HST Observations and Related Models of Cometary Knots in the Helix Nebula

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Session 38 -- Planetary Nebulae and Novae
Display presentation, Wednesday, June 14, 1995, 9:20am - 6:30pm

[38.03] HST Observations and Related Models of Cometary Knots in the Helix Nebula

Kerry Patrick, C.R. O'Dell (Rice University)

We report here on the results of an early analysis of a set of HST WFPC2 images of NGC 7293 made in the filters isolating H$\alpha$, [NII] 6583 $\AA$, and [OIII] 5007 $\AA$. The images were made in a single field on the inner ring, directly north of the central star. The density of Cometary Knots is much greater than detected on ground images, with about 35 objects seen in this field that covers only about one tenth of the inner ring. There is a remarkable similarity of form of most of these Cometary Knots. They are characterized in both H$\alpha$ and [NII] by a bright cusp with the portion nearest the central star being of maximum surface brightness. The widths across the cusps (perpendicular to the central star-Cometary Knot line) are about 2". The tips of these cusps trail into long, radial, almost parallel, streamers. In the [OIII] filter there is little evidence of emission from the Cometary Knots and most are seen as obscuring the background emission arising from the planetary nebula. We have tested the hypothesis that these objects are all optically thick to Lyman continuum ionizing radiation. In such a case there should be a well defined relation between the peak H$\alpha$ surface brightness of a Cometary Knot and its distance from the central star. Such a relationship is seen, supporting the hypothesis. The absolute scale of the relation indicates that the overall planetary nebula intercepts only about one tenth of the Lyman continuum photons. We have examined the small scale structure of the cusps in detail and establish that their surface brightness drops exponentially from its maximum in the direction of the central star. This implies that the density distribution is also exponential which would be the case for a static distribution in equilibrium with a repulsive force arising from the central star. There is no spectroscopic evidence for a stellar wind from the central star, so the force must arise either through radiation pressure acting on the dust mixed in with the gas, or through the flow of the gas constituting the visible planetary nebula.

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