Herschel spectral mapping of the Helix nebula (NGC 7293) ^⋆,⋆⋆

Extended CO photodissociation and OH⁺ emission

¹ Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Torrejón de Ardoz 28850 Madrid Spain
e-mail: m.etxaluze@icmm.csic.es
² Grupo de Astrofísica Molecular, Instituto de Ciencias de Materiales de Madrid, CSIC-ICMM Cantoblanco, 28049 Madrid, Spain
³ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁴ Department of Physics & Astronomy, University College London, London WC1E 6BT, UK
⁵ RAL Space, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK
⁶ University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Wien, Austria
⁷ LOMC-UMR 6294, CNRS-Université du Havre, 25 rue Philippe Lebon, BP 540, 76058 Le Havre, France
⁸ Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, UK
⁹ Institute for Space Imaging Science, Dept. of Physics & Astronomy, University of Lethbridge, Lethbridge, AB T1K3M4, Canada
¹⁰ Institute of Astrophysics, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
¹¹ Department of Physics and Astronomy, University of Denver, 2112 E. Wesley Ave., Denver CO 80210, USA
¹² Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yosinodai, Chuo-ku, 252-5210 Sagamihara, Kanagawa, Japan

Received: 29 October 2013
Accepted: 28 March 2014

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 (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.