  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.

Astrophysics