碰撞冷却富金属气体的非平衡冷却速率
Non-equilibrium cooling rate for a collisionally cooled metal-enriched gas
摘要：
我们提出了自洽计算的非平衡（随时间变化）冷却速率的无尘碰撞控制气体在宽的温度（$10 K\le T\le 10^8 K$）和金属丰度（$10^{-4} Z_\odot \le Z \le 2 Z_\odot$）范围。我们证实，分子氢占主导地位的冷却在$10^2 \simlt T\simlt 10^4$ K和$Z\simlt 10^{-3} Z_\odot$。我们发现H $_2$对$T\sim (4-5)\times 10^3$ K附近冷却速率的贡献在金属丰度范围$Z\simlt 10^{-2} Z_\odot$内刺激了热不稳定性。等压冷却速率通常低于等容冷却速率，因为气体密度的相关增加导致更有效的氢重组和精细结构水平种群的平衡。等容冷却使电离分数在$T\simlt10^4$ K下保持相当高：在$T\simeq 10^3$ K和$Z\simlt 0.1 Z_\odot$下高达$\sim0.01$，并且在更高的金属丰度下甚至更高，与等压冷却相反，其中它至少低一个数量级。尽管电离分数增加，但分子氢的气相形成速率（通过H $^-$）随着金属丰度降低，因为较高的金属丰度缩短了演化时间。我们实现我们的自洽冷却速率到多维并行代码ZEUS-MP，以模拟超新星遗迹的演变，并将其与类似的模型与以前的作品中发表的列表冷却速率进行比较。我们发现这两种描述之间存在显著差异，例如，在星周介质中混合喷射出的金属。
英文摘要：
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.
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