The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds

Abstract

The ionization fraction plays a key role in the chemistry and dynamics of molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the Photodissociation Region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+ J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and new HOC+ and CO+ observations. We compare self-consistently the observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a nonlocal radiative transfer calculation. The chemical network includes deuterated species, 13C fractionation reactions and HCO+/HOC+ isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated. The detection of HOC+ reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few times ~10(-9) in the core. PAH^- anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ([PAH] >10(-8)).

13 pages, 7 figures, 6 tables. Accepted for publication in A&A (english not edited)

References in corpus (17)

• Water, O2 and Ice in Molecular Clouds
• A PDR-Code Comparison Study
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• Analysis of the emission of very small dust particles from Spitzer spectro-imagery data using blind signal separation methods
• Formation and destruction of polycyclic aromatic hydrocarbon clusters in the interstellar medium
• Low sulfur depletion in the Horsehead PDR
• Chemical Rates on Small Grains and PAHs: C^+ Recombination and H_2 Formation
• The penetration of FUV radiation into molecular clouds
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation
• Aromatic emission from the ionised mane of the Horsehead nebula
• Dust processing in photodissociation regions - Mid-IR emission modelling
• A general model for the identification of specific PAHs in the far-IR
• On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs
• Deuterium fractionation in the Horsehead edge
• SCUBA observations of the Horsehead Nebula - what did the horse swallow?
• Simultaneous Determination of the Cosmic Ray Ionization Rate and Fractional Ionization in DR21(OH)
• Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201

Cited by in corpus (57)

• Cool Outflows in Galaxies and their Implications
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions
• The IRAM-30m line survey of the Horsehead PDR: II. First detection of the l-C3H+ hydrocarbon cation
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• New extended deuterium fractionation model: assessment at dense ISM conditions and sensitivity analysis
• GRB090426: the farthest short gamma-ray burst?
• The IRAM-30m line survey of the Horsehead PDR: III. High abundance of complex (iso-)nitrile molecules in UV-illuminated gas
• The IRAM-30m line survey of the Horsehead PDR: IV. Comparative chemistry of H2CO and CH3OH
• H2CO in the Horsehead PDR: Photo-desorption of dust grain ice mantles
• The chemistry of phosphorus-bearing molecules under energetic phenomena
• Observing a column-dependent zeta in dense interstellar sources: the case of the Horsehead Nebula
• Chemical complexity in the Horsehead photodissociation region
• Orbital and physical properties of the $\sigma$ Ori Aa,Ab,B triple system
• Photodissociation chemistry footprints in the Starburst galaxy NGC 253
• Spectral line survey of the ultracompact HII region Mon R2
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
• Spatially resolved images of reactive ions in the Orion Bar
• Deuterium fractionation and the degree of ionisation in massive clumps within infrared dark clouds
• Bottlenecks to interstellar sulfur chemistry: Sulfur-bearing hydrides in UV-illuminated gas and grains
• Molecular Tracers of Turbulent Shocks in Giant Molecular Clouds
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• Mapping photodissociation and shocks in the vicinity of Sgr A*
• Complex organic molecules in diffuse clouds along the line of sight to Sgr B2
• Reactions forming C(0,+)n=2,10, Cn=2,4H(0,+) and C3H(0,+) in the gas phase: semi empirical branching ratios
• A (sub)millimetre study of dense cores in Orion B9
• 3 mm spectral line survey of two lines of sight toward two typical cloud complexes in the Galactic Center
• Starburst Energy Feedback Seen Through HCO${}^{+}$/HOC${}^{+}$ Emission in NGC 253 from ALCHEMI
• A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry
• Anomalous HCN emission from warm giant molecular clouds
• Tracers of the ionization fraction in dense and translucent gas: I. Automated exploitation of massive astrochemical model grids
• The first CO+ image: Probing the HI/H2 layer around the ultracompact HII region Mon R2
• Far-Infrared detection of neutral atomic oxygen toward the Horsehead Nebula
• Dust evolution across the Horsehead Nebula
• Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
• Impact of PAH photodissociation on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
• HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
• Non-ideal MHD simulations of subcritical prestellar cores with non-equilibrium chemistry
• A CSO Search for $l$-C${}_3$H${}^{+}$: Detection in the Orion Bar PDR
• The extremely sharp transition between molecular and ionized gas in the Horsehead nebula
• Probing the massive star forming environment - a multiwavelength investigation of the filamentary IRDC G333.73+0.37
• Gas kinematics around filamentary structures in the Orion B cloud
• JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging
• Gas Kinematics in the HII regions G351.69-1.15 and G351.63-1.25
• The ionization fraction in OMC-2 and OMC-3
• ATLASGAL-selected massive clumps in the inner Galaxy: VIII. Chemistry of photodissociation regions
• Chemical variations across the TMC-1 boundary: molecular tracers from translucent phase to dense phase
• Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions
• Impact of HAC evolution on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
• On the effects of UV photons/X-rays on the chemistry of the Sgr B2 cloud
• The molecular environment of the pillar-like features in the HII region G46.5-0.2
• Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
• Kinematic study of the molecular gas associated with two cometary globules in Sh2-236
• H3+ in irradiated protoplanetary disks: Linking far-ultraviolet radiation and water vapor
• Weak, extended water vapor emission in the Horsehead nebula
• Non-ideal MHD and protostellar feedback effects on disc formation and evolution in numerical simulations of star cluster formation
• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon