the pathways for the formation and destruction processes of ring molecules in interstellar medium have been proposed and their abundances have been calculated under steady state conditions and calculated abundances are of the order of 10−12 n(H), where n(H) is the abundance of hydrogen. Also their detection proposal has been presented.

Measurements of the abundances of OH, HCO+ and DCO+ are employed to derive the ionization rate in molecular clouds.

The chemical evolution of gas and dust flowing in a molecular cloud from the outer layers to the centre according to a circulation model is studied by calculating the chemical rate equations and the heavy element depletion time-dependently. the cloud structure and the circulation velocity are discussed in paper I (Boland and De Jong, 1984, Astron. Astrophys. 134, 87). We assumed that the circulating matter is in pressure equilibrium with the surrounding gas at all depths into the cloud. In the outer parts of the cloud (AV 2) the gas is almost in chemical equilibrium because the chemical timescales are short compared to the circulation timescale. In the dense cloud cores the time required to convert C, N and O into molecular form becomes larger than the circulation time and the depletion time so that appreciable deviations from chemical equilibrium occur. Particularly the abundances of C, HC3N, H2CO, CH4 and C2H2 are significantly enhanced and those of CO, O2 and N2 are reduced compared with their equilibrium concentrations at the same depth. Comparison of the present results with those for chemical equilibrium circumstances presented in Paper I shows that the departures of chemical equilibrium affect the predicted column densities of H/T, H2CO and HC3N significantly. It appears that the time-dependent model can account for the HC3N column densities observed in L183 and TMC-1.

The goal of evolutionary models of interstellar chemistry is to understand how interstellar clouds came to be the way they are, how they will change with time, and to place them in an evolutionary sequence with other celestial objects such as stars. To this end, we present an improved Mark II version of our earlier model of chemistry in dynamically evolving clouds. The Mark II model suggests that the conventional elemental C/O ratio less than one can explain the observed abundances of CI and the non-detection of O2 in dense clouds. Coupled chemical-dynamical models seem to have the potential to generate many observable discriminators of the evolutionary tracks. This is exciting, because, in general, purely dynamical models do not yield enough verifiable discriminators of the predicted tracks.

The chemistry of an 667 Mo interstellar cloud was studied using 142 reactions for 40 species during the contraction under self gravity in two steps. At first the contraction is allowed without gas phase reactions untill certain optical depth is reached. Secondly, at this optical depth the chemical reactions are started for sufficient cycles in a time dependant scheme till only very small additionally changes in the abundances occur. The so obtained, relative abundances and coulmn densities for different species represent a continous function of the optical depths. The values arround τ=6.3 represent the observations for H2, H2+, H3+, OH, OH+, CH, CH+, CH2, CH2+, CH3+, H2O and H3O+. The region of τ between 1 and 5 i.e. of particle density between 4 102–6 103 is the preferable formation place for the majority of molecules.

In its passage through the interstellar gas, a shock imposes massive and irreversible changes on the chemical composition of the gas. The synthesis of molecules from atoms and atomic ions can be highly efficient behind non-dissociative (i.e. slow) shocks. Results of kinetic calculations behind non-dissociative shocks are reviewed here, with emphasis given to the dependence of the postshock molecular composition on initial cloud properties. It is shown that a dense cloud shock can convert essentially all neutral atoms into various molecules. Even in diffuse and unshielded regions, a variety of molecules can attain a high abundance behind shocks. The suggestion that the widespread diffuse cloud species CH+ is shock synthesized is critically examined in the light of new calculations.

Several new interstellar species have recently been detected in the molecular gas, including rotationally-excited CH, vibrationally-excited HCN, and a new molecular ion, HCNH+. These detections have raised some interesting questions concerning the relative importance of “shock” or “high temperature” chemistry vs. ion-molecule reactions in the synthesis of interstellar molecules in dense clouds.

The reactions between O and N atoms and certain interstellar molecules (H2S, NH3, H2CO, CH3 OH, C2 H5 OH) are studied using the dynamicalflowing after glow method. It is found that NH and OH radicals are formed which can be used as a diagnostic probe to differentiate between interstellar shocked and heated regions.

The internal energy distributions of reactants in shocked interstellar clouds are discussed. Rate coefficients appropriate for the oxygen chemistry of shocks differ significantly from those deduced by simple extrapolation of thermal measurements. A one-fluid MHD shock model examines nonthermal effects for a 10 km s−1 shock propagating through clouds of initial densities of 10 and 105 cm−3, using oxygen-hydrogen reaction rates that are specific to the vibrational, rotational, and fine structure temperatures of the reactants.

The pumping of the upper levels of the 2π3/2 J=3/2 state of OH at sufficient rates to account for the observed strengths of interstellar hydroxyl masers requires that OH contains a substantial fraction of the oxygen. Shocks have been suggested as the sites of the OH production, but a better understanding of OH photodissociation and observations of hydroxyl maser kinematics have led to the realization that the original hydroxyl maser shock model must be revised. the possibility that interstellar hydroxyl masers occur in dense molecular photodissociation zones, heated by photoabsorption and perhaps by ion-neutral friction in the magnetic precursor of a shock, is considered. Possible infrared line emission from interstellar hydroxyl maser regions is mentioned.

There is evidence to show that OH and H2O maser emission is associated with the early stages of star formation. But the complexity of both the HII regions and the distribution of maser sites, makes it difficult determine how they are associated. Recent high resolution radio observations of Cep A, obtained using the VLA (Hughes 1985), together with accurate positioning of OH and H2O masers using MERLIN (Cohen et al. 1985), may help in this matter.

Our time dependent model of chemistry of dense interstellar clouds has been extended to study the formation of nitrogen bearing molecules. Here we present results for the calculations, under a variety of density, temperature, and elemental conditions, of the abundances of the following observationally important species: CN, HCN, HNC, NH3, NO, and N2H.

We show that deuteration in DCN and HDCO probably derives from CH2D+ in warm clouds, and that the levels of deuteration observed in those clouds require that radiative association play an important role in the chemistry of interstellar clouds.

A survey of about 100 lines of sight was made using the coudé auxiliary telescope and the coudé spectrograph of the Shane 3m telescope of the Lick Observatory. the data acquisition required 7 observing seasons. the spectra were recorded photographically at 17Ao/mm using a Varo tube intensifier. Each plate was separately calibrated for intensity and wavelength. the plates were measured using the PDS microdensitometer of the Royal Greenwich Observatory, Herstmonceux, and that data reduced on STARLINK using procedures developed by D.W.T. Baines. Care was taken to treat all photographic material in a consistent manner throughout the duration of the survey. the reduced data may therefore be considered self-consistent. the emulsion types used were (principally) Kodak IIIaF and (more rarely) 103aD.

A new law of star formation is proposed which suggests an explicit dependence on the abundance of metals and H2.

Dusty knots are known to exist within supernova remnants (SNRs). the dust in these knots could be sufficiently heated by the X-ray emitting plasma to radiate in the far-infrared (FIR) wavelengths. We have searched the IRAS Point Source Catalogue for FIR sources associated with 117 SNRs from the catalogue of van den Bergh. We selected IRAS sources with definite flux densities at 60 μm or 100 μm and the flux density increasing with wavelength beyond 25 μm. For each SNR, background source density was determined in an area surrounding the SNR and the number of excess sources within the SNR area (with 10% radial extension) was determined.

This review presents an updated listing of infrared observations of circumstellar molecules. The compendium is restricted to the more abundant species with infrared spectra accessible to groundbased observations. No analysis is offered on the chemical importance of a particular species, and no attempt is made to correlate the infrared work with the much larger body of data available from radio frequency observations. Some discussions of these topics may be found in the more comprehensive reviews listed below. The observations described here include both the successful and the unrewarded efforts. Often the negative results from attempted observations are not widely disseminated, even though such results can sometimes be of significant theoretical value. Discussions of infrared stellar and circumstellar spectroscopy from other vantage points may be found in the following reviews. in 1979 Merrill and Ridgway surveyed the contributions of infrared spectroscopy to studies of stellar photospheres and the near circumstellar environment. The following year Zuckerman (1980) summarized the current state of knowledge on circumstellar molecular clouds with attention given to both infrared and microwave observations. Ridgway and Keady (1981) subsequently noted the importance of infrared spectroscopy for studies of circumstellar gas and dust in the star IRC + 10216. in 1983 Hinkle reviewed the high-resolution spectroscopy of late-type circumstellar shells in general but emphasized chemistry and kinematics. Finally, in the proceedings of the 1984 UCLA meeting on mass-loss, Omont (1985) reviewed the physical and chemical structure of circumstellar envelopes and IRC + 10216 in particular, and Wannier (1985) discussed the significance of infrared spectroscopy for studies of mass-loss in late-type stars.

A correlation has been found between strong molecular hydrogen emission and the morphological type of a planetary nebula. Those with an equatorial toroid and bipolar extensions have H2 1−0 S(1) stronger than Brackett γ. H2 maps of several objects, and NGC 2346 in particular, are consistent with a fast stellar wind interacting with an anisotropic medium.

We review observations to date of silicon-bearing molecules in circumstellar gas and dust around red giants. New constraints on circumstellar chemistry are provided by the discovery of (i) a systematic decrease in the [SiO]/[H2] ratio with mass loss rate, ii) SiO in carbon-rich CIT 6. The presence/absence of SiO maser emission in oxygen-rich/carbon-rich envelopes is linked to differences in grain composion and the grain-condensation radius.

We present evidence for a 12 micron absorption band due to the librational mode of the H2O molecule in the form of ice in some IRAS LRS spectra. This identification is based on the 3 micron spectrophotometry of these objects showing the presence of the 3 micron ice band. From these spectra, physical characteristics of the ice can be discussed.