This introductory article aims to give a brief overview of our current view of interstellar dust studies with a particular emphasis on the properties of dust at low temperatures and long wavelengths. An understanding of these dust properties will be key in unravelling the wealth of data that will be forthcoming from the imminent, space-based Herschel and Planck missions, and from the ALMA telescopes.

Herschel is the fourth cornerstone mission in the European Space Agency (ESA) science programme. It will perform imaging photometry and spectroscopy in the far infrared and submillimetre part of the spectrum, covering approximately the 55–672 µmm range and thus bridging the traditional space infrared range with the groundbased capabilities.
The key science objectives emphasize fundamental issues connected to the formation and evolution of galaxies and stars and stellar systems. However, Herschel will be an observatory facility and its unique capabilities will be available to the entire astronomical community for a wide range of observations. Herschel is equipped with a passively cooled 3.5 m diameter classical Cassegrain telescope. The science payload complement – two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution heterodyne spectrometer (HIFI) – is housed in a superfluid helium cryostat. The ground segment is jointly developed by the ESA, the three instrument consortia, and NASA/IPAC. Herschel is scheduled to be launched into a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 ECA (shared with the ESA cosmic background mapping mission Planck) in 2009. Once operational about half a year after launch, Herschel will offer 3 years of routine science operations. Almost 20 000 hours of observing time will nominally be made available for astronomy, 32% is guaranteed time, the remainder is open time which is offered to the worldwide general astronomical community through a standard competitive proposal procedure.

IR spectroscopy is the premier tool to study the composition of interstellar dust. Broad absorption and emission bands provide direct identification of the solid compounds present in space and allow measurement of accurate abundances. Systematic studies of large samples of sources allow then inferences on the origin and evolution of dust in space. Overall, the observed infrared spectra of interstellar and circumstellar dust reveals an incredibly rich and varied composition. This chapter briefly reviews the principles of infrared spectroscopy. This is then applied to the composition of circumstellar oxides and minerals and interstellar Polycyclic Aromatic Hydrocarbon molecules.

Dust is present in almost every astrophysical environment, ranging from circumstellar shells and disks to spiral, elliptical, starburst, and active galaxies, and to pre-galactic objects such as QSO absorption-line and damped Lyα systems. Dust leaves its imprint on interstellar extinction curves, IR spectra, and the elemental depletion patterns in the ISM of galaxies. Understanding the origin and the complex evolutionary cycle of dust is therefore an important goal in astrophysics. In this contribution, we present models to describe the evolutionary history of interstellar dust in a diverse set of astrophysical environments, ranging from normal star-forming galaxies like the Milky Way to high-redshift galaxies undergoing extreme rates of star formation. In particular, we show how the chemical evolution models can explain the correlations of dust abundances with galactic metallicities, and the presence of large amounts of dust in young dusty hyperluminous infrared galaxies in which supernovae are the only source of newly-condensed dust.

When the most primitive types of meteorite (chondrites) and interplanetary dust particles (IDPs) formed in the solar protoplanetary disk (solar nebula), all of them incorporated at least two types of presolar material: interstellar organic matter and circumstellar grains. The organic matter probably formed in the protosolar molecular cloud. To date, 10 types of circumstellar grain have been found and their isotopic compositions show that they formed around RGB/AGB stars, supernovae and, possibly, novae and WR stars. It has been proposed that interstellar silicates are also present in IDPs, but the evidence for this is less compelling.
The range of isotopic compositions exhibited by the circumstellar grains require a minimum of 35–40 stellar sources, suggesting that they are a fairly representative sample of circumstellar dust production in the Galaxy. In general, the relative abundances of circumstellar grains from different sources are similar to what is expected from astronomical estimates provided that the contribution from supernovae is relatively small. The one exception is graphite grains from AGB stars that are highly depleted.
Collisional erosion, radiation damage and sputtering are all expected to affect grains in the ISM on relatively short timescales. Yet, for reasons that are unclear, examination of the circumstellar grains has found little evidence for any of these processes.
The circumstellar grains also provide information about grain nucleation and growth in stellar winds. For instance, only graphite grains from AGB stars seem to have formed by heterogeneous nucleation. Graphite and SiC grains from AGB stars as large as 10–20 µm have been found in meteorites, probably requiring regions of high gas density rather than the usual assumption of uniformly expanding outflows. However, few graphite and SiC grains have similar C isotopic compositions, suggesting that they tend to form at different stages of AGB evolution.

Silicates form a major component of cosmic dust. This paper summarizes the main structural and spectroscopic properties of amorphous and crystalline silicates. Furthermore, it reviews our knowledge of cosmic silicates in the Galaxy. Recent in-situ studies of presolar silicates extracted from primitive meteorites and anhydrous interplanetary dust grains will be discussed.

Laboratory measurements aimed at studying the properties of carbonaceous materials and their evolution under simulated space conditions support the idea that interstellar carbon dust evolves through exposure to UV and cosmic rays, gas and heat. After summarizing the principal aspects of the laboratory approach to the problem of cosmic carbon dust, we discuss recent laboratory results showing how carbon grains influence the properties and evolution of ices mantles and the formation of molecular hydrogen.

Polycyclic Aromatic Hydrocarbons (PAHs) are the most likely carriers proposed to account for the Aromatic Infrared Bands observed in emission between 3.3 and 12.7 μm, fulfilling both the chemical (presence of CC and CH aromatic bonds) and excitation constraints. We explain here how the emission spectrum of such species in astronomical environments can be modelled, to obtain for each infrared band both its intensity and its detailed profile.
The PAH model suffers from the lack of identification of individual species. Two strategies are described here that are currently being used to progress in this identification process. The first idea is that features in the far IR are much more specific to the exact molecular identity. Therefore their search with the coming Herschel Space Observatory will provide an unambiguous way to identify these molecules through their IR emission. Second, a reflection on where interstellar PAHs come from and how they evolve due to environmental conditions (UV irradiation, gas and dust interactions) is also necessary to get more insights into the nature of interstellar PAH candidates. In particular, recent studies have emphasized a chemical link between PAHs and very small grains in photodissociation regions.
Finally, we illustrate in this paper how progress in this field can only be attained by a synergy between astronomical observations, fundamental studies, both theoretical and experimental, and models.

Irradiation is one of the main processes which influences the evolution of dust in astrophysical environments. This article reviews the basics of particle-matter interactions and gives an introduction to the induced modifications including phase transformation, sputtering, implantation, charging effects and transmutation. We also discuss the role of the nature of the incident particle (energetic photons, electrons and ions) as a function of its energy. Finally, we briefly review the main results obtained by irradiation studies on ices, carbonaceous matter and silicates.

All stars end through a phase where they form copious amounts of dust either in a stellar outflow or following explosive events. The most efficient dust factories seem to be stars that evolve through the Red Giant stage and form for several 105 yrs, dust in a cool stellar wind. The dust returned by them triggers the interstellar dust cycle and therefore is vital for the matter cycling in space. We discuss the theoretical basis for calculating the dust production by such stars. First, some elements of the theory of chemical equilibrium condensation calculations are presented and it is then discussed how the concept of activities can be used to also study non-equilibrium condensation. Then we discuss how dust particle growth in stellar winds can be calculated and which kind of input data are required for such calculations. Finally some results for dust production by AGB stars are discussed.

This article is intended to be used as an introduction to the problem of protoplanetary dust aggregation. The basic concept of dust aggregation and its description by Smoluchowski's equation are presented. Analytic solutions to Smoluchowski's equation are explored which show some characteristics of the more general solutions, such as self-similarity and scaling behavior. The physics of dust aggregation is treated through interaction forces between the particles and empirical data on sticking efficiencies and particle morphologies in dust-dust collisions. Aggregate morphologies are described by the fractal concept, and its implications for collision cross sections and collision velocities are discussed. Theoretical and experimental work on collision-induced restructuring and fragmentation of fractal and non-fractal dust aggregates is presented. Finally, the latest modelling approaches for protoplanetary dust growth are discussed.

The processing of dust in shocks in the interstellar medium can lead to changes in the dust to gas mass ratio and also to a radical evolution of the dust size distribution. Here we focus on the important results of numerical models that simulate the impact of shocks on the interstellar dust size distribution in molecular clouds and in the warm intercloud medium. We emphasize the necessity of detailed modelling of the grain charging processes and the grain inertia in order to unravel the complexities and consequences of the dust dynamics in shock waves.

The observational constraints on interstellar dust are summarized. A dust model, consisting of a mixture of amorphous silicate, graphite, and PAH material that reproduces the observed interstellar extinction, is described. It is used to calculate the infrared emission expected when such dust is heated by radiation with the spectrum of interstellar starlight with various intensities. By adopting a suitable size distribution for the smallest carbonaceous grains (PAHs), and a distribution of starlight intensities, one can reproduce the observed global emission spectrum of galaxies. This allows the total dust mass, and the PAH abundance, to be estimated for any galaxy with a spectral energy distribution measured by the IRAC and MIPS cameras on Spitzer Space Telescope. For the Spitzer Infrared Nearby Galaxies Survey (SINGS) sample, galaxies with metallicities Z > 0.3 Z๏ tend to have a major fraction of their refractory elements in dust grains, just as in the Milky Way. For lower metallicity galaxies with extended H I envelopes, the overall dust-to-metals ratio remains uncertain. The PAH abundance index qPAH (the fraction of the total dust mass in PAH particles with NC < 103 C atoms) is dependent on metallicity: galaxies with Z <0.3 Z๏ have median qPAH = 1.1%, whereas galaxies with Z > 0.3 Z๏ have median qPAH = 3.5%. The reasons for the pronounced dependence of qPAH on metallicity remain unclear.

Good knowledge of the far-infrared and millimeter emission from dust in the interstellar medium is important to get reliable estimates of the dust mass, to trace and understand the evolution of pre-stellar structures, and to accurately subtract the foreground emission in the cosmological background anisotropy measurements. Up to now the modeled dust emission profile in FIR and millimeter wavelength range is deduced from the wings of some mid-infrared fundamental lattice-resonances inside the silicate material, which is known to be the dominant constituent of this dust component. However recent astronomical observations have shown that the dust emission profile could be significantly more complicated than expected. In addition, spectroscopic studies in the laboratory on analogues of amorphous interstellar grains have revealed that additional processes can occur in that spectral range, which are strongly temperature-dependent. We propose a new model for far-infrared and millimeter dust emission which takes into account results from the solid state physics, used to interpret these laboratory data. This model explicitly incorporates the effect of the disorder in the internal structure of the dust grain. We show that this model can give a satisfactory interpretation for the astronomical observations. It opens new perspectives to derive some new dust characteristics from the shape of the dust emission spectrum.

This contribution focuses on the current state-of-play in multi-dimensional continuum radiative transfer modeling of gaseous astrophysical media containing dust. The fundamentals of the radiative transfer problem are briefly reviewed. The different solution methods are discussed in terms of applicability, advantages, and error control. To illustrate the problems and requirements for future analysis making use of dust data, two recent transfer models for prominent sources are shown: (i) the silhouette structure SO-1 in M 17 turned out to be the largest circumstellar disk known sofar; (ii) with inverse multi-wavelengths continuum radiative transfer modeling, the full 3D density and temperature structure of the molecular cloud core ρ Oph D could be determined.