Recent years have seen substantial progresses in our understanding of solar type protostellar structure, and particularly of the chemical structure of the protostellar envelopes. On the one hand, the cold outer regions keep intact the memory of the previous pre-collapse phase, when the dust is so cold and dense that almost all molecules freeze out onto the dust grain mantles. The gas-phase chemical composition undergoes dramatic changes, the most spectacular aspect of which is the huge increase of the molecular deuteration degree, which can reach 13 orders of magnitude with respect to the elemental D/H ratio. On the other hand, in the innermost regions of the envelope – the so-called hot corinos – the grain mantles evaporate when the dust temperatures exceed 100 K, injecting into the gas phase hydrogenated molecules, such as formaldehyde and methanol. Those molecules probably undergo chemical reactions that form more complex organic molecules, which have now also been observed in low-mass hot corinos. In this contribution, I review what we have and have not recently understood concerning both the cold envelopes and the hot corinos of solar-type protostars.

Our understanding of the physical and chemical structure of pre-stellar cores, the simplest star-forming sites, has significantly improved since the last IAU Symposium on Astrochemistry (South Korea, 1999). Research done over these years has revealed that major molecular species like CO and CS systematically deplete onto dust grains in the interior of pre-stellar cores, while species like N2H+ and NH3 survive in the gas phase and can usually be detected toward the core centers. Such a selective behavior of molecular species gives rise to a differentiated (onion-like) chemical composition, and manifests itself in molecular maps as a dichotomy between centrally peaked and ring-shaped distributions. From the point of view of star-formation studies, the identification of molecular inhomogeneities in cores helps to resolve past discrepancies between observations made using different tracers, and brings the possibility of self-consistent modelling of the core internal structure. Here I present recent work on determining the physical and chemical structure of two pre-stellar cores, L1498 and L1517B, using observations in a large number of molecules and Monte Carlo radiative transfer analysis. These two cores are typical examples of the pre-stellar core population, and their chemical composition is characterized by the presence of large ‘freeze out holes’ in most molecular species. In contrast with these chemically processed objects, a new population of chemically young cores has begun to emerge. The characteristics of its most extreme representative, L1521E, are briefly reviewed.

Several new multiply deuterated species have been detected over the past three years, including ND3 (van der Tak et al. 2002; Lis et al. 2002), CHD2OH, CD3OH (Parise et al. 2002, 2004), D2S (Vastel et al. 2003), HD2+ (Vastel et al. 2004) and D2CS (Marcelino et al. 2005). In addition, mono-deuterated species have been observed with abundances >10% of their un-deuterated analogues (e.g. CH2DOH observed by Parise et al. 2002; NH2D observed by Saito et al. 2000 and Hatchell 2003). These are remarkable results, given that the underlying abundance of deuterium in the local interstellar medium (ISM) is ∼10−5 times lower than that of hydrogen (Linsky 1998; Sonneborn et al. 2000).

Such large enhancements in the abundances of deuterium-bearing molecules can either be due to gas-phase or to grain-surface fractionation. Grain-surface reactions are undoubtedly important in producing saturated species such as methanol, water, ammonia, and hydrogen sulphide. Water ice is observed to be abundant and ubiquitous throughout the ISM, and enhanced abundances of gas-phase NH3, CH3OH, H2CO and H2S (among others) are observed in warmer regions around protostars where grain mantles have evaporated.

Recent observational and theoretical evidence suggests that the deuterium fractionation in star-forming regions is set by gas-phase and grain-surface reactions during the cold, dense pre-protostellar phase. For species which form on grain surfaces via H atom addition to CO, N, O and S, the deuterium fractionation on grains comes from the relative amounts of atomic D and H which are accreting from the gas. The observations of deuterated methanol and D2S require that the gas-phase atomic D/H ratio at the time the molecules formed was ≥ 0.1.

This paper presents results from chemical models of the prestellar core phase of star formation, showing how this high atomic D/H ratio can be produced, and discusses how models can also be used to look at deuterium fractionation in the protostellar stages of star formation.

We present a coupled dynamical and chemical model for collapsing pre-stellar cores (Li et al. 2002; Shematovich et al. 2003a,b; Pavlyuchenkov et al. 2003). It treats the dynamics of thermally and magnetically supported cores in 1D, with an extended chemical network incorporated. The latest version of the model includes UV-irradiation of the core envelope. We have also developed a 2D Monte Carlo model of radiative transfer to compute molecular line profiles for comparison with observations.

The model allowed us to constrain evolutionary scenarios for collapsing pre-stellar cores, to calculate molecular line profiles from the spatial distribution of chemical species and the velocity field, and to characterize the chemical properties of dense cores.

We have determined line profiles along multiple lines of sight through a given pre-stellar core. This allowed us to compare model predictions with the observational maps of molecular lines available for L1544 and other well studied cores. The comparison of synthetic and observed line profile maps contributed to the understanding of the velocity field and pattern of chemical differentiation observed in individual cores.

Molecular cloud cores—whether stellar, non-stellar, proto-stellar, or pre-proto-stellar—have come to be recognized as key elements in the study of star formation. For both high-and low-mass stars, these cores hold key information for the final make-up of the stellar cluster and the physical processes by which the cluster and/or the individual stars form. In the case of massive stars, the chemical effects extend even beyond the molecular core, reaching into the photo-dissociated region produced by the harder spectrum of the massive stars. We discuss the directions in which these studies are taking us, with special attention given to those aspects which are unique to high-mass star-forming regions.

The distribution and composition of the dust and gas surrounding Young Stellar Objects (YSOs) is of continuing interest. Fortunately, rapid advances in observational capabilities have led to data of high spatial and spectral resolution, as well as the opportunity to observe in previously unavailable windows using satellites. Such high-quality data have motivated enhancements in theoretical models. Key to these models is the chemical evolution of the gas. Since the chemical evolution depends upon temperature, density, and time, the state and history of the source is encoded in the spatial distribution of the chemical abundances.

It is possible, using both parametric and detailed physical-chemical modeling, to constrain many source properties, and identify potential reactions of further laboratory interest. Using specific examples, I discuss some successes toward constraining the source properties, as well as challenges posed by current problems. Finally, I discuss the potential effect of infall dynamics and recent laboratory measurements of temperature programmed desorption of ices from grains on inferring source properties.

Until recently hot cores were the first indirect manifestation of a young massive star, but recent observational successes are now providing us with the first candidates for the very early phases of massive star formation, probably the precursors of hot cores. The characteristic hot core chemistry is believed to arise from the evaporation of the icy mantles that accumulate on the dust grains during the collapse the leads to the formation of the massive star. The duration over which the grains are warmed is determined by the time taken for a pre-stellar core to evolve to the Main Sequence. Hence, hot cores and their precursors contain an integrated record of the physics and chemistry occurring during the collapse that led to the star. In this article I will review recent advances in the chemical modelling of hot cores and their precursors in light of the recent TPD experiments on a variety of ices and I will briefly discuss the implications of such studies for the interpretation of high mass protostellar objects.

I review recent progress in determining the rate coefficients appropriate to modelling interstellar chemistry, give some information on appropriate databases from which rate coefficients can be obtained, and point to the importance of the gas-grain interaction in determining molecular abundances. Although many of the fundamental gas-phase reactions have been studied in the laboratory, the failure of the models to explain the observations of water and methanol in cold clouds indicates that grains may have an important role, both in acting as a surface for freeze-out and in the synthesis of complex molecules. The major challenge in astrochemistry is to develop a more quantitative model for the role of grains and, in some cases, to incorporate a better, probably more complex, physical model for interstellar clouds.

The detection of interstellar molecules relies on the precise knowledge of spectral line positions from laboratory measurements. Technical developments of recent years have led to an extension of the accessible spectral range towards shorter wavelengths. New telescopes like SOFIA, the HIFI instrument aboard the Herschel satellite, and ALMA will be used for astrophysical observations in the terahertz region. The Cologne group has developed precise spectrometers to study molecules of astrophysical importance under laboratory conditions and to obtain characteristic spectra for their possible detection in space. We present recent results on light hydrides, carbon-chain molecules and more complex species.

Over the last few years, we have applied the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique to the study of neutral-neutral reactions and energy transfer processes in the gas phase. This has enabled the rates of a wide range of reactions between electrically neutral species to be measured down to temperatures as low as $\sim $10 K. These results have generated significant interest amongst theoretical chemists, and especially amongst astrochemists. Our measurements of low-temperature rate coefficients have had a significant impact on the models used to simulate the chemistry of dense interstellar clouds. In this article the motivations for the experimental study of reaction kinetics at low temperatures are described, and a detailed description of the CRESU technique is given, followed by a brief overview of the results obtained in Rennes and Birmingham. Four recent low-temperature kinetic studies are then described: the reaction O + OH and the interstellar oxygen problem, reactions of the C$_2$ radical, energy transfer in collisions of C($^3{\rm P}_J$) with He and H$_2$, and preliminary results on the reaction kinetics of the C$_4$H radical.

The paper focuses on collisional excitation rates of molecules by He and H2 relevant to the interstellar medium. It discusses currently available data, presents very recent work and outlines new work being carried out by various teams.

The branching ratios of the different reaction pathways and the overall rate of the dissociative recombination of CD$_{3}$OD$_{2}^{+}$ were measured at the CRYRING storage ring located at the Manne Siegbahn Laboratory in Stockholm, Sweden. A preliminary analysis of the data yielded that formation of methanol accounts for only 6 $\pm$ 2% of the total reaction rate. Largely, dissociative recombination of CD$_{3}$OD$_{2}^{+}$ involves fragmentation of the C–O bond, the major process being the three-body break-up forming CD$_{3}$, OD and D (branching ratio 0.59). A non-negligible formation of interstellar methanol by the previously proposed mechanism is therefore very unlikely.

Understanding deuterium fractionation is currently one of the greatest challenges in astrochemistry. In this contribution deuteration experiments of the series CH$_n^+$, n=2–5, in a low temperature 22-pole ion trap are used to systematically test a simple chemical rule predicting which molecular ion undergoes deuterium exchange in collisions with HD. CH$_4^+$ turns out to be a problem case, where prediction fails. The method of laser induced reaction (LIR) is used to determine the population ratio of the lowest ortho-to-para states of H$_2$D$^+$ relaxed in collisions with H$_2$. Preliminary results indicate that the ortho-to-para ratio of H$_2$D$^+$ is substantially reduced in para-H$_2$. This points at the important role of nuclear spin in deuterium fractionation, in particular at the destruction of ortho-H$_2$D$^+$ in collisions with ortho-H$_2$. More systematic LIR experiments are needed for a chemical model of deuterium fractionation including state-to-state modifications of the species involved.

We review progress in the area of the modelling of shocks in molecular clouds. In particular, we consider what has been learnt about shock structure evolution in situations where the steady-state assumption is no longer valid. We discuss the interpretation of the observed water abundance from SWAS, Odin, and ISO. We also consider the erosion of grains in shocks as well as the effect of the presence of grains on shock structure.

I present an overview of theory and observations of FUV-photon and X-ray dominated regions (PDRs and XDRs), with a discussion of some recent results.

We present recent results on the carbon chemistry in photodissociation regions. We show that carbon chains and rings (CCH, c-C$_3$H$_2$ and C$_4$H) are tightly spatially correlated with each other, and with the mid-infrared emission due to PAHs (7 and 15 $\mu$m), mapped by ISOCAM. Neither the spatial distribution, nor the abundances of these species can be fit by state-of-the-art PDR models, which calls for another production mechanism. We discuss model predictions for carbon clusters and simple hydrocarbons. We show how selected abundance ratios can be used as a diagnostic of the physical conditions. We stress the need for more theoretical and laboratory work on fundamental processes relevant for the interstellar medium, which should be taken into account in the astrochemical models, but whose rates are not known accurately enough.

We report the first astronomical detection of the CF$^+$ (fluoromethylidynium) ion, obtained by recent observations of its $J = 1-0$ (102.6 GHz), $J = 2-1$ (205.2 GHz), and $J = 3-2$ (307.7 GHz) pure rotational emissions toward the Orion Bar. Our search for CF$^+$ — carried out using the IRAM 30m and APEX 12m telescopes—was motivated by recent theoretical models that predict CF$^+$ abundances of a $\rm few \times 10^{-10}$ in UV-irradiated molecular regions where C$^+$ is present. The measurements confirm the predictions. They provide support for our current theories of interstellar fluorine chemistry, which suggest that hydrogen fluoride should be ubiquitous in interstellar gas clouds.

In the past several years, great progress has been made on the spectroscopy of polyatomic molecules in diffuse interstellar clouds. In this talk, I will review recent developments involving H$_3^+$, C$_3$, and the Diffuse Interstellar Bands (DIBs).

The simplest polyatomic molecular ion, H$_3^+$, has long been recognized as the cornerstone of ion-neutral chemistry in dense molecular clouds (Herbst & Klemperer 1973; Watson 1973). However, in diffuse clouds (where electrons are abundantly produced from photoionization of atomic carbon) the H$_3^+$ number density was expected to be considerably lower than in dense clouds, owing to the efficiency of electron recombination. It was, therefore, a surprise when a large column density of H$_3^+$ was detected (McCall et al.1998) in the diffuse line of sight towards Cygnus OB2 12, and subsequently in a sample of heavily reddened diffuse sightlines (McCall et al.2002). Recently, we have detected H$_3^+$ even in the classical diffuse cloud sightline towards $\zeta$ Persei; together with a new measurement of the electron recombination rate coefficient, this result suggests that the cosmic-ray ionization rate is much higher in diffuse clouds than in dense clouds (McCall et al. 2003a)!

In 2001, interstellar C$_3$ was first detected by J. P. Maier and colleagues (maier et al. 2001) in three diffuse cloud sightlines. This was quickly followed up by another detection (Roueff et al. 2002) and a survey conducted at low-resolution (Okaet al. 2003). This was followed by a high-resolution survey (Ádámkovics, Blake, & McCall 2003) that yielded rotationally resolved spectra of C$_3$ in 10 sightlines. Much like C$_2$, C$_3$ has no permanent dipole moment, and therefore its rotational distribution serves as a sensitive diagnostic of both temperature and density.

The existence of larger polyatomic molecules in diffuse clouds is clear from the presence of the DIBs, which have remained an enigma since their discovery some eight decades ago. A recent survey of the DIBs at the Apache Point Observatory has resulted in a uniform sample of DIB spectra with much higher signal-to-noise ratios than previously available. Some early results from the analysis of the survey data include the identification of a set of DIBs that are related to C$_2$ (Thorburn et al. 2003) as well as some well-correlated pairs of DIBs. I will discuss our recent results.

The benefits of placing ultraviolet (UV) spectrographs in space were first pointed out by Spitzer (1946) and Spitzer & Zabriskie (1959), with much of the emphasis on absorption-line measurements of interstellar atoms and molecules. The first UV observations of the diffuse interstellar medium took place in the early 1970s, using sounding rockets, which were followed by a series of orbital instruments starting in the later 1970s and extending almost to the present time. Many of these observations have provided information on the chemistry of the diffuse ISM, and will be emphasized here. The most abundant molecule in space, H$_2$, is not readily observed in other wavelength bands and was first detected by a UV sounding rocket experiment (Carruthers 1970). Thereafter a series of orbital instruments, starting with the Copernicus satellite in 1972 and culminating with the launch of FUSE in 1999, carried out broad surveys of molecular hydrogen in a variety of ISM environments (e.g., Spitzer et al. 1973; Snow et al. 2000; Shull et al. 2000; see also the review by Shull & Beckwith 1982). The UV observations of electronic transitions of H$_2$, including lines arising from excited rotational levels of the ground state, have provided a wealth of information on not only the chemistry of the diffuse ISM, but also the composition and physical conditions. The second most abundant molecule in the diffuse ISM, CO, also has numerous electronic transitions in the UV, and has been widely observed by many different instruments, most usefully the Hubble Space Telescope (HST) (e.g., Lambert et al. 1994; Sheffer et al. 2002). Valuable information on diffuse ISM chemistry, isotopic composition, and physical conditions has been provided by these observations. A few additional molecules, all of them diatomics such as OH and N$_2$, have been detected through UV absorption-line observations, and many others have been sought unsuccessfully. The Cosmic Origins Spectrograph, expected to be installed aboard the HST in 2007, will have enhanced UV sensitivity and will provide information on molecular abundances in denser clouds than previously studied. One of the high priorities will be the search for electronic transitions of complex organic molecules such as PAHs.

Millimeter-wave (mm-wave) absorption profiles toward extragalactic sources consistently find just diffuse (occasionally perhaps translucent) neutral gas—low/moderate density and extinction—along even some very long, dark lines of sight. CO, often heavily fractionated and mimicking the appearance of dark gas in emission, occasionally absent in emission even when present in absorption, is not the dominant form of carbon in these regions (presumably it is C+) yet the abundances of many other molecules resemble those seen in TMC-1. Some species (OH, HCO+, C2H and C3H2) turn on with high abundances just when H2 does; others (HCN, HNC, CN) require slightly higher $N$(H2) and yet others (CS and other sulfur-bearing species,NH3 and H2CO) even higher $N$(H2). The systematics and implications of these recent discoveries are discussed here.