Diffuse interstellar clouds

A much more detailed and time-dependent study of complex hydrocarbon and carbon cluster formation has been prepared by Bettens and Herbst,83 84 who considered the detailed growth of unsaturated hydrocarbons and clusters via ion-molecule and neutral-neutral processes under the conditions of both dense and diffuse interstellar clouds. In order to include molecules up to 64 carbon atoms in size, these authors increased the size of their gas-phase model to include approximately 10,000reactions. The products of many of the unstudied reactions have been estimated via simplified statistical (RRKM) calculations coupled with ab initio and semiempirical energy calculations. The simplified RRKM approach posits a transition state between complex and products even when no obvious potential barrier... 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

The late 30 s brought a further important step in the investigation of the interstellar medium — the discovery of the first molecular species. In the optical region, the electronic spectra of the diatomic radicals CH, CH+, and CN, seen in absorption against the continuum spectra of bright background stars, furnished the first evidence that the interstellar medium was not devoid of molecules but contained at least some simple ones. However, the intensities of the molecular spectral peaks seen via optical absorption studies were quite weak compared with the spectra of atoms, indicating that the sources observed in these early studies were not rich in molecules. These sources, now labeled diffuse interstellar clouds, possess very low gas densities (n 102 cm-3) and are of limited interest chemically. 

Isotopes in diffuse interstellar clouds Ultravioletradiation can fractionate O isotopes in thin, or diffuse, molecular clouds. Ultraviolet starlight can penetrate such... 

Isotopes in diffuse interstellar clouds Ultraviolet radiation is observed to fractionate O isotopes in diffuse molecular clouds (see l60). 

We will therefore consider in the following sections some basic ideas and observations of the chemistry pertaining to diffuse interstellar clouds and cold dark and dense clouds. 

The chemical composition of diffuse interstellar clouds is simple and essentially limited to diatomic molecules which have been summarized in Table 2, together with the wavelength region where detection was made. It has to be noted, however, that there is a marginal detection of H2O at about the 2a level by Snow (1980). If confirmed a new H O mechanism has to be thought of. 

So far, unsuccessful searches for Hj in BN, GL 2591, LkH 101, NGC 2024/IRS, W33IR, NGC 2264 and AFGL 2591 have been reported. Black et alP observed spectral lines of CO simultaneously with their search for Hj. The abundance of CO thus obtained together with the upper limit of the Hj column density set a limit on the rate of the cosmic ray ionization C through eqn. (7). van Dishoeck and Black have proposed on chemical grounds that the abundance of Hj may be equally high in diffuse interstellar clouds. ... 

Two other factors, however, combine to make a pressing need to reach a definitive experimental value for the thermal DR rate coefficient. The observation of Hj in diffuse interstellar clouds has made it possible to determine the abundance of The factor of 10 spread in the... 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

CHgCNir. Many of the reactions involve radiative association. Dissociative electron-ion recombination then yields neutrals such as CH (methane), C2HgOH (ethanol) and CH CN (acetonitrile) [158). It is often joked that diffuse interstellar clouds contain enough grain alcohol to keep space travellers happy on their long journeys. In diffuse clouds, the reaction scheme is more varied and leads to smaller molecules in general. 

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 diffuse interstellar 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 submillimetre 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... 

A search for PHg in interstellar and circumstellar sources is described and the results are discussed with respect to the depletion of P in dense and diffuse interstellar clouds [62]. Laboratory experiments on the gas-phase ion-molecule chemistry of phosphorus using the ion cyclotron resonance technique predicted that the PHg molecule is formed only with difficulty in dense interstellar clouds [63]. 

Rosenfeld 1937, McKellar 1940, Douglas and Herzberg 1941). A summary of the presently known interstellar molecules detected by optical techniques in diffuse interstellar clouds has been given by Winnewisser et al. 1979. 

RADIATIVE PUMPING AND COLLBIONAL EXCITATION OF MOLECULES IN DIFFUSE INTERSTELLAR CLOUDS. 

ABSTRACT. Recent work on radiative processes and collisional excitation in molecular Hydrogen and its deuterated isotopic substitute and in molecular Carbon is reviewed. Particular attention is drawn to non-adiabatic coupling effects on the intensities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of Hj and to the role of nuclear spin on ortho-para transitions in Hj due to collisions. The inter-relation between those processes and state to state chemistry is stressed out. We discuss the implications of these new data in a recent comprehensive model of diffuse interstellar clouds (Viala et al., 1987). 

We have developed recently a comprehensive model of the f Oph diffuse interstellar cloud (Viala et al, 1987) where the populations of the first ten rotational levels of and HD have been calculated by taking into account UV pumping to excited electronic states followed by fluorescence and radiative decay inside the ground state, collisional processes, and chemical... 

We have learned a lot with the excitation of Hj and Cj. From this study, we find rather complementary constraints on the physical parameters prevailing in the f Oph diffuse interstellar cloud, at the opposite of van Dishoeck Black (1986), since we are rather confident in the relevant molecular data. However, we do not discuss here the abundances of the vari-... 

The rapid increase of knowledge on up to now poorly known processes will certainly improve our understanding of diffuse interstellar clouds although some recent values challenge our present view. 

ABSTRACT. The current status of models of diffuse interstellar clouds is reviewed. A detmled comparison of recent gas-phase steady-state models shows that both the physical conditions and the molecular abundances in diffuse clouds are still not fully understood. Alternative mechanisms are discussed and observational tests which may discriminate between the various models are suggested. Recent developments regarding the velocity structure of diffuse clouds are mentioned. Similarities and differences between the chemistries in diffuse clouds and those in translucent and high latitude clouds are pointed out. 

Despite their relative simplicity, however, the structure and chemistry of diffuse interstellar clouds remain elusive. As summarized recently by Black (1987) and Crutcher and Watson (1985), many of the basic questions are still unanswered such as (1) What are the typical temperatures and densities in diffuse clouds Are the densities of the order of a few hundred cm or a few thousand cm (2) Can the abundances of most species indeed be described by quiescent steady-state gas-phase reactions, or do shocks play a crucial role in the formation of most species (3) What is the role of the grains in the cloud Can molecules other than H2 be formed efficiently on grain surfaces and returned to the gas-phase (4) What is the structure of diffuse clouds Can they indeed be represented... 

Oxygen exists mostly in neutral atomic form in diffuse interstellar clouds. In this case, the ionization necessary to initiate the ion-molecule reactions is provided by cosmic rays which penetrate the clouds and ionize H and H2 to form H and Hj. H2 subsequently reacts very rapidly with H2 to form Hj". The rate at which atomic hydrogen is ionized is denoted by Since the ionization potential of O is accidentally close to that of H, the charge transfer reaction... 

Since many of the key chemical processes were identified more than a decade ago, most progress in our understanding of diffuse cloud chemistry has come from detailed laboratory and theoretical work on important reaction rates. A recent compilation of ion-molecule reaction rate coefficients has been given by Anicich and Huntress (1986). References to rate coefficients for other processes are given elsewhere in this volume. Some of the important developments for the chemistry in diffuse interstellar clouds are summarized below. 

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