Rotational excitation of simple polar molecules by H2 and electrons in diffuse clouds

Context. Emission from strongly-polar molecules could be a probe of physical conditions in diffuse molecular gas. Aims. We wish to provide basic information needed to interpret emission from molecules having higher dipole moments than CO, originating in diffuse clouds where the density is relatively low and the temperature and electron fraction are relatively high compared to dark clouds. Methods. 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. Results. 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.