Electron Excitation of High Dipole Moment Molecules Reexamined
  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.
