Tracers of the ionization fraction in dense and translucent molecular gas: II. Using mm observations to constrain ionization fraction across Orion B

Abstract

The ionization fraction ($f_{\mathrm{e}}=n_{\mathrm{e}}/n_{\mathrm{H}}$) is a crucial parameter of interstellar gas, yet estimating it requires deep knowledge of molecular gas chemistry and observations of specific lines, such as those from isotopologs like HCO${}^{+}$ and N${}_2$H${}^{+}$, which are detectable only in dense cores. Previous challenges in constraining $f_{\mathrm{e}}$ over large areas stemmed from the limitations of observational tracers and chemical models. Recent models have identified molecular line ratios that can trace $f_{\mathrm{e}}$ in different environments within molecular clouds. In this study, we analyze various molecular lines in the 3-4 mm range to derive the ionization fraction across the Orion B giant molecular cloud. We focus on dense and translucent gas, exploring variations with gas density ($n$) and the far-ultraviolet (FUV) radiation field ($G_0$). Our findings show that the ionization fraction ranges from $10^{-5.5}$ to $10^{-4}$ in translucent gas and $10^{-8}$ to $10^{-6}$ in dense gas. Notably, $f_{\mathrm{e}}$ is sensitive to $G_0$ in dense, UV-illuminated regions, decreasing with increasing volume density ($f_{\mathrm{e}}\propto n^{-0.227}$ for dense and $f_{\mathrm{e}}\propto n^{-0.3}$ for translucent gas) and increasing with $G_0$. In translucent gas, differing line ratios yield consistent fe values, indicating the importance of electron excitation of HCN and HNC. For dense gas, we recommend using the CN(1-0)/N${}_2$H${}^{+}$(1-0) ratio for upper limits on fe and C${}^{18}$O(1-0)/HCO${}^{+}$(1-0) for lower limits. In translucent environments, CCH(1-0)/HNC(1-0) effectively traces $f_{\mathrm{e}}$. The higher fe values in translucent gas align with the C${}^{+}$/CI/CO transition, while values in dense gas are adequate for coupling with the magnetic field.

Accepted for publication to Astronomy and Astrophysics