ar X iv : a st ro - p h / 04 07 55 6 v 1 2 7 Ju l 2 00 4 Unraveling the Helix Nebula : Its Structure and Knots 12

Through HST imaging of the inner part of the main-ring of the Helix Nebula together with CTIO 4-m images of the fainter outer parts, we have an unprecedented-quality view of the nearest bright planetary nebula. These images have allowed determination that the main-ring of the nebula is composed of an inner-disk of about 499 diameter (0.52 pc) surrounded by an outer-ring (in reality a torus) of 742 diameter (0.77 pc) whose plane is highly inclined to the plane of the disk. This outer-ring is surrounded by an outermost-ring of 1500 (1.76 pc) diameter which is flattened on the side colliding with the ambient interstellar medium. The inner-disk has an extended distribution of low density gas along its rotational axis of symmetry and the disk is optically thick to ionizing radiation, as is the outer-ring. Published radial velocities of the knots provides support for the two-component structure of the main-ring of the nebula and to the idea that the knots found there are expanding along with the nebular material from which it recently originated. These velocities indicate a spatial expansion velocity of the inner-disk of 40 km s and 32 km s for the outer-ring, which yields expansion ages of 6,560 yrs and 12,100 yrs respectively. The outermost-ring may be partially ionized through scattered recombination continuum from the inner parts of the nebula, but shocks certainly are occurring in it. This outermost-ring probably represents a third period of mass-loss by the central star. There is one compact, outer object which is unexplained, showing shock structures indicating a different orientation of the gas flow from that of the nebula. There is a change in the morphology of the knots as a function of the distance from the local ionization front. This supports a scenario in which the knots are formed in or near the ionization front and are then sculpted by the stellar radiation from the central star as the ionization front advances beyond them. Subject headings: planetary nebulae:individual(NGC 7293) 1