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

Abstract

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.

arXiv commentAccepted for publication in Astronomy and Astrophysics

References in corpus (29)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The Herschel-SPIRE instrument and its in-flight performance
• Effect of episodic accretion on the structure and the lithium depletion of low-mass stars and planet-hosting stars
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6-0.4
• Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction
• The chemistry of vibrationally excited H2 in the interstellar medium
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• The Chandra X-ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions
• Submillimetre line spectrum of the Seyfert galaxy NGC1068 from the Herschel-SPIRE Fourier Transform Spectrometer
• First interstellar detection of OH+
• Trigonometric Parallaxes of Central Stars of Planetary Nebulae
• The Enigmatic X-ray Point Sources at the Central Stars of NGC 6543 and NGC 7293
• Knots in the Helix Nebula found in H2
• Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold dust
• Unraveling the Helix Nebula: Its Structure and Knots
• Far-Infrared Spectroscopy of the Galactic Center. Hot Molecular Gas: Shocks versus Radiation near SgrA*
• High Resolution CO and H2 Molecular Line Imaging of a Cometary Globule in the Helix Nebula
• The creation of the Helix planetary nebula (NGC 7293) by multiple events
• Infrared Observations of The Helix Planetary Nebula
• Morphology and Composition of the Helix Nebula
• Spatially extended OH+ emission from the Orion Bar and Ridge
• Detection of OH${}^{+}$ and H${}_2$O${}^{+}$ towards Orion~KL
• A Multi-Instrument Study of the Helix Nebula Knots with the Hubble Space Telescope
• The Multitude of Molecular Hydrogen Knots in the Helix Nebula
• Neutral atomic carbon in the globules of the Helix
• VLT / Infrared Integral Field Spectrometer Observations of Molecular Hydrogen Lines in the Knots in the Planetary Nebula NGC 7293 (the Helix Nebula)
• Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary Nebulae
• Flows along cometary tails in the Helix planetary nebula NGC 7293

Cited by in corpus (6)

• OH+ in astrophysical media: state-to-state formation rates, Einstein coefficients and inelastic collision rates with He
• Molecular ions in the O-rich evolved star OH231.8+4.2: HCO${}^{+}$,H${}^{13}$CO${}^{+}$ and first detection of SO${}^{+}$, N${}_2$H${}^{+}$, and H${}_3$O${}^{+}$
• An Imaging Spectroscopic Survey of the Planetary Nebula NGC 7009 with MUSE
• Herschel Planetary Nebula Survey (HerPlaNS) - First Detection of OH+ in Planetary Nebulae
• Herschel imaging of the dust in the Helix Nebula (NGC 7293)
• OH${}^{+}$ emission from cometary knots in planetary nebulae