以CO示踪的GMF G214.5-1.8：I——低宇宙射线电离率导致的云尺度CO冻结
GMF G214.5-1.8 as traced by CO: I -- cloud-scale CO freeze-out as a result of a low cosmic-ray ionisation rate
AI总结：
本研究利用IRAM 30m数据分析了银河系外缘巨型分子丝G214.5-1.8，发现其极低温度和$^{13}$CO丰度下降源于云尺度CO冻结，并指出该现象由低于典型值一个数量级的宇宙射线电离率驱动，可能促进双极扩散，对宁静区域的CO观测与解释具有重要意义。
摘要：
我们利用IRAM 30m望远镜的$^{12}$CO、$^{13}$CO和C$^{18}$O数据，对银河系外缘巨型分子丝（GMF）G214.5-1.8（G214.5）进行了分析。我们发现由$^{12}$CO (1-0)和(2-1)导出的激发温度几乎相同且极低，中位数为8.2 K，表明整个云体的气体极其寒冷。通过研究G214.5中$^{13}$CO的丰度，我们发现在整个13 pc长的丝状结构主轴上，丰度显著偏低，延伸至半径约0.8 pc处，对应于$A_v \gtrsim 2$ mag和$T_{dust} \lesssim 13.5$ K。基于此，我们将丰度的下降归因于CO冻结，使G214.5成为迄今为止最大尺度的冻结观测案例。我们构建了考虑冻结效应的G214.5 $^{13}$CO体积密度轴对称模型，发现要重现观测到的剖面，需要从低体积密度（$n\gtrsim2000$ cm$^{-3}$）开始就出现显著的耗竭。只有在宇宙射线电离率约为$1.9 \times 10^{-18}$ s$^{-1}$（比典型值低一个数量级）的情况下，才可能在这种低数密度下发生冻结。基于时标论证，我们推测如此低的电离率可能使得双极扩散成为沿G214.5整个主轴的重要物理过程。我们提出，如果低宇宙射线电离率在银河系外缘及其他宁静区域更为普遍，那么在低柱密度和低数密度下发生的云尺度CO冻结可能也会更加普遍，这将对CO观测及其解释产生影响。
英文摘要：
We present an analysis of the outer Galaxy giant molecular filament (GMF) G214.5-1.8 (G214.5) using IRAM 30m data of $^{12}$CO, $^{13}$CO and C$^{18}$O. We find that the $^{12}$CO (1-0) and (2-1) derived excitation temperatures are near identical and are very low, with a median of 8.2 K, showing that the gas is extremely cold across the whole cloud. Investigating the abundance of $^{13}$CO across G214.5, we find that there is a significantly lower abundance along the entire 13 pc spine of the filament, extending out to a radius of $\sim 0.8$ pc, corresponding to $A_v \gtrsim 2$ mag and $T_{dust} \lesssim 13.5$ K. Due to this, we attribute the decrease in abundance to CO freeze-out, making G214.5 the largest scale example of freeze-out yet. We construct an axisymmetric model of G214.5's $^{13}$CO volume density considering freeze-out and find that to reproduce the observed profile significant depletion is required beginning at low volume densities, $n\gtrsim2000$ cm$^{-3}$. Freeze-out at this low number density is possible only if the cosmic-ray ionisation rate is $\sim 1.9 \times 10^{-18}$ s$^{-1}$, an order of magnitude below the typical value. Using timescale arguments, we posit that such a low ionisation rate may lead to ambipolar diffusion being an important physical process along G214.5's entire spine. We suggest that if low cosmic-ray ionisation rates are more common in the outer Galaxy, and other quiescent regions, cloud-scale CO freeze-out occurring at low column and number densities may also be more prevalent, having consequences for CO observations and their interpretation.
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