The surprising emission distribution within the Helix Nebula cometary knots

We compare the morphology of the cometary knots in the Helix Nebula in diff erent emission lines using Hubble Space Telescope Wide Field Planetary Came ra 2 and Space Telescope Imaging Spectrograph observations. We find that th e [N II] 6658 Angstrom line emission from the cometary heads is displaced w ith respect to H alpha, peaking at a position that is closer to the central ionizing star. This result seems at first sight to be in conflict with sim ple photoionization models, which predict that the [N II] emission is close r to the ionization front OF) because it is confined to a thin H+-He-0 laye r, a prediction confirmed by calculations with both our own and the CLOUDY programs. However, the ratio of [N II] to H alpha is very temperature sensi tive, and the observations can be explained if the knots are modeled as pho toevaporating globules. In this case, there is a strong temperature gradien t across the IF, resulting in the [N II] emission being depressed in the pa rtially neutral zones. We also find a strong correlation between the streng th of the [N II] and [O III] emission in individual knots, with both being higher in the knots that are closer to the central star. On the current evi dence, it is unclear whether this is due to inter-knot variations in metal abundances or in gas temperature.