Journal Article

Modelling the warm H<sub>2</sub> infrared emission of the Helix nebula cometary knots

Isabel Aleman, Albert A. Zijlstra, Mikako Matsuura, Ruth Gruenwald and Rafael K. Kimura

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 1, pages 790-800
Published in print September 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19094.x
Modelling the warm H2 infrared emission of the Helix nebula cometary knots

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Molecular hydrogen emission is commonly observed in planetary nebulae. Images taken in infrared H2 emission lines show that at least part of the molecular emission is produced inside the ionized region. In the best studied case, the Helix nebula, the H2 emission is produced inside cometary knots (CKs), comet-shaped structures believed to be clumps of dense neutral gas embedded within the ionized gas. Most of the H2 emission of the CKs seems to be produced in a thin layer between the ionized diffuse gas and the neutral material of the knot, in a mini-photodissociation region (mini-PDR). However, PDR models published so far cannot fully explain all the characteristics of the H2 emission of the CKs. In this work, we use the photoionization code Aangaba to study the H2 emission of the CKs, particularly that produced in the interface H+/H0 of the knot, where a significant fraction of the H2 1–0 S(1) emission seems to be produced. Our results show that the production of molecular hydrogen in such a region may explain several characteristics of the observed emission, particularly the high excitation temperature of the H2 infrared lines. We find that the temperature derived from H2 observations, even of a single knot, will depend very strongly on the observed transitions, with much higher temperatures derived from excited levels. We also proposed that the separation between the Hα and [N ii] peak emission observed in the images of CKs may be an effect of the distance of the knot from the star, since for knots farther from the central star the [N ii] line is produced closer to the border of the CK than Hα.

Keywords: astrochemistry; circumstellar matter; ISM: molecules; planetary nebulae: individual: NGC 7293; planetary nebulae: individual: Helix nebula; infrared: ISM

Journal Article.  7634 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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