Abstract. In the following we present material in tabular and graphical form, with the aim to allow the nonspecialist to obtain a realistic estimate of the diffuse night sky brightness over a wide range of wavelengths from the far UV longward of Lyα to the far-infrared. At the same time the data are to provide a reference for cases in which background brightness has to be discussed, including the planning for space observations and the issue of protection of observatory sites. We try to give a critical presentation of the status at the beginning of 1997.

We describe the European Large Area ISO Survey (ELAIS). ELAIS was the largest single Open Time project conducted by ISO, mapping an area of 12 deg2 at 15 μm with ISOCAM and at 90 μm with ISOPHOT. Secondary surveys in other ISO bands were undertaken by the ELAIS team within the fields of the primary survey, with 6 deg2 being covered at 6.7 μm and 1 deg2 at 175 μm. This paper discusses the goals of the project and the techniques employed in its construction, as well as presenting details of the observations carried out, the data from which are now in the public domain. We outline the ELAIS ‘preliminary analysis’ which led to the detection of over 1000 sources from the 15 and 90‐μm surveys (the majority selected at 15 μm with a flux limit of ∼3 mJy), to be fed into a ground‐based follow‐up campaign, as well as a programme of photometric observations of detected sources using both ISOCAM and ISOPHOT. We detail how the ELAIS survey complements other ISO surveys in terms of depth and areal coverage, and show that the extensive multi‐wavelength coverage of the ELAIS fields resulting from our concerted and on‐going follow‐up programme has made these regions amongst the best studied areas of their size in the entire sky, and, therefore, natural targets for future surveys. This paper accompanies the release of extremely reliable subsets of the ‘preliminary analysis’ products. Subsequent papers in this series will give further details of our data reduction techniques, reliability and completeness estimates and present the 15‐ and 90‐μm number counts from the ‘preliminary analysis’, while a further series of papers will discuss in detail the results from the ELAIS ‘final analysis’, as well as from the follow‐up programme.

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Context. Cold cores in interstellar molecular clouds represent the very first phase in star formation. The physical conditions of these objects are studied in order to understand how molecular clouds evolve and how stellar masses are determined. Aims. The purpose of this study is to probe conditions in the dense, starless clump Ophichus D (Oph D). Methods. The ground-state (1 10 → 1 11 ) rotational transition of ortho-H 2 D + was observed with APEX towards the density peak of Oph D.Results. The width of the H 2 D + line indicates that the kinetic temperature in the core is about 6 K. So far, this is the most direct evidence of such cold gas in molecular clouds. The observed H 2 D + spectrum can be reproduced with a hydrostatic model with the temperature increasing from about 6 K in the centre to almost 10 K at the surface. The model is unstable against any increase in the external pressure, and the core is likely to form a low-mass star. Conclusions. The results suggest that an equilibrium configuration is a feasible intermediate stage of star formation even if the larger scale structure of the cloud is thought to be determined by turbulent fragmentation. In comparison with the isothermal case, the inward decrease in the temperature makes smaller, i.e. less massive, cores susceptible to externally triggered collapse.

Abstract.Odin is a satellite with a combined astronomy and aeronomy mission. It is designed for observations of species difficult or impossible to observe from ground, especially water and oxygen. The main instrument is a radiometer, operating in the frequency range 486-581 GHz and at 118.75 GHz. Its double-reflector telescope has a 1.1 m primary and the front-end amplifiers are cooled for maximum sensitivity. A 3-axis-stabilisation system provides a pointing accuracy better than 10 . Odin was developed on behalf of the space agencies in Sweden, Canada, France and Finland and was launched into a sun synchronous circular orbit in February 2001. The Odin Science Team, composed of astronomers and aeronomers from the partner countries, has established the observing programme and is responsible for all scientific matters regarding the Odin project. The spacecraft and instruments are performing well and operations are expected to continue well beyond the nominal two-year lifetime.

Abstract. The Odin satellite, which can observe the 1 10 -1 01 rotational line at 557 GHz of ortho water with a high spectral resolution (80 m s −1 ) and a spatial resolution of 2

Abstract. We present global IR properties of the Small Magellanic Cloud using an ISOPHOT 170 µm map and re-visited IRAS data. The integrated 170 µm flux is 14 950 ± 2300 Jy, and the integrated IR flux is F 1−1000 = 7.48 × 10 −10 W m −2 . Discrete sources contribute 28%, 29%, and 36% to the integrated flux at 60 µm, 100 µm, and 170 µm, respectively. Most of the total emission arises from diffuse emission regions surrounding the HII regions. The SED can be modelled by the composition of a 45 K, a 20.5 K and a 10 K blackbody component with emissivity index n = 2. A color temperature map is constructed from the ISOPHOT 170 µm and the IRAS 100 µm HiRes maps. The average dust temperature is T D = 20.3 K. The total dust mass is found to be M D = 7.8 × 10 5 M , yielding a gas-to-dust mass ratio M gas /M dust ≈ 540, a value 25 times lower than found by former studies. The global star formation rate is estimated to SFR total ≈ 0.05 M yr −1 . The comparison with other irregular galaxies reveals the SMC as quiet with no peculiar FIR properties.

Context. Using state-of-art near-infrared (NIR) instrumentation the near-infrared light scattered from interstellar clouds can be mapped over large areas. Measurement of the surface brightness provides information on the line-of-sight dust column density. Scattered light therefore provides an important tool to study the mass distribution of quiescent, interstellar clouds at high, even subarcsecond resolution. Aims. We test the assumption that light scattering is the dominant contributor to the surface brightness in all NIR bands. Furthermore, we want to show that scattered light can be used for an accurate estimation of dust column densities in clouds with extinction in the range A V = 1-15 m . Methods. We have obtained NIR images of a quiescent filament in the Corona Australis molecular cloud. The observations provide maps of diffuse surface brightness in the J, H, and Ks photometric bands. Assuming that the main contributor is indeed scattered light, we convert surface brightness data into a map of dust column density. The same observations provide colour excesses for a large number of background stars. These data are used to derive an extinction map of the cloud. The two, largely independent tracers of the cloud structure are compared. Results. In regions where the extinction is below A V ∼ 15 m , both diffuse surface brightness and background stars lead to similar column density estimates. The existing differences can be explained as a result of normal observational errors and bias in the sampling of extinctions provided by the background stars. There is no indication that thermal dust-emission would have a significant contribution even in the Ks band. The results show that, below A V ∼ 15 mag , scattered light provides a reliable way to map cloud structure. Compared with the use of background stars, it can provide data of a significantly higher spatial resolution.