分子云中的水、O2和冰
Water, O2 and Ice in Molecular Clouds
摘要：
我们模拟了分子云的温度和化学结构随云深度的变化，假设云的密度n恒定，由外部FUV（6 eV < E < 13.6 eV）通量G_0（标度因子为局部星际场的倍数）照射。扩展以前的光解区域模型，我们包括冻结的物种，简单的粮食表面化学，和解吸（包括FUV光解吸）的冰。我们还处理不透明的云内部与时间相关的化学。在这里，在某些条件下，气相元素氧冻结为水冰，元素C/O丰度比可以超过1，导致复杂的碳化学。气相H_2O和O_2在距地面约3-8 A_V处的中间深度达到最大值，深度与ln（G_0/n）成正比。在靠近表面的地方，分子发生光解离。在云层深处，分子冻结在颗粒表面。在中间深度，光解离速率被尘埃消光衰减，但光解吸阻止总冻结。当G_0 < 500时，H2O和O2的丰度在约10^（-7）处达到峰值，每平方厘米产生约10^（15）个柱，与G_0和n无关。峰值丰度主要取决于水冰的光解吸产率和每个H核的颗粒表面积的产品。在较高的G_0值下，O原子从晶粒的热脱附略微增加了气相H2O峰丰度和柱，而气相O2峰丰度上升到~10^（-5），柱上升到~ 2x 10 ^（16）/cm ^2。我们提出了简单的解析方程的丰度作为深度的函数，澄清参数的依赖性。该模型适用于观测的H2O，O2和水冰在一些来源，包括B68，NGC 2024，和Rho Oph。
英文摘要：
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 FUV (6 eV < E < 13.6 eV) flux G_0 (scaling factor in multiples of the local interstellar field). Extending previous photodissociation region 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 A_V~3-8 from the surface, the depth proportional to ln(G_0/n). Closer to the surface, molecules are photodissociated. Deeper into the cloud, molecules freeze to grain surfaces. At intermediate depths photodissociation rates are attenuated by dust extinction, but photodesorption prevents total freezeout. For G_0 < 500, abundances of H2O and O2 peak at values ~10^(-7), producing columns ~10^(15) per cm^2, independent of G_0 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 G_0, thermal desorption of O atoms from grains enhances the gas phase H2O peak abundance and column slightly, whereas the gas phase O2 peak abundance rises to ~10^(-5) and the column to ~2x10^(16) per cm^2. We present simple analytic 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 Rho Oph.
机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。
