted at 37° to the sightline but with a toroidal waist itself expanding at 14 km s^-1.

These observations are compared with the expectations of the interacting two-winds model for the formation of planetary nebulae. Only after the fast wind has switched off could this global velocity structure be generated. Ablated flows must complicate any interpretation.

It is suggested that the clumpy nature of much of the material could play a part in creating the radial `spokes' shown here to be apparently present close to the central star. These `spokes' could in fact be the persistent tails of cometary globules whose heads have now completely photoevaporated.

A halo arc projecting from the north-east of the bright core has a counterpart to the south-east. Anomolies in the position-velocity arrays of line profiles could suggest that these are part of an expanding disc not aligned with the central helical structure, although bipolar lobes expanding along a tilted axis are not ruled out.