= Optically thin gas cooling time
{title2=$t_{\rm cool}=3\chi k_BT/(2n_H\Lambda)$}
= Radiative gas cooling time
{synonym}
For a nonrelativistic ideal gas with $n_{\rm tot}$ thermal particles per volume, divide its thermal energy $3n_{\rm tot}k_BT/2$ by the optically thin volume radiation loss. In the $n_H^2$ cooling convention this becomes $3\chi k_BT/(2n_H\Lambda)$, where $\chi=n_{\rm tot}/n_H$. Density scaling and ionization-dependent particle count must be kept consistent.
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