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

TL;DR

This paper provides detailed calculations of non-equilibrium cooling rates for metal-enriched gases across wide temperature and metallicity ranges, highlighting the role of molecular hydrogen and differences between isobaric and isochoric cooling.

Contribution

It introduces self-consistent non-equilibrium cooling rates for collisionally cooled, metal-enriched gases and compares their effects in supernova remnant simulations with previous tabulated models.

Findings

Molecular hydrogen dominates cooling at 100-10,000 K and low metallicity.

H2 contributes to thermal instability around 4-5×10^3 K at low metallicity.

Significant differences in supernova remnant evolution when using new vs. tabulated cooling rates.

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.