Observations of Interstellar Molecules

Within the last four years, nearly 30 different molecules have been identified in the interstellar gas clouds of our Galaxy. This advance has been made possible in part by improved techniques of radio astronomy, and has added a large variety of new interstellar molecules to the list of the three radicals CN, CH, and CH+, known from their ultraviolet spectra since before 1940 (Adams, 

1947 Swings and Rosenfeld, 1937 McKellar, 1940). The discovery in 1963 of the XI8 cm radio spectrum of OH by Weinreb et al. was the first identification of an interstellar molecule by radio astronomy. Thus, up until 1968, only four interstellar molecules were known to exist, leading to the generally accepted conclusion that simple free radicals were the main interstellar molecular constituents in a highly dilute gas ( 1 particle cm-3), subject to ionizing ultraviolet radiation. 

However, the detection of radio spectral lines in the frequency range of about 22 to 23 GHz from the polyatomic molecules NH3 and H20 (in 1968 and 1969) by Cheung et al. and the discovery of the organic molecule H2CO (in 1969) by Snyder et al. marked the beginning of a long series of discoveries. From these and all subsequent discoveries it became evident that dense and cool condensations of the interstellar matter are particularly rich in molecules. 

It is in the dark and black clouds (described in Section II. D) that rather complex organic molecules are being found. Fig. 13 presents the electromagnetic spectrum from about 1 A(108cm 1)to 1010 A (10 2 cm-1). Shown schematically are atmospheric absorption, the different techniques employed by astronomy and the various molecular effects occurring throughout this range. 

BALLON. TELESCOPE RADIO TELESCOPE AIRCRAFT. ROCKET  

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Besides being of interest by themselves, interstellar molecules hdve become essential tools for astronomers, physicists and chemists interested in the study of the general properties of the interstellar medium. Areas which have been deeply influenced by the observation of interstellar molecules and where substantial new insight into physical and chemical processes have been gained can be placed into four large groups ... 

The following sections deal with II the physical conditions in interstellar space, III observations of interstellar molecules and their interpretation, including relevant laboratory measurements, and Section IV experimental and theoretical investigations of the processes of formation and destruction relevant to interstellar molecules. This review covers primarily the interstellar matter proper and refers only briefly to observations of molecules in circum-stellar shells, or molecule formation in protostellar nebulae. Table 1 sets out certain quantities which are used in astronomy and are necessary to an understanding of this paper. 

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. 

In 1981, Geiss and Reeves 100) suggested that the deuterium enrichment observed in the organic macromolecular compounds could be explained on the basis of a suggestion made by the same authors in 1972 101 , i.e. the survival of interstellar molecules [synthesised at low temperatures (10—50 K) by ion-molecule gas-phase reaction] in chondritic matter. 

Recent theoretical gas phase model calculations leading to the observed molecular complexity are discussed together with a critical evaluation of gas phse versus grain syntheses of interstellar molecules. Finally the question of how large interstellar molecules can be is addressed, seen in the light of chemical evolution. 

Most molecules observed to date in interstellar space can be dissociated by UV radiation of wave lengths longer than 912 A. In fact, their average lifetimes in interstellar space are <100 yr, unless they are protected by a dust layer (Section IV. E). This, and the fact that surface reactions on dust grains play an important role in the formation of interstellar molecules (see Section IV. 

In Section B we have discussed how the basic quantities of line emission and absorption, the excitation temperature Tex and optical depth r can be determined from observations. Energies required for rotational excitation are generally low enough (< 200 cm-1) so that the rotational levels are expected to be populated even at the very low kinetic temperatures of the interstellar molecular clouds. On the other hand, with a few exceptions such as H20 and NH3, one may assume that only the lowest energy levels of interstellar molecules are populated. Under these conditions the observable fractional column density Nx may not deviate appreciably from the total column density N of a molecule, which can be computed by means of Eq. (17) on the assumption of LTE. 

The wide variety of interstellar molecules detected so far in our Galaxy (see Table 6) are composed of the most abundant chemically reactive elements, i.e. H, C, N, 0, Si and S. The selection of detected molecules is influenced by molecular and observational considerations i) the molecules must be polar ii) they must have sufficient vapor pressure for their laboratory spectra to be known, iii) of the known spectra, only the most intense transitions can be expected to be observable in interstellar space, and iv) the frequencies of these transitions have to be located within the Earth s atmospheric windows Only molecules which satisfy these conditions are amenable to radio techniques. 

With the rapid increase in the numbers of identified types of interstellar molecules in the early 1970s, comprehensive models of interstellar chemistry were developed by a number of authors, including Klemperer, Solomon and Herbst. These models showed that much Important chemistry occurs in reactions between ions and molecules, with the ionization being maintained by cosmic rays or by photoionization. Such reactions can account for the observed interstellar ions (now numbering... 

In line with the spirit of this book, I will present mainly the highlights of our contributions and those of other laboratories on this subject, with autobiographical comments, following an approximate chronological order. I will leave aside the general aspects of the theory of chemical evolution and will mention briefly some of the more specific observations of organic molecules in interstellar space and comets. 

Although thermodynamics has its uses in respect of astrochemistry, it is clear that the abundances of interstellar molecules are not determined by thermodynamics but rather by kinetics [27]. The relatively large observed abundances of free radicals and unsaturated hydrocarbon molecules clearly support this statement. It is useful to start a discussion of kinetics by recalling (1.16), writing it in logarithmic form, and then differentiating that equation with respect to ( IRT) ... 

One of the most exciting applications since the 1970s has been the observation of microwave (rotational) spectra of interstellar molecules. Common species such as formaldehyde, ammonia and methylamine and more exotic species such as HCO and H—CC-C=C—CN have been detected in various interstellar media. The experimental technique differs substantially from that outlined in Figures 1 and 2. In this case the interstellar molecular spectra are detected by collecting microwave emissions from interstellar space with large radio telescopes equipped with sensitive... 

The first observations of diatomic species in the diffuse interstellar medium several decades ago posed serious challenges for theorists because of the extremely low densities which are found there. Radiative association seemed unable to produce any of the observed species, most importantly CH, and this meant that exotic mechanisms were initially held responsible for the presence of such molecules. Work on the abundance of H2, following the observation of the molecule in the diffuse medium in the ultraviolet by the Copernicus satellite in the mid-1970s, and the discovery of... 

The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

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. 

The first step in interstellar chemistry is the production of diatomic molecules, notably molecular hydrogen. Observations of atomic hydrogen in dense clouds show that this species cannot be detected except in a diffuse halo surrounding the cloud, so that an efficient conversion of H into H2 is necessary. In the gas phase this might be accomplished by the radiative association reaction,... 

A fifth success concerns carbon monoxide, the dominant interstellar molecule from an observer s point of view. Despite all the uncertainties and problems with the model calculations, which will be amply brought out in this review, the predicted fractional abundance of CO is large and in the range of 10"5 to 10-4, in excellent agreement with observation. 

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. 

One of the oldest problems in molecular astronomy concerns identification of the molecules responsible for diffuse interstellar bands (DIBs). Since their first observation in 1922 some 127 bands have been detected all over the electromagnetic spectrum, shown schematically in Figure 3.19, but the origins of the transitions, the so-called carriers of DIBs, have not been determined. 

Binary ion-molecule reactions are indicated by thin arrows (c.t. indicates charge transfer), the radiative association reaction of C+ with H2 is indicated by the thick arrow and the dissociative recombination reactions are indicated by dashed arrows leading to the neutral molecules inside the compound brackets (e indicates free electrons). The molecules indicated in bold are known (observed) interstellar molecules. 

Yi-Jehng Kuan el al. (2004). Searches for interstellar molecules of potential prebiotic importance. Advances in Space Research 33 31-39 Yung Y., Allen M. and Pinto J. (1984). Photochemistry of the atmosphere of Titan comparison between model and observations. Astronomy Astrophysics Supplement... 

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