A summary of the conference is presented along with some outstanding problems and challenges for future work.

I feel a bit embarrassed to be talking about things which happened before most of you were born. But this is what Ewine asked me to do.

We review current ideas and models in the problem of star formation from molecular cloud cores that are relatively isolated from the influences of other forming stars. We discuss the time scales, flow dynamics, and density and temperature structures applicable to each of the four stages of the entire process: (a) formation of a magnetized cloud core by ambipolar diffusion and evolution to a pivotal state of gravomagneto catastrophe; (b) self-similar collapse of the pivotal configuration and the formation of protostars, disks, and pseudo-disks; (c) onset of a magnetocentrifugally driven, lightly ionized wind from the interaction of an accretion disk and the magnetosphere of the central star, and the driving of bipolar molecular outflows; (d) evolution of pre-main-sequnce stars surrounded by dusty accretion disks. For each of these stages and processes, we consider the characteristics of the molecular diagnostics needed to investigate the crucial aspects of the observational problem.

Recent advances in the observational characterization of young stellar objects (YSOs) with millimeter-wave aperture synthesis arrays and (sub)millimeter single dish telescopes are reviewed. Studies of circumstellar material with ∼arc second resolution, which have only become possible at these frequencies within the past few years and which can potentially probe all stages of the star formation process, are emphasized. Molecules that are sensitive to different routes of formation and modification are outlined, including comments about their utility in distinguishing between a variety of environments and histories in star-forming cloud cores.

IR spectra of sources associated with molecular cloud material show a variety of absorption features attributed to simple molecules, such as H2O, CO, CH3OH, CO2, CH4, and OCS in icy grain mantles. These identifications are reviewed. These molecules are formed through accretion and reaction of gas phase species on grain surfaces. The high abundance of CH3OH and CO2 point towards the importance of hydrogenation and oxidation reactions in this process. Observations also show that thermal outgassing is of great importance for the composition of interstellar ice mantles. Both these processes are discussed in some detail.

Recent observational results toward the “Hot Core” sources (Orion KL, SgrB2, G34.3+0.15, W51, and so on) are summarized. Several saturated organic molecules are commonly observed among these sources, and these results favor formations on the grain mantles followed by evaporation. Some results of surveys in “hot core molecules” are presented. Such molecules may be used as diagnostics to establish the presence of “hot core” regions.

We review the properties of hot molecular cores with specific reference to chemical modelling. While most theoretical modelling has been generic, we discuss models for two specific sources, G34.3+0.15 and W3 (H2O). We show that the hot core gas in both these regions has an age of 5 103 — 104 yrs and that the abundances of many molecules are consistent with formation in the gas-phase following evaporation of simple molecular ices from the grains. We end with a discussion of current problems and future directions.

There are now many observations of high velocity, molecular emission associated with outflows from young stars. This emission might come from molecules that are formed in the outflowing material, or from entrained, ambient molecular gas. The present paper explores the latter possibility, describing the efforts that have been made to model the “lateral entrainment” that occurs in the turbulent mixing layer formed along the edges of a jet-like flow (or, equivalently, at any interface between a fast moving flow and the surrounding environment). A simple, analytic approach based on model equations is used to provide a qualitative picture of the dynamical, thermal and chemical structure of such a turbulent mixing layer. Finally, a review of the efforts up to date of modelling the dynamics and chemistry of turbulent mixing layers is presented.

We discuss recent high-sensitivity observations carried out with the IRAM instruments of the shock chemistry associated with bipolar outflows. We highlight the recent mm-wave studies of L1157, a highly-collimated outflow driven by a Class 0 source. These observations illustrate nicely the chemical segregation produced by the outflow. Different molecular lines are observed to trace different components of the gas. Some molecules are abundant in the quiescent medium but are not observed in the shock (e.g. C3H2, N2H+, H13CO+, DCO+), whereas some other molecules are very enhanced at the shocked region (e.g. SiO, CH3OH, H2CO, HCN, CN, SO, SO2, H2CS). We briefly discuss the chemistry of the most important molecules, devoting special attention to the species which are thought to be abundant in interstellar ice mantles.

We present recent results for the yields of Si and O, produced in the sputtering of SiO2 by ions of different masses, and show the importance of sputtering by heavy particles at low streaming velocities. These data are incorporated in a C-shock model to study the erosion of interstellar grains and the release of silicon through non-thermal sputtering within the shock. Once in the gas phase, the atomic silicon reacts with O2 and is rapidly transformed into SiO. The column densities of SiO thus calculated are compared with the observations of molecular outflows with a satisfactory agreement. In the postshock gas, SiO disappears from the gas phase through the reaction SiO(OH,H)SiO2 and SiO2 remains, unseen, in the cold dense gas. This could explain the extremely low upper limits of SiO deduced from observations of dark clouds.

Since the construction of the first specialized submillimeter telescopes nearly a decade ago, there has been extensive mapping of astronomical sources in the previously difficult to observe ground-state fine-structure transition of [C I] at 492 GHz. We discuss recent CSO observations of [C I] and 13CO (2 → 1) lines toward bright photon-dominated regions (PDRs). These observations have revealed an interesting effect. In many sources and over a broad range of column densities, the observed intensity of the [C I] line is proportional to that of the 13CO (2 → 1) line with exceptions mainly in dense, active star-forming regions, where C I appears to be under-abundant relative to 13CO. In this paper we quantitatively display this effect.

A brief account is presented of the chemical characteristics of regions controlled by the absorption of X-rays. Recent studies of the infrared emission from molecular hydrogen and time-dependent effects in evolving photon-dominated regions are summarized.

X-rays from young stellar objects can influence the physical properties of their surroundings. A particularly interesting example is the ionization of the accretion disk by hard X-rays at distances of the order of 1 AU or more. We show that X-rays induce a layered ionization structure in which the outer layer is sufficiently ionized to permit the Balbus-Hawley instability to operate and to allow accretion.

Maser emission from the OH, H2O, and CH3OH molecules is found toward hundreds of molecular cloud cores with recent and on-going star formation. The often very high intensities of the observed emission imply substantial abundances of the maser molecules, which are difficult or impossible to explain with ion-molecule gas phase chemistry. We summarize relevant observations which show that OH and Class II CH3OH masers form in the envelopes of ultracompact HII regions in which the high abundances of these molecules are produced by evaporation of methanol and water from icy dust grain mantles. In contrast, H2O masers form in outflows from protostellar objects, where high H2O abundances are produced by endothermic neutral-neutral reactions in hot postshock gas. Finally, we briefly discuss Class I CH3OH masers, which also arise in interstellar outflows.

Interstellar dark clouds have provided a laboratory which has been successfully used to identify a number of species, perform high resolution spectroscopy, and characterize the processes in the interstellar medium. We have successfully solved a number of problems and achieved significant progress through dark cloud observations. With the improvement of observing techniques and theories, however, we are now having new problems and are waiting for further progress. The dark clouds, because of their quietness and large column densities, still provide a very useful testing ground of many uncertain interstellar processes. Here we summarize some of the recent progress made in dark cloud observations and discuss related problems. We focus especially on the importance of nearby objects, possible signs of gas-grain interactions in dark clouds, and further sensitive observations.

The condensation of gas-phase molecules onto grain surfaces in cold molecular clouds is widely expected, and the presence of the resultant icy mantles well established, but quantitative study of the gas-phase depletions has not proved easy. This paper reviews the methods for determining depletions and the associated problems. Further observations are critical to testing our expectations for depletions and for the chemical evolution of forming stellar systems.

Bart Bok speculated back in the 1940's that dark cloud cores (i.e. Bok globules) are sites of star formation. The means for probing the dark cloud cores, molecules, were not discovered until much later. Today, we can finally say Bart Bok was right! Evidence for collapse in dark cloud cores will be discussed in general, along with specific examples of collapse candidates. We also describe methods to search for new candidates and the complications in identifying the infall motion.

The collision of comet D/Shoemaker-Levy 9 with Jupiter in July 1994 was an unprecedented opportunity to witness a phenomenon that was ubiquitous in the early solar system and which continues to shape its evolution. The tidal breakup of the comet, and the subsequent evolution of its fragments, have provided important insights into the nature of cometary nuclei, although conclusions regarding the comet's fundamental properties remain controversial. Detailed models describing the passage of the fragments through the jovian atmosphere have been fairly successful in explaining many aspects of the observations, including the appearance of giant plumes extending >3000 km above Jupiter's limb, the huge infrared signals following the splashback of material into the jovian atmosphere, and the formation of dark impact scars over large regions at mid-southern jovian latitudes. New chemical species (e.g., CO, H2O, S2, CS2, CS, OCS, HCN, C2H4, and possibly H2S) were created in Jupiter's atmosphere due to the shock heating of the mixture of cometary and jovian gases. The large NH3 enhancement in the jovian stratosphere was apparently caused by heating of the ambient NH3 cloud followed by upwelling. The presence of metal atoms and ions in the jovian atmosphere was an unmistakable signature of the comet. Photochemistry may have played an important role in the temporal evolution of the newly-created species. The appearance of expanding rings emanating from the impact sites was originally explained as the propagation of gravity waves, but this required an oxygen abundance in the deep jovian atmosphere that was ∼10 times solar, an hypothesis that has been apparently contradicted by recent results from the Galileo probe. Although the impacts clearly produced dramatic effects in Jupiter's atmosphere, most traces of the trauma were barely discernible one year later.

Comets are believed to be the most pristine material surviving from the origin of the Solar System. The nature of their volatiles should in principle provide chemical and physical clues to their formation environment in the solar nebula. Another important aspect of the study of comets is the extent to which they may incorporate relatively unprocessed interstellar material.

Our knowledge of the composition of cometary ices has been greatly improved in this last decade. The years 1995-1996 were rich in new results, with the close approach to Earth of comet C/1996 B2 Hyakutake and the discovery of the bright comet C/1995 O1 Hale-Bopp at large heliocentric distance. We will present the current knowledge of the nature and abundances of cometary volatiles and mention some of the outstanding issues related to the origin of comets and their links to interstellar material.