Electron Excitation of High Dipole Moment Molecules Re-examined

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 H₂ molecules, ≃10⁵ for HCN, yields the requirements for electron excitation to be of practical importance if $n\left(H_2\right)⩽{10}^{5.5}$ ${cm}^{-3}$ and $X\left(e^{-}\right)⩾{10}^{-5}$, where the numerical factors reflect the critical values $n_c\left(H_2\right)$ and $X^{\ast }\left(e^{-}\right)$. This indicates that in regions where a large fraction of carbon is ionized, $X\left(e^{-}\right)$ 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, $X^{\ast }\left(e^{-}\right)$, defined by the value required for equal effect from collisions with H₂ and e⁻. Electron excitation is, for example, unimportant for CO and C⁺. Electron excitation may be responsible for the surprisingly large spatial extent of emission from dense gas tracers in some molecular clouds. 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.