The Multitude of Molecular Hydrogen Knots in the Helix Nebula

We present HST/NICMOS imaging of the H_2 2.12 \mu m emission in 5 fields inthe Helix Nebula ranging in radial distance from 250-450" from the centralstar. The images reveal arcuate structures with their apexes pointing towardsthe central star. Comparison of these images with comparable resolution groundbased images reveals that the molecular gas is more highly clumped than theionized gas line tracers. From our images, we determine an average numberdensity of knots in the molecular gas ranging from 162 knots/arcmin^2 in thedenser regions to 18 knots/arcmin^2 in the lower density outer regions. Usingthis new number density, we estimate that the total number of knots in theHelix to be ~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\~1.5x10^{-5} M_\odot for the individual knots. The H_2 intensity, 5-9x10^{-5}erg s^{-1} cm^{-2} sr^{-1}, remains relatively constant with projected distancefrom the central star suggesting a heating mechanism for the molecular gas thatis distributed almost uniformly in the knots throughout the nebula. Thetemperature and H_2 2.12 \mu m intensity of the knots can be approximatelyexplained by photodissociation regions (PDRs) in the individual knots; however,theoretical PDR models of PN under-predict the intensities of some knots by afactor of 10.Comment: 26 pages, 3 tables, 10 figures; AJ accepte