We present HST/NICMOS imaging of the H _ { 2 } 2.12 \mu m emission in 5 fields in the Helix Nebula ranging in radial distance from 250–450″ from the central star .
The images reveal arcuate structures with their apexes pointing towards the central star .
These molecular hydrogen knots are most highly structured in the fields closest to the central star and become increasingly less structured with increasing radius .
Comparison of these images with comparable resolution ground based images reveals that the molecular gas is more highly clumped than the ionized gas line tracers .
From our images , we determine an average number density of knots in the molecular gas ranging from 162 knots/arcmin ^ { 2 } in the denser regions to 18 knots/arcmin ^ { 2 } in the lower density outer regions .
The decreasing number density of H _ { 2 } knots in the outer regions creates a lower filling factor of neutral and molecular gas emission in the radio observations of CO and HI and may explain why these outer regions , where we clearly detect H _ { 2 } 2.12 \mu m , fall below the detection limit of the radio observations .
Using this new number density , we estimate that the total number of knots in the Helix to be \sim 23,000 which is a factor of 6.5 larger than previous estimates .
The total neutral gas mass in the Helix is 0.35 M _ { \odot } assuming a mass of \sim 1.5 \times 10 ^ { -5 } M _ { \odot } for the individual knots .
The H _ { 2 } emission structure of the entire Helix nebula supports the recent interpretation of the Helix as a nearly pole-on poly-polar planetary nebula .
The H _ { 2 } intensity , 5–9 \times 10 ^ { -5 } erg s ^ { -1 } cm ^ { -2 } sr ^ { -1 } , remains relatively constant with projected distance from the central star suggesting a heating mechanism for the molecular gas that is distributed almost uniformly in the knots throughout the nebula .
The temperature and H _ { 2 } 2.12 \mu m intensity of the knots can be approximately explained by photodissociation regions ( PDRs ) in the individual knots ; however , theoretical PDR models of PN under-predict the intensities of some knots by a factor of 10 .
The brightest H _ { 2 } emission ( \sim 3 \times 10 ^ { -4 } erg s ^ { -1 } cm ^ { -2 } sr ^ { -1 } ) may be enhanced by a larger than unity area filling factor of H _ { 2 } knots or may be an individual H _ { 2 } knot exposed to direct starlight causing rapid photoevaporation compared with the more embedded knots of the disk .