2007/01/23 by C. R. O'Dell, C. R. O’dell, W. J. Henney +1 · 4 citations
Physics and Astronomy · #Astro and Planetary Science #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Infrared #Ion #Ionization #Knot (papermaking) #Nebula #Optics #Physics #Population #Radiation #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/513011
published as Astron.J.133:2343-2356,2007 · Astronomical Journal, in press. Some figures are shown at reduced resolution. A full resolution version is available at http://www.ifront.org/wiki/Helix_Nebula_2007_Paper
arxiv created 2007/01/23 · openalex publication_date 2007/04/04 · arxiv updated 2009/12/01 · openalex created_date 2020/11/23 · openalex updated_date 2026/08/05
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 H 2 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 H 2 0-0 S (7) line is not a reason for making the unlikely-to-be-true assumption that H 2 emission is caused by shock excitation. We derive a total flux from the nebula in H 2 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 H 2 radiation—only the stellar extreme-ultraviolet radiation shortward of 912 Å does. New images of the knot 378-801 in the H 2 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 H 2 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 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.