A review is presented of some of the developments in astrochemistry that have occurred in the past three years, ranging from the chemistry of the early Universe to the chemistry of planetary systems.

The star-formation history of molecular cores is largely determined by gas phase chemical processes that are greatly modified by gas-dust interactions. Substantial elemental depletions may result from the efficient formation of refractory grain material. The depletion of carbon and of refractory elements such as S, Si, Mg, etc. in molecular clouds is well known, but is very poorly constrained. Star forming regions are cold, dark and chemically quiescent, so that in addition to the initial elemental depletions, an ongoing dynamical depletion of molecular material occurs as gas-phase material freezes out onto the surface of dust grains. Observational evidence for anomalous depletions in low mass star forming clumps became apparent in the early 1980s when the narrowness of molecular emission lines (especially those of NH3) suggested that high velocity infalling material is being depleted from the gas phase. This prompted further studies into the chemical effects of differential depletion and together with radiative transfer models has established molecular diagnostics of infall/depletion sources. Recent observations at high spatial resolution show direct evidence for gas-phase depletions in the central regions of protostellar cores.

In addition to the diagnostic and purely chemical implications for collapsing cores, depletion plays a very active role in the physical evolution of star-forming regions. Most gravitationally unstable low mass cores are believed to be magnetically sub-critical. Recent polarimetric studies of elongated cores are also providing evidence of magnetic alignment, giving strong support to the idea that magnetic fields play a dominant role in core evolution. The relaxation of magnetic pressure (whether through ambipolar diffusion, or by the damping of MHD waves) is critically dependent on the ionization structure which, in turn, is highly sensitive to the elemental and molecular depletions.

We have developed comprehensive models, based on the New Standard Model reaction dataset, for the gas-phase chemistry of translucent clouds. These models predict satisfactorily the abundances of 34 of the 38 molecular species observed in both translucent and cold dense clouds. With 3 additional reactions, the models also appear able to explain diffuse cloud abundances.

Ions and electrons play a key role in the chemical and dynamical evolution of interstellar clouds. Gas phase ion–molecule reactions are major chemical routes to the formation of interstellar molecules. The ionization degree determines the coupling between the magnetic field and the molecular gas through ion–neutral collisions, and thus regulates the rate of star formation. In the theoretical determination of the degree of ionization we run into several sources of uncertainty, including the poorly known cosmic ray flux and metal depletion within the cores, the penetration of UV radiation deep into regions of high visual extinction due to cloud inhomogeneities, and the ionization rate increase in the proximity of young stellar objects which may be strong X–ray emitters. Observational estimates of electron (or ion) fractions x(e) (≡ n(e)/n(H2), where n(e) and n(H2) are the electron and molecular hydrogen number densities, respectively) in dense cloud cores are thus of considerable interest. In this paper, I will review recent improvements in the estimates of the ion fraction in dense cores and point out the difficulties in determining x(e).

We discuss recent models of chemical evolution in the developing and collapsing protostellar envelopes associated with low-mass star formation. In particular, the effects of depletion of gas-phase molecules onto grain surfaces is considered. We show that during the middle to late evolutionary stages, prior to the formation of a protostar, various species selectively deplete from the gas phase. The principal pattern of selective depletions is the depletion of sulfur-bearing molecules relative to nitrogen-bearing species: NH3 and N2H+. This pattern is shown to be insensitive to the details of the dynamics and marginally sensitive to whether the grain mantle is dominated by polar or non-polar molecules. Based on these results we suggest that molecular ions are good tracers of collapsing envelopes. The effects of coupling chemistry and dynamics on the resulting physical evolution are also examined. Particular attention is paid to comparisons between models and observations.

We have carried out interferometric observations of pre-protostellar and protostellar envelopes in Taurus. Protostellar envelopes are dense gaseous condensations with young stellar objects or protostars, while pre-protostellar envelopes are those without any known young stellar objects. Five pre-protostellar envelopes have been observed in CCS JN=32–21, showing flattened and clumpy structures of the envelopes. The observed CCS spectra show moderately narrow line widths, ~0.1 to ~0.35 km s–1. One pre-protostellar envelope, L1544, shows a remarkable velocity pattern, which can be explained in terms of infall and rotation. Our C18O J=1–0 observations of 8 protostellar envelopes show that they have also flattened structures like pre-protostellar envelopes but no clumpy structures. Four out the eight envelopes show velocity patterns that can be explained by motions of infall (and rotation). Physical properties of pre-protostellar and protostellar envelopes are discussed in detail.

Based on theoretical considerations, the chemistry around embedded young stellar objects is expected to be governed by the interplay between gas-phase reactions, condensation of molecules onto dust grains, grain surface reactions, and evaporation of altered ice mantles near the star and in the outflow. I discuss the chemical characteristics of embedded young stellar objects, with special emphasis on these processes.

The spectra of dilute molecular gases in astrophysics arise in material far out of thermodynamical equilibrium (non-LTE), often in inhomogeneous atmospheres where Doppler motions from kinematical gradients and turbulence are large compared with thermal motions. Thus line intensities contain information about density and temperature as well as abundances, and line profiles probe the structure of the atmosphere. However, the theoretical description of the atmosphere can be extremely complicated and can make enormous demands for molecular data. Improvements in angular resolution and the possibility to make sensitive spectroscopic measurements at mid-infrared and submillimeter wavelengths pose new challenges for theoretical models.

Recent observational studies of intermediate- and high-mass star-forming regions at submillimeter and infrared wavelengths are reviewed, and chemical diagnostics of the different physical components associated with young stellar objects are summarized. Procedures for determining the temperature, density and abundance profiles in the envelopes are outlined. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios, the abundances of evaporated molecules, and the fraction of heated ices with increasing temperature. Since these diverse phenomena involve a range of temperatures from < 100 K to 1000 K, the enhanced temperatures must be communicated to both the inner and outer parts of the envelopes. This ‘global heating’ plausibly results from the gradual dispersion of the envelopes with time. Similarities and differences with low-mass YSOs are discussed. The availability of accurate physical models will allow chemical models of ice evaporation followed by ‘hot core’ chemistry to be tested in detail.

Recent submillimetre observations of continuum radiation from warm dust and molecular line emission from hot gas in regions of high mass star formation are reviewed. Such regions are characterised by ultracompact HII regions around young OB stars and associated hot molecular cores which appear to harbour high mass protostars at an earlier stage of evolution.

Single dish spectral line surveys of high mass star-forming regions provide spectra with a very high line density, and reveal the presence of many complex molecules. Besides the prototypical Orion BN/KL region, more and more regions get surveyed and we start to get a better idea of the chemical similarities and differences. Yet, single dish studies miss an important aspect of hot cores, which is revealed by higher resolution studies with interferometers: the cores are not chemically homogeneous, but a pronounced chemical substructure exists. As an example of such an interferometric study, we will present one particular set of objects, the UC HII W3(OH) and its neighboring hot core W3(H2O) (otherwise known as the Turner-Welch object), and discuss their chemical properties.

In the recent years revolutionary results concerning the nature of icy dust particles have been obtained with the help of the Infrared Space Observatory (ISO) and ground based observations. To date interstellar ice features of H2O, CO, CO2, CH3OH, CH4, H2CO, OCS and HCOOH as well as other minor species are observed. Interstellar grains act as important catalysts in the interstellar medium. Processes such as UV irradiation, cosmic ray processing and temperature variations determine the grain mantle growth and chemical evolution. ISO has revealed that ice segregation is an important and ubiquitous process in the vicinity of massive protostars and reflects the extensive thermal processing of grains in such environments.

In this paper a recent view on the inventory of interstellar ices is presented. Constraints on the reservoirs of oxygen in dense clouds are discussed, taking into account recent measurements of oxygen-bearing species. Large abundances of CO2 and CH3OH in dense molecular clouds provide challenging perspectives to investigate the differences of ice chemistry in the vicinity of high and low-mass protostars. Accurate abundances of ice species and knowledge on the ice distribution in the protostellar regions are an important tool to define the environmental conditions in molecular clouds. A global understanding of interstellar ice chemistry also allows monitoring the incorporation and evolution of volatiles in planetesimals and comets and to reveal processes predominant in the early Solar System.

It is difficult if not impossible to explain the abundances of assorted interstellar molecules in both the gaseous and condensed phases without the use of grain chemistry. Unfortunately, the chemistry occurring on grains is not well understood because of a variety of uncertainties including the nature, size, and shape of dust particles, the binding energies of key species, the dominant mechanism of surface chemistry, and the correct mathematical treatment of surface processes. Still, intrepid astrochemists have used granular chemistry in chemical models of an assortment of sources including cold clouds, protostellar disks, and hot cores. Indeed, the dominant explanation of the saturated gas-phase molecules observed in hot cores involves grain chemistry during an earlier, low temperature phase. Although gas-grain models have elucidated major features of the chemistry, much more work remains to be accomplished before they can be used to help characterize the physical conditions in star-forming regions and their temporal variations.

The Submillimeter Wave Astronomy Satellite (SWAS) was successfully launched on 5 December 1998 with the goals of studying: (1) the distribution of oxygen in the interstellar medium; (2) the role of H2O and O2 as gas coolants; and (3) the UV-illuminated surfaces of molecular clouds. To achieve these goals, SWAS is conducting pointed observations of dense (n(H2) > 103 cm–3) molecular clouds throughout our Galaxy in either the ground-state or a low-lying transition of five astrophysically important species: H2O, H218O, O2, CI, and 13CO. SWAS has made great strides in each of these areas of investigation. This paper will summarize our H2O and O2 findings one year into the mission.

A review is presented of ISO observations of molecular hydrogen and its isotopic species, HD, toward various Galactic source types, such as shocks (Orion, DR 21, Cep A), photon dominated regions (IC 1396A, S140, Orion Bar) and putative X-ray excited sources (RCW 103, SS 433). In so doing I examine the similarities and differences in the H2 spectrum found under these different excitation conditions and mechanisms, and how the observations impact on some of the latest shock and PDR models. For instance, in addition to the ubiquitous ~ 2000 K component in shocks, seen using ground-based instrumentation, ISO reveals the existence of gas with both lower (150–800 K) and higher (≥ 3000 K) excitation temperatures. Further, ISO allows for the first time a measure of the bulk gas temperature toward PDRs (100–700 K), and H2 is also detected where X-ray emission is observed, providing at least circumstantial evidence of X-ray heating.

Photodissociation regions (PDRs) and shocks give rise to conspicuous emission from rotationally and vibrationally excited molecular hydrogen. This line emission has now been studied with ISO and from the ground in great detail. A remarkable discovery has been that toward the Orion outflow and other shock-excited regions, the H2 level populations show a very high excitation component. We suggest that these high-excitation populations may arise from non-thermal pumping processes, such as H2 formation and high-velocity ion-molecule collision in partially dissociative shocks. In PDRs such as NGC 7023 however, formation pumping is always less important than fluorescent pumping.

We furthermore present two HD emission line detections toward Orion Peak 1. This enables the first comparison of the H2 and the HD excitation, which surprisingly turn out to be identical.

Millimeter wave observations of molecular lines provide a useful tool to study the chemistry of bipolar outflows. In this contribution we discuss recent observational results which have permitted to assess the chemical alterations of the ambient medium produced by the presence of bipolar collimated winds driven by young protostars. Discussed first are the results from molecular line surveys which have allowed to identify several species whose abundances in the outflow lobes are considerably enhanced with respect to that of the ambient medium. These enhancements are useful diagnostics of physical and chemical processes in the outflow lobes, such as sputtering of grain cores and/or desorption of grain mantles. Then we discuss the characteristics of the line profiles and of the spatial distribution of the emission in specific enhanced species, which provide information about the mechanisms responsible for the enhancements. In particular we give emphasis to the results derived from observations of two highly enhanced species, SiO and CH3OH, which are observed to trace different components of the outflows.

Jets and outflows are typical manifestations of the stellar mass loss process during the early stage of star formation. Optical and near-infrared (NIR) imaging of jets and outflows has become extremely popular recently, especially concerning large-scale surveys of Herbig-Haro (HH) objects, NIR imaging of jets and outflows driven by both low- and high-mass stars, and detailed imaging of jets and bow shocks at sub-arcsec resolution.

Optical and IR observations of jets and outflows have revealed (1) cavities evacuated by the outflow motion; (2) bow shocks inside and near the surface of the jets and outflows; (3) exciting sources; (4) circumstellar disks responsible for collimating the jets and outflows; (5) sometimes a cluster instead of a single source from which the jets and outflows initiate; (6) bursts of jets and outflows in some regions. Large-scale surveys of HH objects and outflows have been done in a number of nearby regions including Taurus, Orion, and Perseus. The prominent results of these studies range from the discovery of parsec-scale jets and outflows to their large-scale distribution. These studies also begin to reveal the relation between outflow activity and the large-scale distribution of young stellar objects.

Masers have been well studied as indicators of star formation regions for over three decades. Their small size, high brightness, and narrow velocity width mean that we can measure their position and velocity with enormous accuracy, and so they stand out as high-precision signposts amidst the swirling gas that they sample. Nevertheless, in most cases the complexity of their kinematics has defied attempts to use them to unravel the processes of star formation. However, the last two or three years have seen a resurgence of interest in these masers because of exciting new evidence that, in some cases, they are tracing with high precision the kinematics of material in circumstellar disks around massive stars. The very existence of circumstellar disks around these massive stars is puzzling, and yet the maser results have now been confirmed by other data at radio and infrared wavelengths. In this paper I will review the current status of high-resolution maser observations, discuss some of the puzzles that are now confronting us, and speculate on where our current tentative steps may lead us.