Atomic line cooling
= Atomic line cooling
{title2=$\mathcal C_{\rm line}=\sum n_en_iq_{ij}(T)\,E_{ij}$}
<Atomic line cooling> removes gas kinetic energy by <collisional excitation> followed by photon escape. It is strongest where atoms or ions have bound <Electrons> and thermally accessible transitions. If photons are trapped, or if the relevant atoms are fully ionized, the simple optically thin line-cooling rate no longer gives the same loss.