[This is truncated to suit the whims of the archivers ...] Parameter studies
in LVG models are used to show how the low-lying rotational transitions of
common polar molecules HCO+, HCN and CS vary with number density, column
density and electron fraction; with molecular properties such as the charge
state and permanent dipole moment; and with observational details such as the
transition that is observed. Physically-based models are used to check the
parameter studies and provide a basis for relating the few extant observations.
The parameter studies of LVG radiative transfer models show that lines of polar
molecules are uniformly brighter for ions, for lower J-values and for higher
dipole moments. Excitation by electrons is more important for J=1-0 lines and
contributes rather less to the brightness of CS J=2-1 lines. If abundances are
like those seen in absorption, the HCO+ J=1-0 line will be the brightest line
after CO, followed by HCN (1-0) and CS (2-1). Because of the very weak
rotational excitation in diffuse clouds, emission brightnesses and molecular
column densities retain a nearly-linear proportionality under fixed physical
conditions, even when transitions are quite optically thick; this implies that
changes in relative intensities among different species can be used to infer
changes in their relative abundances.Comment: To appear in A&
