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Herschel spectral mapping of the Helix nebula (NGC 7293) - Extended CO photodissociation and OH+ emission

Etxaluze, M., Cernicharo, J., Goicoechea, J. R., van Hoof, P. A. M., Swinyard, Bruce, Barlow, M. J., van de Steene, G. C., Groenewegen, M. A. T., Kerschbaum, F., Lim, T. L., Lique, F., Matsuura, Mikako, Pearson, C., Polehampton, E. T., Royer, P. and Ueta, T. 2014. Herschel spectral mapping of the Helix nebula (NGC 7293) - Extended CO photodissociation and OH+ emission. Astronomy and Astrophysics 566 , A78. 10.1051/0004-6361/201322941

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Abstract

Context. The Helix nebula (NGC 7293) is our closest planetary nebulae. Therefore, it is an ideal template for photochemical studies at small spatial scales in planetary nebulae. Aims. We aim to study the spatial distribution of the atomic and the molecular gas, and the structure of the photodissociation region along the western rims of the Helix nebula as seen in the submillimeter range with Herschel. Methods. We used five SPIRE FTS pointing observations to make atomic and molecular spectral maps. We analyzed the molecular gas by modeling the CO rotational lines using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model. Results. For the first time, we have detected extended OH+ emission in a planetary nebula. The spectra towards the Helix nebula also show CO emission lines (from J = 4 to 8), [N ii] at 1461 GHz from ionized gas, and [C i] (3P2–3P1), which together with the OH+ lines trace extended CO photodissociation regions along the rims. The estimated OH+ column density is ~ 1012 − 1013 cm-2. The CH+ (1–0) line was not detected at the sensitivity of our observations. Non-LTE models of the CO excitation were used to constrain the average gas density (n(H2) ~ (1 − 5) × 105 cm-3) and the gas temperature (Tk ~ 20−40 K). Conclusions. The SPIRE spectral-maps suggest that CO arises from dense and shielded clumps in the western rims of the Helix nebula, whereas OH+ and [C i] lines trace the diffuse gas and the UV and X-ray illuminated clump surfaces where molecules reform after CO photodissociation. The [N ii] line traces a more diffuse ionized gas component in the interclump medium.

Item Type: Article
Date Type: Published Online
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Uncontrolled Keywords: planetary nebulae: individual: NGC 7293 ; infrared: ISM ; photon-dominated region (PDR) ; ISM: molecules ; ISM: lines and bands
Publisher: EDP Sciences
ISSN: 0004-6361
Date of Acceptance: 28 March 2014
Last Modified: 20 Feb 2019 14:46
URI: http://orca.cf.ac.uk/id/eprint/69883

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