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

Abstract

<i>Context. <i/>The ionization fraction (i.e., the electron abundance) plays a key role in the chemistry and dynamics of molecular clouds.<i>Aims. <i/>We study the H<sup>13<sup/>CO<sup>+<sup/>, DCO<sup>+<sup/> and HOC<sup>+<sup/> 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.<i>Methods. <i/>We analyze an IRAM <i>Plateau de Bure Interferometer<i/> map of the H<sup>13<sup/>CO<sup>+<sup/> –0 line at a resolution, complemented with IRAM-30 m H<sup>13<sup/>CO<sup>+<sup/> and DCO<sup>+<sup/> higher-<i>J<i/> line maps and new HOC<sup>+<sup/> and CO<sup>+<sup/> 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, <sup>13<sup/>C fractionation reactions and HCO<sup>+<sup/>/HOC<sup>+<sup/> isomerization reactions. The role of neutral and charged PAHs in the cloud chemistry and ionization balance is investigated.<i>Results. <i/>The detection of the HOC<sup>+<sup/> reactive ion towards the Horsehead PDR proves the high ionization fraction of the outer UV irradiated regions, where we derive a low [ HCO<sup>+<sup/>] /[ HOC<sup>+<sup/>] 75–200 abundance ratio. In the absence of PAHs, we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...], lower than (with respect to H), and a cosmic-ray ionization rate of s<sup>-1<sup/>. The inclusion of PAHs modifies the ionization fraction gradient and increases the required metal abundance.<i>Conclusions. <i/>The ionization fraction in the Horsehead edge follows a steep gradient, with a scale length of ~0.05 pc (or ~25<i>''<i/>), from [ e<sup>-<sup/>] 10<sup>-4<sup/> (or –5 cm<sup>-3<sup/>) in the PDR to a few times ~10<sup>-9<sup/> in the core. PAH<sup>-<sup/> anions play a role in the charge balance of the cold and neutral gas if substantial amounts of free PAHs are present ().