Non-equilibrium cooling rate for a collisionally cooled metal-enriched gas

We present self-consistent calculations of non-equilibrium (time-dependent) cooling rates for a dust-free collisionally controlled gas in wide ranges of temperature (10 ≤ T ≤ 108 K) and metallicity (10−4 ≤ Z ≤ 2 Z). We confirm that molecular hydrogen dominates cooling at 102 T 104 K and Z 10− 3 Z. We find that the contribution from H2 into the cooling rate around T ∼ (4-5) × 103 K stimulates thermal instability in the metallicity range Z 10− 2 Z. Isobaric cooling rates are generally lower than isochoric cooling rates, because the associated increase of gas density leads to both more efficient hydrogen recombination and equilibration of the fine-structure level populations. Isochoric cooling means that the ionization fraction remains quite high at T 104 K - up to ∼0.01 at T 103 K and Z 0.1 Z, and even higher at higher metallicity - unlike isobaric cooling, where it is at least an order of magnitude lower. Despite this increase in ionization fraction, the gas-phase formation rate of molecular hydrogen (via H−) decreases with metallicity, because higher metallicity shortens the evolution time. We implement our self-consistent cooling rates into the multidimensional parallel code zeus-mp in order to simulate the evolution of a supernova remnant. We compare it to an analogous model with tabulated cooling rates published in previous studies. We find significant differences between the two descriptions, which might appear, for example, in the mixing of the ejected metals in the circumstellar medium.