Interstellar medium: physical conditions

Chemistry without numbers is poetry astrochemistry without numbers is myth. A molecule placed around a star, in a nebula, lost in the interstellar medium, on a planet or within a cell has the potential for very complex and beautiful chemistry but unless we can understand the local conditions and how the molecule interacts with them we have no idea what chemistry is really happening. To understand astrochemistry we need to understand the physical conditions that occur within many diverse molecular environments. The exploration of the molecular universe will take us on a long journey through the wonders of astronomy to the new ideas of astrobiology... 

Laboratory astrophysics is not an oxymoron. Laboratory studies are one of the very few means to understand the chemical and physical processes that occur in the outlandish environments (by terrestrial standards) observed by astronomers. Such processes often occur under conditions that are far removed from what is considered normal by most chemists and physicists, and the application of a terrestrially honed chemical intuition to processes in astrophysical systems is likely to yield incorrect interpretations of the observations unless the environmental effects are carefully considered. The design of experiments that address processes active in astrophysical environments must also account for differences between the laboratory and reality. Often one finds that it is neither necessary or even desirable to carry out experiments under typical astrophysical conditions. Few funding agencies will support experiments lasting ten million (or even ten ) years to duplicate processes in the interstellar medium, and few institutions will hire or promote a... 

Solid icy surfaces are observed both in the interstellar medium as mantles on silicatic or carbonaceous grains and on many objects in the Solar System." In space, these icy targets are continuously bombarded by energetic ions from solar wind and flares, planetary magnetospheres, stellar winds and galactic cosmic rays. When an energetic ion collides with an icy target produces physico-chemical modifications in the latter. The study of those effects is based on laboratory ion irradiation experiments carried out under physical conditions as close as possible to the astrophysical ones. 

The distribution of elements in the cosmos is the result of many different physical processes in the history of the Universe, from Big Bang to present times. Its study provides us with a powerful tool for understanding the physical conditions of the primordial cosmos, the physics of nucleosynthesis processes that occur in different objects and places, and the formation and evolution of stars and galaxies. Cosmochemistry is a fundamental topic for many different branches of Astrophysics as Cosmology, Stellar Structure and Evolution, Interstellar Medium, and Galaxy Formation and Evolution. 

Clearly all of these reactions (and perhaps others) may contribute to the formation of C-C3H3 In the interstellar medium, but their relative contributions will depend on the physical and chemical conditions which prevail. 

The operating conditions for the Haber-Bosch process are far from the physical conditions in those regions in the interstellar medium where the majority of molecules are found. On the other hand, the low temperatures in cold, dark molecular clouds do encourage the occurrence of physisorption, and it is also necessary to recognise that many of the species that may be physisorbed are unsaturated molecules or radicals for which low energy reaction pathways may exist on dust grains. 

The structure of the interstellar medium phases, and especially how they are spatially organized, is not fully elucidated. Large scale surveys of tracers of the neutral, ionized and coronal gas have led to determination of the global filling factors, and of the average physical conditions as listed in Table 2.1. The coronal gas, with temperatures in excess of 10 K is mostly formed by the bubbles created in... 

The analysis of radiation across the electromagnetic spectrum has allowed astronomers to uncover the composition of the neutral interstellar medium, its spatial variation and how it relates to the local environment (physical conditions) as well as the overall evolutionary stage of the object. 

The presence of reactive ions like CH in the diffuse interstellar medium has been a challenge for interstellar chemistry since CH is easily destroyed by reactions with H2 but slow to form under the known physical conditions of the diffuse interstellar medium. It now appears that a warm chemistry can develop in the tiny dissipative structures of the interstellar turbulence, enabling the formation of transient species like CH and SH [43], The opening up of the sub-millimetre sky by the Herschel telescope has led to the discovery of several new reactive ions, enabling a better characterization of their chemistry. In the future, these tracers should bring interesting constraints on the properties of the interstellar turbulence. 

Abstract We discuss models that astrochemists have developed to study the chemical composition of the interstellar medium. These models aim at computing the evolution of the chemical composition of a mixture of gas and dust under astrophysical conditions. These conditions, as well as the geometry and the physical dynamics, have to be adapted to the objects being studied because different classes of objects have very different characteristics (temperatures, densities, UV radiation fields, geometry, history etc) e.g., proto-planetary disks do not have the same characteristics as proto-stellar envelopes. Chemical models are being improved continually thanks to comparisons with observations but also thanks to laboratory and theoretical work in which the individual processes are studied. 

In Fig. 4.1, we show the results of a model for dense cloud conditions (a temperature of 10 K, a hydrogen atom density of 2 x 10" cm andavisualextinctionof 10), which includes only gas-phase processes, except for the production of molecular hydrogen, which occurs on granular surfaces. The chemical processes that are involved in the chemistry of the interstellar medium are described in Sect 4.3 of this chapter. In addition to the parameters described in Sect. 4.2.1, the geometry of the object, the presence of mixing, and physical dymamics can influence the chemical composition as well (Sects. 4.2.2 and 4.2.3). 

Most, if not all, astrophysical objects are not static over a period of time long enough for the chemistry to reach steady state, except perhaps in the diffuse medium as long as the initial H2/H abundance ratio is assumed to be non-zero. As a consequence, the modifications of the physical conditions with time have to be considered. A good example is the modelling of circumstellar envelopes. The cells of material pushed away from the star encounter lower temperatures and densities. Close to the star, the temperatures are so high that species are only in atomic form. As the material moves away from the star, molecules wiU be formed and survive until they encounter a much thinner medium where the interstellar UV field will destroy them. A schematic view of the physical structure, which is far more complex than discussed here, and the chemical composition of a circumstellar envelope are given by Fig. 4.3. 

Fig. 4.4 Schematic view of the dominant physico-chemical processes in the interstellar medium depending on the physical conditions...

Strong shocks produced by the expanding ionized envelopes of massive stars and supernova remnants heat and compress the interstellar medium, leading to conditions ripe for many high-temperature chemical reactions. Like grain surface chemistry, reactions within the shock chemistry environment are difficult to simulate, but are progressing toward a physical framework that can be compared to observations. While OH and H2O are prominent products of ion-molecule chemistry as well as shock chemistry, SiO, and SiS are predominently produced in shocks. 

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