Determination of the Physical Conditions of the Knots in the Helix Nebula from Optical and Infrared Observations

Abstract

[Abridged] We use new HST and archived images to clarify the nature of the knots in the Helix Nebula. We employ published far infrared spectrophotometry and existing 2.12 micron images to establish that the population distribution of the lowest ro-vibrational states of H2 is close to the distribution of a gas in LTE at 988 +- 119 K. 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 FUV radiation has enough energy to power the H2 radiation, only the stellar EUV radiation shortward of 912 Angstrom does. Advection of material from the cold regions of the knots produces an extensive zone where both atomic and molecular hydrogen are found, allowing the H2 to directly be heated by Lyman continuum radiation, thus providing a mechanism that can explain the excitation temperature and surface brightness of the cusps and tails. New images of the knot 378-801 reveal that the 2.12 micron cusp and tail lie immediately inside the ionized atomic gas zone. This firmly establishes that the "tail" structure is an ionization bounded radiation shadow behind the optically thick core of the knot. A unique new image in the HeII 4686 Angstrom 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.