Electron Excitation of High Dipole Moment Molecules Reexamined

TL;DR

This paper reevaluates the significance of electron excitation in high dipole moment molecules within molecular clouds, highlighting its potential impact on interpreting dense gas tracers and star formation indicators.

Contribution

It provides a detailed analysis of conditions under which electron excitation affects molecular emission, emphasizing its relevance in ionized regions and its implications for galaxy observations.

Findings

Electron excitation is significant for molecules like HCN, HCO+, CN, and CS under certain ionization conditions.

Electron excitation is negligible for CO and C+, contrasting with high dipole moment molecules.

Enhanced electron excitation may explain extended dense gas tracer emissions and abundance ratios in galaxy nuclei.

Abstract

Emission from high-dipole moment molecules such as HCN allows determination of the density in molecular clouds, and is often considered to trace the "dense" gas available for star formation. We assess the importance of electron excitation in various environments. The ratio of the rate coefficients for electrons and H$_2$ molecules, $\simeq$10$^5$ for HCN, yields the requirements for electron excitation to be of practical importance if $n({\rm H}_2) \leq\ 10^{5.5} ~ \rm cm^{-3}$ and $X({\rm e}^-) \geq\ 10^{-5}$, where the numerical factors reflect critical values $n_{\rm{}c}({\rm H_2})$ and $X^*({\rm{}e}^-)$. This indicates that in regions where a large fraction of carbon is ionized, $X({\rm e}^-)$ will be large enough to make electron excitation significant. The situation is in general similar for other "high density tracers", including HCO$^+$, CN, and CS. But there are significant differences in the critical electron fractional abundance, $X^*({\rm e}^-)$, defined by the value required for equal effect from collisions with H$_2$ and e$^-$. Electron excitation is, for example, unimportant for CO and C$^+$. Electron excitation may be responsible for the surprisingly large spatial extent of the emission from dense gas tracers in some molecular clouds (Pety et al. 2017; Kauffmann, Goldsmith et al. 2017). The enhanced estimates for HCN abundances and HCN/CO and HCN/HCO$^+$ ratios observed in the nuclear regions of luminous galaxies may be in part a result of electron excitation of high dipole moment tracers. The importance of electron excitation will depend on detailed models of the chemistry, which may well be non-steady state and non-static.