SIMBAD references

The ionization fraction (i.e., the electron abundance) 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 an IRAM Plateau de Bure Interferometer map of the H¹³CO⁺ J=1-0 line at a 6.8'' x4.7'' resolution, 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 the 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-ray ionization rate of ζ=(5±3)x10^–17s^–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 ∼0.05pc (or ∼25''), from [e^–]≃10^–4 (or n_e ∼1-5cm^–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).