Herschel spectral-mapping of the Helix Nebula (NGC 7293): Extended CO
  photodissociation and OH+ emission
  The Helix Nebula (NGC 7293) is the closest planetary nebulae. Therefore, it
is an ideal template for photochemical studies at small spatial scales in
planetary nebulae. 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. We use 5 SPIRE FTS pointing observations to make atomic and molecular
spectral maps. We analyze the molecular gas by modeling the CO rotational lines
using a non-local thermodynamic equilibrium (non-LTE) radiative transfer model.
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), [NII] at 1461 GHz from ionized gas, and [CI] (2-1), which together
with the OH+ lines, trace extended CO photodissociation regions along the rims.
The estimated OH+ column density is (1-10)x1e12 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)x1e5
cm-3) and the gas temperature (Tk= 20-40 K). The SPIRE spectral-maps suggest
that CO arises from dense and shielded clumps in the western rims of the Helix
Nebula whereas OH+ and [CI] lines trace the diffuse gas and the UV and X-ray
illuminated clumps surface where molecules reform after CO photodissociation.
[NII] traces a more diffuse ionized gas component in the interclump medium.
