Formation and Destruction of Molecules in Interstellar Space

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. 

Interstellar molecules are observed in the cold component of the interstellar gas which, together with the short lifetime of the molecules in unshielded regions of space, shows that formation in stellar atmospheres or protostellar nebulae can not be their principal formation mechanisms provided the molecules are not formed simultaneously with the dust and then blown out into the interstellar medium. 

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. 

This may have excluded from detection so far molecules containing the elements Fe and Mg which are of comparable abundance to Si and S. Furthermore, molecules which are expected to be abundant in interstellar space, such as HCCH, CH4, C02 cannot at all or only with great difficulty be detected in the microwave region. 

On the other hand reactions of hydride radicals with each other lead to the formation of astrophysically important, but often non-polar molecules such as HCCH, H2CCH2, H3CCH3. Correspondingly HNNH, H2NNH2 and in particular HOOH and HSSH, which are all polar, may very well be detectable in interstellar space. Surface reactions of the hydride radicals with other radicals and molecular fragments produce more complex molecules. For example cyano-acetylene, HCC-CN may well have been formed this way. All detected interstellar polyatomic molecules can be explained this way (see Table-8) and some hitherto undetected, but important ones, can be predicted to exist in interstellar space. Their observation or absence in interstellar space may then in conjunction with laboratory results shed more light on possible chemical pathways. 

---