Probing the Astronomical Gas-Grain Interaction: Formation and Morphology of Icy Grain Materials

分子对当今宇宙演化的重要性不应被低估。分子在恒星形成过程中扮演着关键角色，事实上，如果没有复杂的分子，就不太可能有我们自己的恒星所代表的那种小的、长寿命的恒星。如果没有这样的恒星，生命甚至不可能进化到认识到这一事实。分子不仅充当恒星形成区域的标志，或者作为化学钟，我们通过它远程探测恒星形成过程的时间尺度，而且在气团坍塌形成恒星前天体的早期阶段，分子还充当活跃的辐射冷却剂。正是通过后一种作用，化学可以说是主动控制宇宙！在过去25年左右的时间里，通过分子天体物理学和化学物理界的相互作用，星际分子形成和摧毁的主要过程得到了合理的理解。这些分子在追踪恒星形成区域的气体，特别是通过冷却主动控制气体演化方面的作用已经得到了彻底的探索。在过去的十年中，这一领域研究的主要目的是单纯的气相化学不能成功地解释星际化学的多样性和丰富性。它也不能解释分子在天文学中决定气体中的热压和磁压所起的作用。因此，气相化学的贡献必须得到气体-尘埃相互作用的补充。这些相互作用在正在形成恒星的分子云中密度较高的区域尤为重要。主要的相互作用是颗粒表面的直接化学反应，气体物种冻结形成分子冰，分子冰被环境紫外线处理，分子从冰表面解吸，以及热星附近尘埃上的冰膜蒸发。与表面科学界的互动是最近的，通过增加对构成气体-颗粒相互作用的过程的了解，正在带来实质性的好处。然而，我们认为更充分地理解气体-颗粒相互作用所需的基础表面科学最重要的方面尚未得到解决，其中包括在冷颗粒上形成简单的分子冰，这是本提案的重点。

The importance of molecules to the evolution of the cosmos today should not be under-estimated. Molecules play a key role in the star formation process, indeed without complex molecules it is unlikely that there would ever be small, long-lived stars of the type represented by our own star. Without such stars, it is unlikely that life could have even evolved to recognise this fact. Not only do molecules act as markers for regions of star formation, or as chemical clocks through which we remotely probe the timescale of the star forming process, but also as active radiative coolants during the early stages of the collapse of a gas clump to form a pre-stellar object. It is through this latter role that chemistry can be said to actively control the cosmos!Over the past 25 years or so, through interaction of the molecular astrophysics and chemical physics communities, the main processes by which interstellar molecules are formed and destroyed have become reasonably understood. The role of these molecules in tracing the gas in star forming regions, and especially in actively controlling the evolution of the gas through cooling, have been thoroughly explored. The essential thrust of research in this area during the last decade has been that purely gas phase chemistry cannot successfully account for the variety and richness of interstellar chemistry. Nor can it account for the roles that molecules play in astronomy in determining the thermal and magnetic pressures in the gas. Thus, the contribution of gas phase chemistry must be complemented by the gas-dust interactions. These interactions are particularly important in denser regions of molecular clouds where star formation is occurring. The main interactions are direct chemical reactions at grain surfaces, freeze-out of gaseous species to form molecular ices, processing of molecular ices by ambient UV, desorption of molecules from the ice surface, and evaporation of ice films on dust near hot stars. Interaction with the surface science community is recent and is bringing substantial benefits through an increased understanding of the processes that constitute the gas-grain interaction. However, what we consider to be the most important aspects of fundamental surface science required for a fuller understanding of the gas-grain interaction have yet to be addressed and these include the formation of simple molecular ices on cold grains, which is the focus of this proposal.