A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space   Telescope
  We have conducted a combined observational and theoretical investigation of the ubiquitous knots in the Helix Nebula (NGC 7293). We have constructed a combined hydrodynamic+radiation model for the ionized portion of these knots and have accurately calculated a static model for their molecular regions. Imaging observations in optical emission lines were made with the Hubble Space Telescope's STIS spectrograph, operating in a "slitless" mode, complemented by WFPC2 images in several of the same lines. The NICMOS camera was used to image the knots in molecular hydrogen. These observations, when combined with other studies of molecular hydrogen and CO provide a complete characterization of the knots. They possess dense molecular cores of densities about 1,000,000 per cubic centimeter surrounded on the central star side by a zone of hot molecular hydrogen. The temperature of the molecular hydrogen emitting layer defies explanation either through detailed calculations for radiative equilibrium or for simplistic calculations for shock excitation. Further away from the core is the ionized zone, whose peculiar distribution of emission lines is explained by the expansion effects of material flowing through this region. The shadowed region behind the core is the source of most of the CO emission from the knot and is of the low temperature expected for a radiatively heated molecular region. 
C. R. O'Dell, W. J. Henney, G. J. Ferland