CN excitation and electron densities in diffuse molecular clouds

TL;DR

This study provides detailed electron-impact excitation rates for CN molecules in diffuse clouds, combining advanced calculations with radiative transfer modeling to estimate electron densities consistent with astronomical observations.

Contribution

The paper introduces new rate coefficients for electron-CN collisions up to N=20 levels, calculated with Born-corrected R-matrix and IOS approximation, enhancing modeling accuracy.

Findings

Electron-impact rates exceed H2 excitation rates by five orders of magnitude.

Electron densities in diffuse clouds are estimated to be between 0.01 and 0.06 cm$^{-3}$.

Radiative transfer models align with observed CN excitation, supporting the role of electrons.

Abstract

Utilising previous work by the authors on the spin-coupled rotational cross-sections for electron-CN collisions, data for the associated rate coefficients is 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 (IOS) approximation. The dominant hyperfine transitions are those with $\Delta N=\Delta j= \Delta F=1$. For dipole-allowed transitions, electron-impact rates are shown to exceed those for excitation of CN by para-H$_2$($j=0$) 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 (CMB) photons, is considered. Radiative transfer calculations are performed and the results compared to observations. These comparisons suggest that electron density lies in the range $n(e)\sim 0.01-0.06$~cm$^{-3}$ for typical physical conditions present in diffuse clouds.