WATER, O2, AND ICE IN MOLECULAR CLOUDS
WATER, O2, AND ICE IN MOLECULAR CLOUDS
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分子云中的水、氧气和冰
DOI：
10.1088/0004-637x/690/2/1497
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
G. Melnick
中科院分区：
文献类型：
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作者：
D. Hollenbach;M. Kaufman;E. Bergin;G. Melnick
我们将分子云的温度和化学结构建模为进入云的深度的函数，假设在外部远紫外（FUV; 6 eV <hν < 13.6 eV）通量G0（倍数倍数的比例因子）照射下，云的密度为恒定n。扩展了以前的光解离区（PDR）模型，我们包括了物种的冻结，简单的颗粒表面化学和冰的解吸（包括FUV光解吸）。我们还用随时间变化的化学方法来处理不透明的云内部。在这里，在一定条件下，气相单质氧冻结成水冰，单质C/O丰度比可以超过1，导致复杂的碳化学。气相H2O和O2的丰度在进入云层的中间深度处达到峰值，大约距离表面AV ~ 3-8，深度与ln（G0/n）成正比。靠近表面，分子被光解离。在云层深处，分子冻结成颗粒表面。在中等深度，pdr因粉尘消退而衰减，但光解吸防止完全冻干。当G0 < 500时，H2O和O2的丰度在~ 10−7处达到峰值，产生的色谱柱为~ 1015 cm−2，与G0和n无关。峰值丰度主要取决于水冰的光解吸产率和每H核的晶粒表面积。在较高的G0值下，晶粒中O原子的热解吸略微提高了气相H2O的峰丰度和色谱柱，而气相O2的峰丰度上升到~ 10−5，色谱柱上升到~ 2 × 1016 cm−2。我们提出了丰度作为深度函数的简单解析方程，阐明了丰度对参数的依赖性。这些模型被应用于观测许多来源的H2O、O2和水冰，包括B68、NGC 2024和Oph。
We model the temperature and chemical structure of molecular clouds as a function of depth into the cloud, assuming a cloud of constant density n illuminated by an external far-ultraviolet (FUV; 6 eV <hν < 13.6 eV) flux G0 (scaling factor in multiples of the local interstellar field). Extending previous photodissociation region (PDR) models, we include the freezing of species, simple grain surface chemistry, and desorption (including FUV photodesorption) of ices. We also treat the opaque cloud interior with time-dependent chemistry. Here, under certain conditions, gas-phase elemental oxygen freezes out as water ice and the elemental C/O abundance ratio can exceed unity, leading to complex carbon chemistry. Gas-phase H2O and O2 peak in abundance at intermediate depth into the cloud, roughly AV∼ 3–8 from the surface, the depth proportional to ln(G0/n). Closer to the surface, molecules are photodissociated. Deeper into the cloud, molecules freeze to grain surfaces. At intermediate depths, PDRs are attenuated by dust extinction, but photodesorption prevents total freeze-out. For G0 < 500, abundances of H2O and O2 peak at values ∼10−7, producing columns ∼1015 cm−2, independent of G0 and n. The peak abundances depend primarily on the product of the photodesorption yield of water ice and the grain surface area per H nucleus. At higher values of G0, thermal desorption of O atoms from grains slightly enhances the gas-phase H2O peak abundance and column, whereas the gas-phase O2 peak abundance rises to ∼10−5 and the column to ∼2 × 1016 cm−2. We present simple analytical equations for the abundances as a function of depth, which clarify the dependence on parameters. The models are applied to observations of H2O, O2, and water ice in a number of sources, including B68, NGC 2024, and ρ Oph.