Destruction of Interstellar Molecules

The destruction of interstellar molecules by interaction with soft X-rays and subcosmic rays is even less important, because of the general decrease of the decomposition cross section with increasing energy of the collision partner. 

The detection of molecules via their rotational spectra allows astrophysicists to probe interstellar clouds to provide information on their environment, star formation, interstellar chemistry, mechanisms for synthesis and destruction of interstellar molecules, isotopic distributions, etc. 

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 existence of a large number of molecules and their concentrations in interstellar space suggest that the interstellar medium is quite heterogeneous. Although the chemical processes are likely to be different in the various regions of space, it is possible to consider some common features of the life cyle of an interstellar molecule. The important processes are the formation, the relaxation, and finally the destruction of a molecule. Formation and destruction are not well understood processes. 

Photodissociation and exchange reactions are the primary destruction process for interstellar molecules. If photodecomposition is considered to be the only destruction mechanism, then the lifetime of interstellar molecules depends upon three factors the absorption cross section, the quantum yield of dissociation and the interstellar radiation field. A quantitative discussion of this destruction mechanism has been given by Stief (1971) for two diatomic and eight polyatomic molecules. 

The photodissociation rates for a number of important astrophysical molecules are summarized in Table 1 for the unattenuated interstellar radiation field given by Draine (1978 Eq. (3)). In order to calibrate the significance of a rate of 3 x 10 s , say, it is useful to note that this rate corresponds to a lifetime of a molecule of 10 years, which is short in comparison with dynamical lifetimes of interstellar clouds (often estimated to be 10 - 10 years), and with most other chemical timescales. In particular, destruction of a molecule by reaction with an ion in a cloud of density 10 cm with a fractional ionization less than 10 will occur with a characteristic lifetime of 300 years or more. 

While dense IS regions are generally well-shielded against high-energy photons, the interaction between molecular ions and photons is relevant to the question of the survival of such ions in the diffuse interstellar medium, where UV irradiation might be expected to be a powerful destructive force. Such effects are, of course, important also for the fate of neutral molecules in the diffuse interstellar radiation field, but UV photo absorption by molecules of moderate size is often more likely to lead to photoionization (itself an important topic, but not covered herein) than to photodissociation. 

The aim of this article is to give a short outline of current theories of molecule formation and destruction in interstellar clouds, together with a short summary of the observational material which has been accumulated up to early 1981. Although this article will address itself predominantly to simple molecules a section on complex molecules has been added. We will, therefore, discuss some general aspects of cosmochemistry and then turn to molecule formation in diffuse clouds followed by a discussion of the chemistry of dense interstellar clouds. A section has been added to summarize recent observational results and theoretical proposals in understanding the formation of intermediate and complex molecules, an area of considerable current activity. Finally the article closes with a short summary of the molecular species found in planetary atmospheres and a short discussion of what the relation might be to the interstellar molecules. 

Potential energy curves and transition dipole moment functions for the NH molecule have been computed by Goldfield and Kirby (1987). The photodissociation cross sections into the excited and H states give rise to an unshielded rate of about 5 x 10 s (Kirby and Goldfield 1988), which is comparable to that of OH, and about a factor of two smaller than that of CH (van Dishoeck 19876). The photodissociation rate of NH is based on cross sections calculated by van Dishoeck (1986). Although many of the exdted electronic potentials are repulsive, most of them have vertical excitation energies larger than 13.6 eV, so that the destruction by interstellar radiation is not very rapid. 

The first attempts to explain the observed abundances of molecules on the basis of non-therinodynamic equilibrium models were by Kramers and ter Haar (1946) and Bates and Spitzer (1951). These investigations considered in some detail the basic formation and destruction processes through radiative association, photodissociation, photoionization and dissociative recombination, but were unable to reproduce the observed abundances of the then known interstellar molecules CH and CH. Because of the difficulties encountered with the early gas-phase reaction schemes, subsequent models focussed on grain surface... 

Major concerns in astrochemistry are (1) the identification of the molecules that exist in the cosmos and the characterisation of the physical conditions in those regions of the uiuverse where the observed molecules are found (2) laboratory measurements on the spectroscopy of potential molecules and on the rates and products of homogeneous and heterogeneous processes that may contribute to the formation and destruction of molecules, under the generally extreme interstellar conditions where molecules are found (3) the creation of computer models that use laboratory data and seek both to reproduce what is found in the molecular universe , and to suggest what other molecules may be present and which processes appear to be especially important, so as to focus the efforts of laboratory scientists. These topics are dealt with, in turn in chapters by Maryvonne Gerin (in Chap. 2), Michael Pilling (in Chap. 3), and by Valentine Wakelam, Herma Cuppen and Eric Herbst (in Chap. 4). 

HNC is intricately linked to the formation and destruction of numerous other molecules of importance in the interstellar medium - aside from the obvious partners HCN, HCNH, and CN, HNC is linked to the abundances of many other compounds, either directly of through few degrees of separation. As such, an understanding of the chemistry of HNC leads to an understanding of countless other species - HNC is an integral piece in the complex puzzle representing interstellar chemistry. 

A very wide range of astrophysical problems can be tackled with ISOCAM, for example - solar system studies structure of zodisM l bands and cometary trails, spectral maps of comets - interstellar matter and staff formation nature of 12 m emission of large molecules or grains, grain formation and destruction, interaction between supemovae and the interstellar medium... 

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