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physics.system - September 4 2007 15:49 World Scientic...

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September 4, 2007 15:49 World Scientific Review Volume - 9in x 6in ws-ehtm.final Chapter 1 The Chemistry of Cold Interstellar Cloud Cores Eric Herbst Department of Physics, The Ohio State University, Columbus, OH 43210 USA * Tom J. Millar Astrophysics Research Centre, School of Mathematics and Physics Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland We review the chemical processes that occur in cold (10 K) and dense ( n 10 4 cm - 3 ) interstellar cores, which are the coldest objects in larger assemblies of gas and dust known as dense interstellar clouds. We show how these processes produce the wide variety of exotic and normal molecules detected in these portions of the interstellar medium. Although much of the chemistry occurs in the gas phase, a significant portion takes place on the surfaces of the dust particles. Both types of chemistry are discussed. Some emphasis is placed on deuterium isotopic fractionation, which enhances the abundances of deuterium-containing isotopologues by very large factors compared with the low deuterium- to-hydrogen elemental abundance ratio of 10 - 5 . The strengths and weaknesses of large time-dependent models of the chemistry are dis- cussed. 1.1. Introduction Baryonic matter in the universe is localised to a great extent in large assem- blies of stars and interstellar matter known as galaxies. Galaxies come in a variety of shapes and sizes (e.g. spiral, elliptical, irregular, giant, dwarf); ours, the Milky Way, is a rather typical specimen of the spiral variety and has a baryonic mass of roughly 10 11 solar masses. Except for a halo around the galactic centre, the Milky Way is rather flat, with its spiral arms trac- * Also Departments of Astronomy and Chemistry 1
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September 4, 2007 15:49 World Scientific Review Volume - 9in x 6in ws-ehtm.final 2 E. Herbst and T. J. Millar ing out high densities of material. Approximately 10% of the matter lies in between the stars. In external galaxies, the amount of interstellar matter can be significantly less. Careful study of the spectra of stellar atmospheres in which the matter is mainly atomic yields the relative abundances of each element. Although all stars are not the same in this regard, average or “cosmic abundances” are well-known for our galaxy and tell us that the Milky Way consists mostly of hydrogen, with the abundance of helium roughly 10% of that of hydrogen by number. Both of these elements were produced in the Big Bang. All heavier elements, which are produced in stellar interiors, have much lower abundances. For example, the biogenic elements carbon, nitrogen, and oxygen have abundances relative to hydro- gen of 3 × 10 - 4 , 9 × 10 - 5 , and 7 × 10 - 4 , respectively. The abundance of deuterium is somewhat variable but is roughly 1-2 × 10 - 5 that of hydrogen throughout much of our galaxy. Elemental abundances need not be the same in other galaxies; for example, the abundances of elements heavier than helium, known collectively as the “metallicity” despite the fact that the most abundant elements are certainly not metals, can be much lower in nearby irregular galaxies.
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