[en] The successes and failures of models of interstellar chemistry are discussed with particular reference to molecular clouds, such as Orion, which are in the immediate neighborhood of HII regions. Some recent laboratory measurements at temperatures below 100 K are reviewed and their astrophysical consequences discussed. From a survey of abundance measurements towards Orion, it is concluded that composition differences between hot HII region clouds and cold dark clouds are not great. The influence upon the chemistry of shocks and mass outflow from young protostars is discussed. It is concluded that although some anomalies do exist, gas phase ion-molecule type schemes can account for most of the qualitative features of the observations. (Auth.)

[en] The ionization equilibrium of carbon and heavier elements in a neutral gas cloud close to an early type star is discussed using a simple model for the transfer of the ultra-violet radiation. The balance between atomic and molecular hydrogen is also considered. It is concluded that the H II region exciting star may be, but should not be assumed to be, the source of the photons which ionize carbon in the region giving rise to the observed recombination lines. The origins of the narrow hydrogen and heavier element lines are also discussed. 1 table, 2 figs, 29 refs

[en] The author gives a qualitative discussion of the factors determining the relative importance of collisional broadening and stimulated emission. He then gives the results of some model calculations, and finally, surveys some of the observational data and discusses the applications of recombination line measurements to the determination of the physical properties of HII regions. (Auth.)

[en] We have observed with approximately the same spatial resolution HCN (J = 1-0), HNC (J = 1-0), their ¹³C isotopic species, and HC₃N (J = 2-1 and J = 10-9) toward six locations in the Taurus dark cloud complex, one dark cloud (L183) outside Taurus, and two warm clouds with associated H II regions. The column densities of HCN, HNC, and HC₃N are typically less than those of NH₃ by factors of 10^-1-10^-3. NH₃ is believed to be a tracer of the total H₂ abundance and a chemical precursor of the cyanogenated molecules. HNC is systematically about a factor of 10 more abundant than HC₃N. Limits on HCN column densities are given; upper limits were determined by limits on H¹³CN emission, and lower limits were derived from the weakest hyperfine component, F = 0-1, of HCN (J = 1-0). The HNC/HCN column-density ratios lie in the range approx.3 to approx.10 in cold dark clouds. They are uncertain because HCN column densities are uncertain, but they are definitely greater than unity. The fact that this ratio does not deviate by large factors from unity can be understood in a gas-phase chemical network if ''chemical shuffling'' between HNC and HCN is important. Inclusion of reactions that convert HCN to HNC and vice versa forces the ratio HNC/HCN to a narrow range about unity if the chemical shuffling rates are fast relative to other HCN and HNC destruction rates

[en] A search for the lowest energy rotational transition (J = 5--4) of iron monoxide (FeO) near 153 GHz in the lowest and first excited vibrational states has been conducted toward a variety of objects. FeO was not detected. Our upper limit on the FeO column density in Orion and other warm molecular clouds is N(FeO)<2 x 10¹² cm^-2. In Orion, <2 x 10⁷ of the cosmic iron abundance can be in the form of FeO and [FeO]/[H₂]<10^-11. It is likely that most Fe is incorporated into interstellar dust grains. The remaining fraction in the gas phase probably favors atomic iron rather than FeO

[en] Calculations are presented for the populations of very high electronic states (principal quantum numbers 200--900) associated with radio recombination lines of neutral carbon atoms at temperature < or approx. =100 K. These make it possible to assess the likelihood for and significance of very low frequency, radio recombination lines of carbon in the interstellar medium

[en] Level populations have been computed and are tabulated for hydrogenic atoms with principal quantum numbers in the range 20-80 and electron densities in the range 10⁴ - 10^8.5 cm^-3. The consequences of these results for the interpretation of millimeter recombination line observations are briefly discussed

[en] The difficulty of understanding molecular formation processes in star forming regions lies in the multiplicity of processes which contribute the observed abundance distribution. This is exemplified by the Orion Kleinmann-Low region for which many high angular resolution of maps are now available. It seems probable that in Orion, both shock processes and grain-mantle evaporation play a role in the chemistry. In this review, the author discusses methods of distinguishing between them. In particular, he considers the chemistry of deuterated species in regions of star formation and the extent to which, from the observed deuterium fractionation, one can derive information about timescales and grain mantle chemistry. He also discusses various diagnostics for shock processes such as the observed [HCN]/[HNC] abundance ratio

[en] A sensitive search for J = 1-0, v = 1 and v = 0 SiO in a variety of stellar and interstellar sources has been performed with the 100 m telescope of the MPIfR. Numerous new detections of SiO maser emission from Mira variables, short-period variables, and OH/IR stars were made which increase the number of known SiO sources by nearly two-thirds. Strong linearly polarized velocity features were observed in the v = 1 emission of R Leo. Maser emission in the v = 0 line was seen in at least four Mira variables; thermal or quasi-thermal v = 0 emission was observed in several other sources. First detections of thermal v = 0 emission in the J = 1-0 line of SiO were made in the star forming regions W3 and NGC 7538

[en] A survey of the distribution of long carbon chain molecules in interstellar clouds shows that their abundance is correlated. The various formation schemes for these molecules are discussed. It is concluded that the ion-molecule type formation mechanisms are more promising than their competitors. They have also the advantage of allowing predictions which can be tested by observations. Acetylene C₂H₂ and diacetylene HCCCCH, may be very abundant in interstellar clouds. (Auth.)