Helix Nebula 2007 Paper

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

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

Authors

C. R. O'Dell¹, W. J. Henney², G. J. Ferland³

¹Department of Physics and Astronomy, Vanderbilt University, Nashville, TN
²Centro de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán, México
³Department of Physics and Astronomy, University of Kentucky, Lexington, KY

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Abstract

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 ro-vibrational states of H₂ is close to the distribution of a gas in local thermodynamic equilibrium (LTE) 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. 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 will probably explain the excitation temperature and surface brightness of the 2.12 μm cusps and tails.

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. The new 2.12 μm image together with archived Hubble images is used to establish a pattern of decreasing surface brightness and increasing size of the knots with increasing stellar distance. Although the contrast against the background is greater in 2.12 μm than in the optical lines, the higher resolution and signal of optical images remains the most powerful technique for searching for knots.

A unique new image of a transitional region of the nebula's inner disk in the HeII 4686 Å 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.

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