  We present new computations of the equilibrium and non-equilibrium cooling
efficiencies and ionization states for low-density radiatively cooling gas
containing cosmic abundances of the elements H, He, C, N, O, Ne, Mg, Si, S, and
Fe. We present results for gas temperatures between 1e4 and 1e8 K, assuming
dust-free and optically thin conditions, and no external radiation. For
non-equilibrium cooling we solve the coupled time-dependent ionization and
energy loss equations for a radiating gas cooling from an initially hot, >5e6K
equilibrium state, down to 1e4K. We present results for heavy element
compositions ranging from 1e-3 to 2 times the elemental abundances in the Sun.
We consider gas cooling at constant density (isochoric) and at constant
pressure (isobaric). We calculate the critical column densities and
temperatures at which radiatively cooling clouds make the dynamical transition
from isobaric to isochoric evolution. We construct ion ratio diagnostics for
the temperature and metallicity in radiatively cooling gas. We provide
numerical estimates for the maximal cloud column densities for which the gas
remains optically thin to the cooling radiation. We present our computational
results in convenient on-line figures and tables (see
http://wise-obs.tau.ac.il/~orlyg/cooling/).
