Journal Article

CN excitation and electron densities in diffuse molecular clouds

Stephen Harrison, Alexandre Faure and Jonathan Tennyson

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 435, issue 4, pages 3541-3546
Published in print November 2013 | ISSN: 0035-8711
Published online September 2013 | e-ISSN: 1365-2966 | DOI:
CN excitation and electron densities in diffuse molecular clouds

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Utilizing previous work by the authors on the spin-coupled rotational cross-sections for electron–CN collisions, data for the associated rate coefficients are presented. Data on rotational, fine-structure and hyperfine-structure transition involving rotational levels up to N = 20 are computed for temperatures in the range 10–1000 K. Rates are calculated by combining Born-corrected R-matrix calculations with the infinite-order-sudden approximation. The dominant hyperfine transitions are those with ΔN = Δj = ΔF = 1. For dipole-allowed transitions, electron-impact rates are shown to exceed those for excitation of CN by para-H2(j = 0) by five orders of magnitude. The role of electron collisions in the excitation of CN in diffuse clouds, where local excitation competes with the cosmic microwave background photons, is considered. Radiative transfer calculations are performed and the results compared to observations. These comparisons suggest that electron density lies in the range n(e) ∼ 0.01–0.06 cm−3 for typical physical conditions present in diffuse clouds.

Keywords: astrochemistry; molecular data; molecular processes; scattering; ISM: abundances; ISM: molecules

Journal Article.  3362 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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