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

摘要

Context. The ionization fraction~-(i.e., the electron abundance) plays a key role in the chemistry and dynamics of molecular clouds.Aims. We study the H~(13)CO~+, DCO~+and HOC~+line emission towards the Horsehead, from the shielded core to the UV irradiatedcloud edge, i.e., the photodissociation region (PDR), as a template to investigate the ionization fraction gradient in molecular clouds.Methods. We analyze an IRAM Plateau de Bure Interferometer map of the H~(13)CO~+J =1-0 line at a 6.8" × 4.7" resolution, comple-mented with IRAM-30 m H~(13)CO~+ and DCO~+higher-J line maps and new HOC~+and CO~+ observations. We compare self-consistentlythe observed spatial distribution and line intensities with detailed depth-dependent predictions of a PDR model coupled with a non-local radiative transfer calculation. The chemical network includes deuterated species, ~(13)Cfractionation reactions and HCO~+/HOC~+isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated.Results. The detection of the HOC~+reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UVirradiated regions, where we derive a low HCO~+/HOC~+ = 75-200 abundance ratio. In the absence of PAHs, we reproduce theobservations with gas-phase metal abundances, Fe+Mg+..., lower than 4 × 10~(-9)(with respect to H), and a cosmic-ray ionization rateof ζ = (5 ± 3) × 10~(-17)s~(-1). The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance.Conclusions. The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25"), frome~- = 10~(-4)(orn_c ~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 thecold and neutral gas if substantial amounts of free PAHs are present (PAH > 10~(-8)).