Tracers of the ionization fraction in dense and translucent molecular gas: II. Using mm observations to constrain ionization fraction 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 and 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 limitations of observations of these tracers and chemistry models. Aims. Recent models provided a set of molecular lines which ratios (intensities and column densities) can be used to trace fe in di erent 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 derive the ionization fraction for dense and translucent gas, and we investigate its variation with the gas density n and the strength of the far-ultraviolet (FUV) radiation field G0 with their ratio G0=n. Methods. We present results for the ionization fraction across 1 square degree in Orion B derived using analytical models and 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), and ratios of C2H(1–0)/HNC(1–0), C2H(1–0)/HCN(1–0), and C2H(1–0)/CN(1–0) in translucent gas (2 mag  Av   6 mag). Results. We find that the ionization fraction is in the range of 10􀀀5:5, 10􀀀4 for the translucent medium, and 10􀀀8 to 10􀀀6 for the dense medium. Our results show that the inferred fe values depend on the selected line intensity ratio and the value of G0, especially in the dense, highly UVilluminated gas. We also find that the ionization fraction in dense gas decreases with increasing volume density, and increases with G0, which is a consequence of how sensitive the emission of CN and HCO+ is to the stellar radiation field. In the case of the translucent medium, we do not find any significant di erence in the ionization fraction computed from di erent line ratios. However, the range of fe found in translucent gas implies that electron excitation of HCN and HNC becomes important 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 and 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 su cient to couple the gas with the magnetic field.