The nature of the cometary knots in the Helix Nebula as determined by HST images
Authors: O'Dell C. R. , Burkert A. 
Published: July 2016

In a brief paper at the first meeting of this series, Vorontzov-Velyaminov (1968) presented the first examination of the fine scale structure found in the Helix Nebula. At that time he used the term “filaments” to describe them, although the results of improved spatial resolution during the ensuing decades (Malin 1982, Meaburn et al. 1992, Walsh & Meaburn 1993) show that the term “Cometary Knot”(CK) communicates a clearer picture of their form. The most useful groundbased images are those of Meaburn et al.(1992) made with the New Technology Telescope, which showed that each CK has a characteristic form, composed of a central dark knot, accompanied by a luminous cusp on the substellar side, and accompanied by a thin sheath of luminous material extending from the edge of the cusp to large distances. Owing to their much higher density, the Cusps are of lower ionization than the nebula as a whole, so that one sees the central dark knots, which evidently contain significant amounts of dust, in silhouette against the background nebular emission. Radio observations of the CO line show that the CK are also sources of molecular emission (Huggins et al.1992). The groundbased emission line images show that the “tails” trailing away from the Cusps lie closely on radial lines passing between the Cusp and the Central Star. Although these objects appear to be diving towards the Central Star, such an interpretation is naive, as their form must be determined by a central repulsive force arising from the star, much as true comets in our Solar System may trail their tails behind while approaching the Sun but actually follow their tails as they recede from the Sun. We also now know, from yet another excellent study by Meaburn et al. (1996) that the CK as a group are expanding away from the Central Star, although this group velocity of about 10 km s–1 is about half that value of 21 km s–1 characteristic of the nebula as a whole (Taylor 1977,Terrett 1979). The greatest breakthrough in imaging was the Hubble Space Telescope's WFPC2 (O'Dell & Handron 1996, hereafter O&H) program on a single field of view (FOV) in the north ring of the Helix in Hα, [NII], and [OIII]. One year later the same field was reimaged in [OI] and with the f547m filter, the newer observations allowing calibration of the emission line images with correction for the underlying continuum. A second, contiguous field, was also imaged, in all five filters. The results of both sets of observations are used in this paper and a combined color image is shown in the rear section of this volume. The method of calibration was the same as that used in studies of the Orion Nebula (Hu 1993). Details of the calibration and digital copies of the images can be obtained from the first author. The new observations show heretofore unseen structure which is discussed in the following sections. They indicate that there are about 3500 detectable CK in the entire Helix Nebula, with the actual number probably being much larger as strong observational selection effects operate against detection of objects far from the Central Star. The images also show that the previously known orientation along radial lines is followed, but a detailed examination shows that the tails show small local variations, as if additional, non-radial, forces are acting on them (Fig.4). O&H demonstrated that the chord diameters, measured across the cusps, decrease from about 1.8″ at 120″ distance from the Central Star to about 0.6″ at 180″. There are so many known CK that one cannot use a sequential system of designation. In this paper we will use a coordinate based system, dividing the nebula into boxes of 1″ in Declination and 0.1s of Right Ascension and dropping the values common to all of the CK. Therefore, a CK located at 22:29:42.331 −20:47:32.1 would have the designation Helix 423-732. This system will allow future studies to uniquely identify all CK with only six digits within the range of declination −20:45:00 and −20:55:00, with any objects found north or south of this range having either 4 or 5 inserted as an additional digit immediately after the hyphen. Epoch 2000 is used for the positions and the position is judged to be at the center of the dark knot.