银河系半透明云中的水吸收：来自Herschel/HIFI PRISMAS观测的气体的条件和历史
Water absorption in Galactic translucent clouds: conditions and history of the gas derived from Herschel/HIFI PRISMAS observations
摘要：
我们提出了Herschel/HIFI观测\hho和\hheo向六个高质量恒星形成区域的九个转变，作为PRISMAS关键计划的一部分。半透明云中的水蒸气在沿每一视线的吸收中被检测到。我们推导出的柱密度的\hho或\hheo的较低的能级观察到的每个过渡。总水柱密度约为几个$10^{13} \rm{cm^{-2}}$。我们发现水相对于氢核的丰度为$1\times10^{-8}$，与具有高宇宙射线电离率的氧化学模型一致。相对于\hh，$5\times10^{-8}$的水丰度非常恒定。激发态水的丰度最多为15%，这意味着基态跃迁的激发温度$T_{ex}$低于10 K。从两个邻位基态跃迁导出的列密度表明，$T_{ex}\simeq5$ K和密度$n($ \hh $)$在云中是$\le10^4 \rm{cm^{-3}}$。对于大多数云，我们得到一个水的邻-对-对位比符合预期的值3在高温极限的热力学平衡。两个具有大柱密度的云显示出明显低于3的比率。这可能会认为水分子的历史包括一个冷阶段，当分子在冷颗粒上形成时，或者当它们后来在云层的屏蔽低温区域中至少部分地与冷气体（$\sim25$ K）热化时;显然，它们还没有完全与云层的温暖（$\sim50$ K）半透明部分热化。
英文摘要：
We present Herschel/HIFI observations of nine transitions of \hho and \hheo towards six high-mass star-forming regions, obtained as part of the PRISMAS Key Program. Water vapor in translucent clouds is detected in absorption along every sightline. We derive the column density of \hho or \hheo for the lower energy level of each transition observed. The total water column density is about a few $10^{13} \rm{cm^{-2}}$. We find that the abundance of water relative to hydrogen nuclei is $1\times10^{-8}$ in agreement with models for oxygen chemistry with high cosmic ray ionization rates. Relative to \hh, the abundance of water is remarkably constant at $5\times10^{-8}$. The abundance of water in excited levels is at most 15%, implying that the excitation temperature $T_{ex}$ in the ground state transitions is below 10 K. The column densities derived from the two ortho ground state transitions indicates that $T_{ex}\simeq5$ K and that the density $n($\hh$)$ in the clouds is $\le10^4 \rm{cm^{-3}}$. For most clouds we derive a water ortho-to-para ratio consistent with the value of 3 expected in thermodynamic equilibrium in the high temperature limit. Two clouds with large column densities exhibit a ratio significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains, or when they later become at least partially thermalized with the cold gas ($\sim25$ K) in the shielded, low temperature regions of the clouds; evidently, they have not yet fully thermalized with the warmer ($\sim50$ K) translucent portions of the clouds.
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