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

M Etxaluze, Jos Cernicharo, Javier R. Goicoechea, Franz Kerschbaum

Veröffentlichungen: Beitrag in FachzeitschriftArtikelPeer Reviewed

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] ( 3P 2- 3P 1), which together with the OH + lines trace extended CO photodissociation regions along the rims. The estimated OH + column density is ~ 10 12 - 10 13 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(H 2) ~ (1 - 5) × 10 5 cm -3) and the gas temperature (T k ~ 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.

OriginalspracheEnglisch
AufsatznummerA78
Seitenumfang10
FachzeitschriftAstronomy & Astrophysics
Jahrgang566
DOIs
PublikationsstatusVeröffentlicht - Juni 2014

ÖFOS 2012

  • 103003 Astronomie
  • 103004 Astrophysik

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