Rotational Excitation of polar molecules by H2 and electrons in diffuse clouds

Abstract

[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.

To appear in A&A

References in corpus (6)

• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• Formation, fractionation and excitation of carbon monoxide in diffuse clouds
• Hubble Space Telescope Survey of Interstellar ^12CO/^13CO in the Solar Neighborhood
• Electron-impact rotational and hyperfine excitation of HCN, HNC, DCN and DNC
• Low energy H+CO scattering revisited: CO rotational excitation with new potential surfaces
• A CO J=1-0 Survey of common optical/uv absorption sightlines

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