The structure of cometary knots in the Helix Nebula

The H alpha surface brightness distribution of cometary knots in the Helix Nebula is investigated. The brightness profiles of their photoionized surfaces (cusps) drop exponentially with increasing distance r from the center of the knots: Sigma(r) similar to exp (-r/h). An r(-3) profile, which would be expected for a freely expanding ionization region, in general does not provide a good fit. The cusps of all the knots have the same universal exponential brightness scale height h of 2 pixels (6.3 x 10(14) cm), which is also independent of the direction with respect to the central star. The origin of the cometary knots and the formation and nature of their luminous cusps is investigated. We propose that the cusps are in hydrostatic equilibrium rather than freely expanding, but we have been unable to identify a physical mechanism that will constrain them. Ly alpha radiation pressure acting on the dust is evaluated as a mechanism to do the confinement, but it lacks sufficient energy density, and the scale height expected disagrees with what is observed.