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

Autor(en)

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

Abstrakt

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.

Organisation(en)

Institut für Astrophysik

Externe Organisation(en)

Spanish National Research Council (CSIC)

Journal

Astronomy & Astrophysics

Band

566

Anzahl der Seiten

10

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201322941

Publikationsdatum

06-2014

Peer-reviewed

Ja

ÖFOS 2012

103003 Astronomie, 103004 Astrophysik

Schlagwörter

ASJC Scopus Sachgebiete

Astronomy and Astrophysics, Space and Planetary Science

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/43f9ab41-9922-462e-84ea-c924bb5ff054