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

Abstract

We present self-consistent calculations of non-equilibrium (time-dependent) cooling rates for a dust-free collisionally controlled gas in wide temperature ($10 K\le T\le 10^8 K$) and metallicity ($10^{-4} Z_\odot \le Z \le 2 Z_\odot$) ranges. We confirm that molecular hydrogen dominates cooling at $10^2 \simlt T\simlt 10^4$ K and $Z\simlt 10^{-3} Z_\odot$. We find that the contribution from H$_2$ into cooling rate around $T\sim (4-5)\times 10^3$ K stimulates thermal instability in the metallicity range $Z\simlt 10^{-2} Z_\odot$. Isobaric cooling rates are generally lower than isochoric ones, because the associated increase of gas density leads to both more efficient hydrogen recombination and equilibration of the fine-structure level populations. Isochoric cooling keeps the ionization fraction remains quite high at $T\simlt10^4$ K: up to $\sim0.01$ at $T\simeq 10^3$ K and $Z\simlt 0.1 Z_\odot$, and even higher at higher metallicity, contrary to isobaric cooling where it at least an order of magnitude lower. Despite this increase in ionization fraction the gas-phase formation rate of molecular hydrogen (via H$^-$) lowers with metallicity, because higher metallicity shorttens the evolution time. We implement our self-consistent cooling rates into the multi-dimensional parallel code ZEUS-MP in order to simulate evolution of a supernova remnant, and compare it with an analogous model with tabulated cooling rates published in previous works. We find significant differences between the two descriptions, which may appear, e.g., in mixing of the ejected metals in the circumstellar medium.