Formation and evolution of interstellar icy grain mantles
Formation and evolution of interstellar icy grain mantles
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星际冰粒地幔的形成和演化
DOI：
10.1007/978-94-011-5652-3_1
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
W. Schutte
中科院分区：
文献类型：
--
作者：
W. Schutte
我们回顾了致密云层中凝聚在星际颗粒上的冰地幔的演化。被遮蔽天体的红外吸收光谱特征表明，H2O是星际冰中最丰富的成分，CO、CH3OH以及可能的CO2和H2CO都有重要贡献。模型进一步预测O2、N2、NH3和CH4可能会产生显著的贡献。拟合所观察到的红外波段形状与实验室生产的各种成分的冰，它推导出的各种成分是不均匀混合，而是不同的阶段存在不同的化学组成。一个阶段是由水冰为主，而另一个是由非极性物种为主，即，CO和可能的O2、N2和CO2。此外，可能存在富含CH_3OH和H_2O冰的第三相。在浓云化学模型的基础上，讨论了这些阶段中的每一个如何对应于以不同密度和消光为特征的云区中的凝结。考虑到这一点以及对云的动态演化的简单看法，有人认为，冰地幔可能具有洋葱壳状结构，这是由于不同类型的冰层在不同的物理区域中凝结而成的。它讨论了如何处理紫外线和宇宙射线可以修改冰，产生了一些相对复杂的，往往是反应性的物种。我们审查目前的证据修改的气相化学的分子形成的粮食地幔，随后喷出。最后，它被认为是如何未来的观测空间为基础的天文台可以提高我们的知识的星际冰的组成，以及如何这可能会导致更好地了解稠密的云化学一般。
We review the evolution of the icy mantles that condense on interstellar grains in dense clouds. Infrared absorption features in the spectra of obscured objects show that H2O is the most abundant constituent of interstellar ices, with important contributions by CO, CH3OH and possibly CO2and H2CO. Models furthermore predict that O2, N2, NH3and CH4could make significant contributions. Fitting the observed infrared band shapes with laboratory produced ices of various composition, it was derived that the various components are not homogeneously mixed but rather that various phases are present of distinct chemical make-up. One phase is dominated by water ice, while another is dominated by apolar species, i.e., CO and possibly O2, N2and CO2. Furthermore, a third phase rich in CH3OH as well as H2O ice is probably present. On the basis of models of dense cloud chemistry, it is discussed how each of these phases could correspond to condensation in cloud regions characterized by distinct densities and extinction. Considering this as well as a simple-minded view on the dynamic evolution of clouds, it is argued that the icy mantles could have an onion-shell like structure, resulting from the condensation of layers of different types of ices in the physically distinct regions in which the grain resides over the course of time. It is discussed how processing by UV and cosmic rays could modify the ices, giving rise to a number of relatively complex and often reactive species. We review current evidence for the modification of the gas phase chemistry by molecules which were formed in the grain mantles and subsequently ejected. Finally, it is considered how future observations by space-based observatories could enhance our knowledge of the composition of interstellar ices and how this could lead to a better understanding of dense cloud chemistry in general.