Determination of the physical conditions of the knots in the Helix Nebula from optical and infrared observations

C. R. O'Dell, W. J. Henney, G. J. Ferland

Research output: Contribution to journalArticlepeer-review

29 Scopus citations


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 H2 0-0 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 H2 2.12 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 H2 2.12 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 reexamined 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.

Original languageEnglish
Pages (from-to)2343-2356
Number of pages14
JournalAstronomical Journal
Issue number5
StatePublished - May 2007


  • Planetary nebulae: individual (Helix Nebula, NGC 7293)

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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