Abstract
The chemistry that occurs in interstellar clouds consists of both gas-phase processes and reactions on the surfaces of dust grains, the latter particularly on and in water-dominated ice mantles in cold clouds. Some of these processes, especially at low temperature, are very unusual by terrestrial standards. For example, in the gas-phase, two-body association reactions form a metastable species known as a complex, which is then stabilized by the emission of radiation under low-density conditions, especially at low temperatures. In the solid phase, it has been thought that the major process for surface reactions is diffusive in nature, occurring when two species undergoing random walks collide with each other on a surface that has both potential wells and intermediate barriers. There is experimental evidence for this process, although very few rates at low interstellar temperatures are well measured. Moreover, since dust particles are discrete, modeling has to take account that reactant pairs are on the same grain, a problem that can be treated using stochastic approaches. In addition, it has been shown more recently that surface reactions can occur more rapidly if they undergo any of a number of non-diffusive processes including so-called three-body mechanisms. There is some experimental support for this hypothesis. These and other unusual gaseous and solid-state processes will be discussed from the theoretical and experimental points of view, and their possible role in the synthesis of organic molecules in interstellar clouds explained. In addition, their historical development will be reviewed.
Highlights
INTRODUCTIONThe interstellar medium is exceedingly diverse (Tielens, 2005). Much of it is composed of so-called interstellar clouds of gas and dust, which range from rather wispy diffuse clouds to dense clouds that are impervious to background light and so can appear black against the sky
The Interstellar MediumThe interstellar medium is exceedingly diverse (Tielens, 2005)
If the complex can be stabilized by emission of radiation with some degree of efficiency, radiative association can be an important process for the growth of molecules in the cold interstellar medium at densities far too low for collisional stabilization
Summary
The interstellar medium is exceedingly diverse (Tielens, 2005). Much of it is composed of so-called interstellar clouds of gas and dust, which range from rather wispy diffuse clouds to dense clouds that are impervious to background light and so can appear black against the sky. The production of low-mass stars arises from the collapse and warm-up of cold cores through regions known as “prestellar cores” followed by “protostellar cores,” a term for stars in the act of formation, during which gas and dust collapse inwards to form so-called “hot corinos” with temperatures exceeding 100 K. During this stage, early-stage disks are formed surrounding the protostar. Some of the unusual gas-phase and granular processes will be discussed within a historical background These reactions include those that can occur efficiently only in low density regions. These products can form from the complex and so compete directly with complex emission or they can be formed from reactants and compete indirectly by division of the total rate coefficient
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