Tracers of the ionization fraction in dense and translucent molecular gas: II. Using mm observations to constrain ionization fraction across Orion B
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2025
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| author | Bešlić, Ivana Gerin, Maryvonne Guzmán, Viviana V. Bron, Emeric Roueff, Evelyne Goicoechea, Javier R. Pety, Jérôme Petit, Franck Le Coudé, Simon Einig, Lucas Mazurek, Helena Orkisz, Jan H. Palud, Pierre Santa-Maria, Miriam G. Ségal, Léontine Zakardjian, Antoine Bardeau, Sébastien Chainais, Pierre Demyk, Karine Magalhaes, Victor de Souza Gratier, Pierre Hughes, Annie Languignon, David Levrier, François Bourlot, Jacques Le Lis, Dariusz C. Liszt, Harvey S. Peretto, Nicolas Roueff, Antoine Sievers, Albrecht Thouvenin, Pierre-Antoine |
| author_facet | Bešlić, Ivana Gerin, Maryvonne Guzmán, Viviana V. Bron, Emeric Roueff, Evelyne Goicoechea, Javier R. Pety, Jérôme Petit, Franck Le Coudé, Simon Einig, Lucas Mazurek, Helena Orkisz, Jan H. Palud, Pierre Santa-Maria, Miriam G. Ségal, Léontine Zakardjian, Antoine Bardeau, Sébastien Chainais, Pierre Demyk, Karine Magalhaes, Victor de Souza Gratier, Pierre Hughes, Annie Languignon, David Levrier, François Bourlot, Jacques Le Lis, Dariusz C. Liszt, Harvey S. Peretto, Nicolas Roueff, Antoine Sievers, Albrecht Thouvenin, Pierre-Antoine |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_19480 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Tracers of the ionization fraction in dense and translucent molecular gas: II. Using mm observations to constrain ionization fraction across Orion B Bešlić, Ivana Gerin, Maryvonne Guzmán, Viviana V. Bron, Emeric Roueff, Evelyne Goicoechea, Javier R. Pety, Jérôme Petit, Franck Le Coudé, Simon Einig, Lucas Mazurek, Helena Orkisz, Jan H. Palud, Pierre Santa-Maria, Miriam G. Ségal, Léontine Zakardjian, Antoine Bardeau, Sébastien Chainais, Pierre Demyk, Karine Magalhaes, Victor de Souza Gratier, Pierre Hughes, Annie Languignon, David Levrier, François Bourlot, Jacques Le Lis, Dariusz C. Liszt, Harvey S. Peretto, Nicolas Roueff, Antoine Sievers, Albrecht Thouvenin, Pierre-Antoine Astrophysics of Galaxies 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. |
| title | Tracers of the ionization fraction in dense and translucent molecular gas: II. Using mm observations to constrain ionization fraction across Orion B |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2507.19480 |