The interstellar medium

Astrochemistry from astronomy to astrobiology. Andrew M. Shaw 2006 John Wiley Sons, Ltd 

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In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

To date, researchers have identified more than 100 different molecules, composed of up to 13 atoms, in the interstellar medium [16]. Most were initially detected at microwave and (sub)millimetre frequencies, and the discoveries have reached far beyond the mere existence of molecules. Newly discovered entities such as difhise mterstellar clouds, dense (or dark) molecular clouds and giant molecular cloud complexes were characterized for the first time. Indeed, radioastronomy (which includes observations ranging from radio to submillunetre frequencies) has dramatically changed our perception of the composition of the universe. Radioastronomy has shown that most of the mass in the interstellar medium is contained in so-called dense... 

Herbst E 1995 Chemistry in the interstellar medium Ann. Rev. Rhys. Chem. 46 27-53... 

Comparison with the empirical Equation (1.4) shows that = /re /S/z eg and that n" = 2 for the Balmer series. Similarly n" = 1, 3, 4, and 5 for the Lyman, Paschen, Brackett and Pfimd series, although it is important to realize that there is an infinite number of series. Many series with high n" have been observed, by techniques of radioastronomy, in the interstellar medium, where there is a large amount of atomic hydrogen. For example, the (n = 167) — ( " = 166) transition has been observed with V = 1.425 GFIz (1 = 21.04 cm). 

Table 5.2 lists some of the molecules which have been detected. It is interesting to note that some of them, such as the linear triatomics C2H, HCO and N2H, were found in the interstellar medium before they were searched for and found in the laboratory. In all molecules, except OH and NH3, the transitions observed are rotational in nature. 

Identification of a molecule known in the laboratory is usually unambiguous because of the uniqueness of the highly precise transition frequencies. However, before frequencies detected in the interstellar medium can be compared with laboratory frequencies they must be corrected for the Doppler effect (see Section 2.3.2) due to the motion of the clouds. In Sagittarius B2 the molecules are found to be travelling fairly uniformly with a velocity of... 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

The hydrogen atom and its spectrum are of enormous importance in astrophysics because of the large abundance of hydrogen atoms both in stars, including the sun, and in the interstellar medium. 

There has been a resurgence of interest in atomic HF calculations because astrophysicists want to study highly ionized atomic species in the interstellar medium. They look to theory for their energy-level data rather than earth-bound experiments where the species are hard to prepare and study. 

And over all, as time and studies around this new interstellar component increase, it reveals to be possibly related to the polycyclic aromatic hydocarbon family (PAHs), those controversial molecules of prime interest which could be omnipresent in the interstellar medium and an essential link between simple molecules and grains. 

The reactions of ground state atomic carbon, C(3Pj), with unsaturated hydrocarbons are another important class of reactions characterized by multiple pathways. These reactions, besides being of fundamental interest, are of great relevance in the chemistry of the interstellar medium and also in combustion.12,93-95... 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions ... 

There are many situations in which scientists need to know how alike a number of samples are. A quality control technician working on the synthesis of a biochemical will want to ensure that each batch of product is of comparable purity. An astronomer with access to a large database of radiofrequency spectra, taken from observation of different parts of the interstellar medium, might need to arrange the spectra into groups to determine whether there is any correlation between the characteristics of the spectrum and the direction of observation. 

Ehrenfreund, P. and Charnley, S.B. (2000). Organic molecules in the interstellar medium, comets and meteorites. Annu. Rev. Astron. Astrophy., 38, 427-483... 

Linked to 1) is of course the enrichment of the interstellar medium, to which they are important contributors in nuclearly processed elements as He, C, N, s-elements (Ba etc). Goal 2) can be pursued with nuclearly unprocessed elements , the best accessible of them being O, Ne, Ar and S. 

Dust is probably present in the DLA and significantly affects the observed abundances. I counted 55 systems for which both Fe and Zn are measured, which are plotted in Fig. 1. The abundance of Fe is always found below that of Zn. By analogy with the interstellar medium, this behaviour is interpreted as the effect of some Fe being locked into dust grains. Other indicators for the presence of dust... 

The idea behind this hypothesis is that an early generation of AGBs might have evolved and polluted the interstellar medium with nuclearly processed material, so that the new generations of stars were born from an already contaminated gas (D Antona et al. 1983 Cottrell Da Costa 1981). 

Investigate the creation of stars and their interaction with the interstellar medium. 

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... 

The temperature of a molecule within any astronomical environment may vary from the intense cold of the interstellar medium with a temperature of 10 K to the temperature within a sun spot 4000 K close to the temperature at which a molecule would fall apart. The relative intensity of transitions along the progression is given by a line strength factor ... 

A similar temperature analysis is possible for the P-branch transitions to determine the temperature, and usually the R-branch and P-branch temperatures agree. It is from analysis of the relative intensities of transitions that temperatures for the interstellar medium and the photospheres of stars may be determined. 

Putting in the rotational constant for CO = 1.9313 cm-1 but converting this to the SI units gives 5.719 x 1010 s-1 and a temperature of 50 K gives a relative intensity of 1.803. The R(l) transition is nearly twice as intense as the R(0) transition. When the temperature drops to 10 K, the R(1)/R(0) ratio is 0.595 and it is this variation in relative intensity that enables the temperature of the interstellar medium to be determined compare this with intensities in Figure 3 7. 

Energy levels within an atom or molecule can be populated in several ways to produce more target species in the higher energy excited state than in the ground state. The population can occur by collisional processes such as between molecules in the interstellar medium and a balance can occur between the excitation process and a number of deactivation processes (Figure 3.17a). The population of level 2 can be subjected to ... 

A recent success in the detection of H species has been that of the molecular ion H3+. All of the models of ion-molecule chemistry in hydrogen-dominated regions are controlled by reactions of H3+ but until recently the H2+ molecular ion had not been detected. However, the modes of vibration of H3"1" provide for an allowed IR transition at 3.668 pin used for its detection. These ro-vibrational transitions have now been observed in a number of places, including the interstellar medium and in the aurorae of Jupiter. Not all astronomical detection and identification problems have been solved, however, and the most annoying and compelling of these is the problem of diffuse interstellar bands. 

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