The first estimation of the ionization fraction in dense and translucent molecular gas across Orion B

¹ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicu na Mackenna 4860, 7820436 Macul, Santiago, Chile
³ LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
⁴ Instituto de Física Fundamental (CSIC). Calle Serrano 121, 28006, Madrid, Spain
⁵ IRAM, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France
⁶ Department of Earth, Environment, and Physics, Worcester State University, Worcester, MA 01602, USA
⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138, USA
⁸ Univ. Grenoble Alpes, Inria, CNRS, Grenoble INP, GIPSA-Lab, Grenoble 38000, France
⁹ Univ. Lille, CNRS, Centrale Lille, UMR 9189 – CRIStAL, 59651 Villeneuve d'Ascq, France
¹⁰ Department of Astronomy, University of Florida, PO Box 112055, Gainesville, FL 32611, USA
¹¹ Université de Toulon, Aix Marseille Univ, CNRS, IM2NP, Toulon, France
¹² Institut de Recherche en Astrophysique et Planétologie (IRAP), Université Paul Sabatier, Toulouse cedex 4, France
¹³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903, USA
¹⁴ Laboratoire d'Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹⁵ Laboratoire de Physique de 1'Ecole normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, Sorbonne Paris Cité, Paris, France
¹⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁷ School of Physics and Astronomy, Cardiff University, Queen's buildings, Cardiff CF24 3AA, UK

^★ Corresponding author: antoine.roueff@univ-tln.fr

Received: 8 January 2025
Accepted: 14 July 2025

Abstract

Context. The ionization fraction (f_e=n_e/n_H) represents a fundamental parameter of the gas in the interstellar medium. However, estimating f_e relies on a deep knowledge of the underlying chemistry of molecular gas as well as observations of atomic recombination lines and electron-sensitive molecular emission, such as deuterated isotopologs of HCO⁺and N₂H⁺, which are only detectable in the dense cores. Until now, it has been challenging to constrain the ionization fraction in the interstellar gas over a large areas because of the observational limitations on these tracers and chemistry models.

Aims. Recent models have provided a set of molecular lines whose ratios (intensities and column densities) can be used to trace f_e in different environments of molecular clouds. Here, we use a set of various molecular lines typically detected in the 3−4 mm range to constrain the ionization fraction across the Orion B giant molecular cloud. In this work, we derived the ionization fraction for dense and translucent gas, and we investigated its variation with the density of the gas, n, and the strength of the far-ultraviolet radiation field, G₀, with their ratio G₀/n.

Methods. We present our results for the ionization fraction across one square degree in Orion B derived using analytical models as well as observational intensity and column density ratios of CN(1−0)/N₂H⁺(1−0), ¹³CO(1−0)/HCO⁺(1−0), and C¹⁸O(1−0)/HCO⁺(1−0) in the dense and shielded medium (A_v ≥ 10 mag). We also used ratios of C₂H(1−0)/HNC(1−0), C₂H(1−0)/HCN(1−0), and C₂H(1−0)/CN(1−0) in the translucent gas (2 mag ≤ A_v ≤ 6 mag).

Results. We find that the ionization fraction is within the range of 10^−5.5−10⁻⁴ for the translucent medium and 10⁻⁸−10⁻⁶ for the dense medium. Our results show that the inferred f_e values are sensitive to the value of G₀, especially in the dense, highly UV-illuminated gas. We also find that the ionization fraction in dense and translucent gas decreases with an increasing volume density (f_e ∝ n^−0.227 for dense gas and f_e ∝ n^−0.3 in translucent gas). It increases with G₀, which is a consequence of how sensitive the emission of selected molecular lines (e.g., CN and HCO⁺) is to the UV radiation field. In the case of the translucent medium, we did not find any significant difference in the ionization fraction computed from different line ratios. The range of f_e values found in translucent gas implies that the electron excitation of HCN and HNC becomes significant in this regime.

Conclusions. In dense and shielded gas, we recommend using CN(1−0)/N₂H⁺(1−0) to derive an upper limit on the ionization fraction f_e, along with C¹⁸O(1−0)/HCO⁺(1−0) to set constraints on the lower limit. In a translucent medium, C₂H(1−0)/HNC(1−0) serves as a good tracer of f_e. The moderately high f_e values found in translucent gas are consistent with the C⁺/CI/CO transition regime, while the values we find in the dense gas are sufficient to couple the gas with the magnetic field.