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

TL;DR

This study investigates the ionization fraction gradient in the Horsehead molecular cloud edge using multi-line observations and detailed PDR modeling, revealing a steep ionization gradient influenced by PAHs and cosmic rays.

Contribution

It provides the first detailed analysis of the ionization fraction gradient across the Horsehead edge, incorporating PAHs and complex chemical reactions into PDR models.

Findings

Ionization fraction varies steeply from the PDR to the core.

Detection of HOC+ confirms high ionization in UV-irradiated regions.

PAHs significantly influence the ionization balance and chemistry.

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)).