2007AJ....133.2343O

We use new Hubble Space Telescope and archived images to clarify the nature of the ubiquitous knots in the Helix Nebula, which are variously estimated to contain a significant to majority fraction of the material ejected by its central star. We employ published far-infrared spectrophotometry and existing 2.12 µm images to establish that the population distribution of the lowest rovibrational states of H₂ is close to the distribution of a gas in local thermodynamic equilibrium at 988±119 K. In addition, we present calculations that show that the weakness of the H₂0-0 S(7) line is not a reason for making the unlikely-to-be-true assumption that H₂ emission is caused by shock excitation. We derive a total flux from the nebula in H₂ lines and compare this with the power available from the central star for producing this radiation. We establish that neither soft X-rays nor 912-1100 Å radiation has enough energy to power the H₂ radiation–only the stellar extreme-ultraviolet radiation shortward of 912 Å does. New images of the knot 378-801 in the H₂2.12 µm line reveal that the 2.12 µm cusp lies immediately inside the ionized atomic gas zone. This property is shared by material in the ``tail'' region. The H₂2.12 µm emission of the cusp confirms previous assumptions, while the tail's property firmly establishes that the tail structure is an ionization-bounded radiation shadow behind the optically thick core of the knot. A unique new image of a transitional region of the nebula's inner disk in the He II λ4686 line fails to show any emission from knots that might have been found in the He⁺⁺ core of the nebula. We also re-examined high signal-to-noise ratio ground-based telescope images of this same inner region and found no evidence of structures that could be related to knots.