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

¹ Laboratorio de Astrofísica Molecular. Centro de Astrobiología. CSIC-INTA. Carretera de Ajalvir, Km 4. Torrejón de Ardoz, 28850 Madrid, Spain e-mail: goicoechea@damir.iem.csic.es
² IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France e-mail: pety@iram.fr
³ LERMA - LRA, UMR 8112, CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris, France e-mail: maryvonne.gerin@lra.ens.fr
⁴ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France e-mail: pierre.hilyblant@obs.ujf-grenoble.fr
⁵ LUTH, UMR 8102 CNRS, Université Paris 7 and Observatoire de Paris, Place J. Janssen, 92195 Meudon, France e-mail: Jacques.Lebourlot@obspm.fr

Received: 10 December 2008
Accepted: 11 February 2009

Abstract

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 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.

Methods. We analyze an IRAM Plateau de Bure Interferometer map of the H¹³CO⁺ –0 line at a resolution, complemented with IRAM-30 m 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.

Results. 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 (with respect to H), and a cosmic-ray ionization rate of  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''), from [ e^-]   10^-4 (or –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 ().