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

Context. The ionization fraction (fe=ne/nH) represents a fundamental parameter of the gas in the interstellar medium. However, estimating fe 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 fe 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 (Av ≥ 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 ≤ Av ≤ 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 fe 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 (fe ∝ n−0.227 for dense gas and fe ∝ 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 fe 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 fe, 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 fe. The moderately high fe 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.