Time-dependent Ionization in Radiatively Cooling Gas

By O. Gnat, A. Sternberg. 2006

291 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e7e1cc6b29c46c796b96ac6df46f8d390651c31f

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Citations in the corpus

THE COS-HALOS SURVEY: PHYSICAL CONDITIONS AND BARYONIC MASS IN THE LOW-REDSHIFT CIRCUMGALACTIC MEDIUM (2014, background): “Collisional ionization of gas at T > 104.6K would yield negligible quantities of ions like MgII, CII, and SiII, even if at solar abundance and with a large total gas column (e.g. Gnat & Sternberg 2007).”
The enrichment of the intergalactic medium with adiabatic feedback – I. Metal cooling and metal diffusion (2009, background): “While the IGM and ISM are exposed to UV radiation, widely-used metal cooling rates (e.g., Sutherland & Dopita 1993; Gnat & Sternberg 2007) were usually calculated assuming collisional ionization equilibrium (CIE), which is only valid when the radiation background is absent.”
The effect of photoionization on the cooling rates of enriched, astrophysical plasmas (2008, methodology): “In Figure 1 we compare our CIE results (i.e., cloudy version 07.02) with those of Gnat & Sternberg (2007) (who used cloudy version 06.02), Sutherland & Dopita (1993) (who used mappings) and also with mappings iii (version r; Groves et al. 2008).”
Theoretical Challenges in Galaxy Formation (2016, background): “If the gas is cooling rapidly, or in the presence of a rapidly changing radiation field, non-equilibrium cooling will be more accurate (see e.g. Gnat & Sternberg 2007, Oppenheimer & Schaye 2013a) and first steps in this direction have been made in galaxy scale simulations (Forbes et al. 2016; Hu et…”
A High-Resolution Survey of Low-Redshift QSO Absorption Lines: Statistics and Physical Conditions of O VI Absorbers (2007, methodology): “Gnat & Sternberg (2007) have recently modeled nonequilibrium radiatively cooling gas, and they provide predictions for these column density ratios as a function of gas temperature.”
The COS-Halos Survey: Metallicities in the Low-redshift Circumgalactic Medium (2017, methodology): “Nevertheless, we caution that low density gas may not be in strict ionization balance (e.g. Gnat & Sternberg 2007).”
The Circumgalactic Medium (2017, methodology): “These two classes of model are not mutually exclusive: a gas that is collisionally ionized may have the ion ratios further affected by incident radiation, and there are numerous possible departures from equilibrium that further complicate modeling (e.g. Gnat & Sternberg 2007b).”
TESTING A PREDICTIVE THEORETICAL MODEL FOR THE MASS LOSS RATES OF COOL STARS (2011, background): “(21) Other examples of the metallicity dependence ofΛ(T) have been given by, e.g., Boehringer & Hensler (1989) and Gnat & Sternberg (2007).”
Galaxy formation theory (2010, background): “Figure 2 shows calculations of effective cooling functions for gas initially in collisional ionization equilibrium (and with no external radiation field) with a fully time-dependent calculation ofcooling and ionization state (Gnat and Sternberg 2007).”
Physical Processes in the Interstellar Medium (2014, background): “Instead, it becomes over-ionized compared to the CIE case (see e.g. Kafatos, 1973; Sutherland & Dopita, 1993; Gnat & Sternberg, 2007).”
Absorption-line systems in simulated galaxies fed by cold streams (2011, methodology): “In our simplest model, we assume collisional ionization at equilibrium (CIE) under the simplifying assumption of a dustfree gas without external radiation (Gnat & Sternberg 2007).”
THE COS-HALOS SURVEY: ORIGINS OF THE HIGHLY IONIZED CIRCUMGALACTIC MEDIUM OF STAR-FORMING GALAXIES (2016, background): “Collisional ionization equilibrium models give TCIE ≈105.4 K where the abundance of O VI peaks (e.g. Gnat & Sternberg 2007; Oppenheimer & Schaye 2013b).”
THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1 (2013, methodology): “We used the collisional ionization equilibrium and non-equilibrium models of Gnat & Sternberg (2007) and shock ionization models of Gnat & Sternberg (2009) to compare the predicted and observed ionic ratios of C II/C III, Si II/Mg II, and O II/Mg II.”
The current status of galaxy formation (2012, background): “For a cooling function Λ(T ) ∝ T β , over the relevant temperature range (105 − 107 K), one can take β ≈ −1/2 for a low metallicity plasma (Gnat & Sternberg, 2007).”
Gravitational quenching in massive galaxies and clusters by clumpy accretion (2007, methodology): “We use the standard cooling function computed by Sutherland & Dopita (1993), which is consistent with the equilibrium state computed by Gnat & Sternberg (2007).”
THE COS-HALOS SURVEY: AN EMPIRICAL DESCRIPTION OF METAL-LINE ABSORPTION IN THE LOW-REDSHIFT CIRCUMGALACTIC MEDIUM (2012, background): “Collisional ionization of gas at T > 104.6 K would yield negligible quantities of ions like Mg+, C+, and Si+, even if at solar abundance and with a large total gas column (e.g., Gnat & Sternberg 2007).”
The Low-z Intergalactic Medium. III. H I and Metal Absorbers at z < 0.4 (2007, background): “…VI are currently the best tracer of the “cooler” portions of these shocks, reaching a peak ionization fraction, fOVI ≈ 0.22 at temperature Tmax = 10 5.45 K in collisional ionization equilibrium or CIE (Sutherland & Dopita 1993) and similar values in non-equilibrium cooling (Gnat & Sternberg 2007).”