Time-dependent ionization in radiatively cooling gas

Orly Gnat*, Amiel Sternberg

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

285 Scopus citations


We present new computations of the equilibrium and nonequilibrium cooling efficiencies and ionization states for low-density radiatively cooling gas containing the elements H, He, C, N, O, Ne, Mg, Si, S, and Fe. We present results for gas temperatures between 104 and 108 K, assuming dust-free and optically thin conditions, and no external radiation. For nonequilibrium cooling we solve the coupled time-dependent ionization and energy loss equations for a radiating gas cooling from an initially hot, ≳5 × 106 K, equilibrium state, down to 104 K. We present results for heavy element compositions ranging from 10-3 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 online figures and tables.

Original languageAmerican English
Pages (from-to)213-230
Number of pages18
JournalAstrophysical Journal, Supplement Series
Issue number2
StatePublished - Feb 2007
Externally publishedYes


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