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

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)).

💡 Research Summary

The paper presents a comprehensive study of the ionization fraction gradient across the Horsehead nebula, using a combination of high‑resolution interferometric and single‑dish observations together with state‑of‑the‑art photodissociation region (PDR) modeling and non‑local radiative transfer calculations. The authors mapped the H¹³CO⁺ J = 1‑0 line with the IRAM Plateau de Bure Interferometer (PdBI) at ~5″ resolution, complemented these data with IRAM‑30 m maps of higher‑J transitions of H¹³CO⁺ and DCO⁺, and added new detections of the reactive ions HOC⁺ and CO⁺. These observations trace the chemistry from the deeply shielded core (A_V ≈ 10) to the UV‑irradiated edge (A_V ≈ 0–2), providing both spatial distribution and line‑intensity information for several key molecular ions.

To interpret the data, the authors employed a depth‑dependent PDR code that solves the thermal balance, chemistry, and radiative transfer in a one‑dimensional slab illuminated by a far‑UV field χ ≈ 60 Draine. The chemical network is unusually complete: it includes deuterated species, ¹³C fractionation reactions, the isomerization cycle HCO⁺ ↔ HOC⁺, and a full set of PAH charge‑exchange reactions (PAH⁰, PAH⁺, PAH⁻). The model is coupled to a non‑LTE line‑radiative‑transfer module, allowing direct comparison of synthetic spectra with the observed line profiles. Free parameters explored are the total gas‑phase metal abundance (