DCO+ is one of the most common deuterated molecules in cold (TK ~ 10 K) molecular cloud cores such as TMC-1. We report the results of a survey for DCO+ and H13CO+ emission regions among a sample of low mass cloud cores. We compare the derived DCO+/HCO+ ratio (0.046±0.014) with current chemistry models for deuterium fractionation.

The latest table of molecular abundances in the cold, dark clouds TMC-1 and L134N is presented. Molecular abundance variations between TMC-1 and L134N, those within TMC-1 and L134N, and those among 49 dark cloud cores surveyed by Suzuki et al. (1991) are interpreted as an effect of chemical evolution.

We report on results from an ongoing spectral scan of four nearby positions in the Sgr B2 molecular cloud using SEST (Swedish-ESO Submillimeter Telescope). The antenna beam size is approximately 22″ in the frequency range 226-245 GHz presently covered. This high angular resolution allows detailed studies of the physical and chemical conditions in the warm and compact cores discovered (Vogel et al. 1987, Goldsmith et al. 1987) inside the region previously surveyed in the 3 mm band with lower angular resolution (Cummins et al. 1986, Turner 1989, beam sizes of 1.5-2.9′ and 1-2′, respectively). The Sgr B2(OH) position used by these investigators is located about 30″ south of our M position, and hence the cores M and N will contribute to the observed spectral line emissions to a larger or lesser extent.

We review what species remain to be seen for several types of astrochemistry: Thermochemical Equilibrium (TE) in circumstellar envelopes (CSEs); photo- and ion-molecule chemistry in CSEs; ion-molecule chemistry in cold interstellar clouds; grain chemistry (passive, catalytic, disruptive); and shock chemistry. In CSEs, a rich Si gas-phase chemistry is now recognized, and two predicted species (SiN, SiH2) have been seen. Others are predicted. In the ISM, a global picture of refractory-element chemistry predicts that compounds of Mg, Na, Fe, and possibly Al occur with detectable gas-phase abundance. Predicted species require laboratory synthesis and spectroscopy. Reactions of hydrocarbon ions with neutral species dominate the formation of the families CnH, HCnN, H2Cn, and CnO in both interstellar (TMC-1) and circumstellar (IRC10216) cases, and readily explain the favored values of n in each case as well as predicting which higher-n species remain to be seen. Confirmation of H3O+ (interstellar) is discussed.

Interferometric observations of the HNO 3.68 mm line in Sgr B2 show clumping and some distribution similarities with HNCO. A search for the HNO 1.23 mm line with the NRAO 12-m telescope was unsuccessful.

In various models of the chemistry of interstellar grains, solid O2 is formed by accretion as well as by surface reactions. In dense molecular cloud models, at a later stage of evolution of an interstellar grain, solid O2 becomes a major grain mantle constituent at the expense of water ice abundance. If molecular oxygen is embedded in a “dirty” ice” matrix, the forbidden fundamental vibration of O2 at 1550 cm−1 may become observable.

The H3O+ ion is a key species in the oxygen chemistry leading to H2O, OH and O2. Chemical models predict O2 and H2O to be the dominant oxygen-bearing molecules in interstellar clouds. However, neither of them can easily be observed in the bulk of the interstellar medium because of blockage from the Earth's atmosphere. Determination of the abundance and distribution of the precursor H3O+ ion might thus provide an important indirect measure of their abundances.

Isotopic molecular abundances are used to interpret Galactic chemical evolution and the properties of interstellar clouds. The isotopic chemistry of carbon plays an important role in the interpretation of these measurements. This paper reviews the recent measurements of the carbon twelve to thirteen ratio across the Galaxy and the isotopic chemistry.

Theoretical models of the chemistry in regions of low-mass star formation are reviewed.

Evolutionary chemical models are ultimately unavoidable for a full understanding of interstellar clouds. They include not only the chemical processes but also the dynamical processes by which the modeled object came to be the way it is. From an evolutionary perspective, dark cores may be ephemeral objects and dynamical equilibrium an exception rather than norm. Evolutionary models have numerous advantages over “classical” fixed condition equilibrium models. They have the potential to provide more elegant explanations for the observed inter-cloud and intra-cloud chemical differences. The problem of the depletion of gas phase molecules by condensation onto the grain may also be less serious in evolutionary models. Hence, these models should be actively developed.

In this article, I list the deuterated species detected so far, together with an estimate of their abundance ratios relative to their hydrogen bearing parent for a variety of astronomical regions, discuss the basic chemical processes which fractionate deuterium together with some simple estimates of the degree of fractionation which can result from these processes. I then discuss in detail the deuteration of hydrocarbon molecules and show that the effects of chemical reactions which can recycle parent species can appreciably affect estimates of fractionation. Finally I shall discuss how structural effects may inhibit fractionation in the cyanopolyynes and related species.

Molecular lines have revealed various supporting motions in dense cores. Line widths and emission region sizes of NH3 and CS in the same kind of cores or of the same line in cores with or without sources are different and can not be explained with the line width- size relationship. Outflows in dense cores show rich characteristics which can account for the NH3 emission difference between the two kinds of the cores; CS emission is consistent with the chemical effects in shocked regions. Rotation exists in both kinds of cores and may be related to the observed polarities and collimations of outflows.

We have obtained column densities for HCO+, HCO, HCS+, C3H2, HC5N, SiO, OCS, HCOOH, CH3CH2 OH, and CH3CCH toward Sgr A. The fractional abundance of SiO relative to molecular hydrogen in Sgr A is comparable to that for the Orion plateau, ~ 10−7 − 10−8, which may be a typical value for hot clouds. The abundances of HCO, CH3CH2OH and CH3CCH all appear to be enhanced relative to other molecular clouds suc as Sgr B2.

We observed 3 mm transitions of several interstellar molecules toward star-forming regions in W58 and GL490 using the 14 m Daeduk Radio Telescope (Korea). We derive molecular abundances for the“broad” components observed with a high spectral resolution, which could represent the abundances of outflowing materials.

Gas phase models of ion molecule chemistry have been rather successful in matching the observed abundances of small interstellar molecules containing carbon, hydrogen, and oxygen. However, the situation is somewhat less clear for nitrogen-containing species, partly because the important initiating reaction N+ + H2 is slightly endothermic; and for sulfur-containing molecules, where it remains uncertain whether it is necessary to invoke surface reactions on grains to match the observed abundances. As a relatively simple species, the abundance of nitrogen sulfide should provide a good test of the models of the coupled chemistry of nitrogen and sulfur. Until very recently only two molecules containing both these elements were known in the interstellar medium, NS and HNCS, and both have been observed only in Sgr B2. We have therefore undertaken a survey for interstellar NS in Galactic molecular clouds using the FCRAO 14-meter telescope. The 2Π1/2, J = 5/2 → 3/2, transition has in fact been detected in many regions of massive star formation (see table).

UV radiation leads to photodissociation and photoionization of molecular species in diffuse clouds and in the extended envelopes of dark regions. In the inner part of heavily-obscured objects, the residual external field may be negligible; alternative mechanisms, such as cosmic-ray-induced H2 emission, appear to be efficient in photodestruction processes. We evaluate the relative importance of these radiation sources, and give some estimates of their effects on the mean lifetime of interstellar molecules, taking into account the cloud structure that significantly affects the penetration depth of UV radiation.

Recent observations of molecular clouds perturbed by interstellar shocks are reviewed. Effects of the shock waves on the chemical composition of these regions are discussed.

We discuss a simple H2O maser pumping mechanism in which the population inversion of the masing levels takes place during the quick cooling of the gas behind a shock wave. The population of the rotational energy levels in the initial hot state and final cool state of the molecular gas, and the decay paths between levels are analysed to calculate the average number of 22 GHz photons emitted per H2O molecule in the cooling process.

Observations (Goldsmith et al., 1986) indicate that the gas in dark clouds with embedded low-mass stars experiences a cycle, driven by stellar winds, between low and high density phases. The cycle time is sufficiently short that the chemistry never attains steady state. Icy mantles accumulating on grains during the collapse and dense core phases are thought to be removed in each cycle by low velocity shocks that terminate the low density phase. Models of this type give detailed point-by-point descriptions of both gas and solid phases in molecular clouds (Charnley et al., 1988; Nejad et al., 1990).

A review is given of the evidence for depletion of heavy elements in dense molecular cloud clumps onto dust grain surfaces. Particular attention is given to the recent controversy over the nature of the dense clumps in NGC 2024. We also discuss recent ammonia observations of“hot core” regions and evidence that the NH3 abundance is greatly enhanced.