Abstract. We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 µm with an angular resolution of 2-3.5 , of a quiescent dense filament (typically A V ∼ 4) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS I 60 µm /I 100 µm flux ratio in comparison with the diffuse interstellar medium. We show, from the combination of the PRONAOS/SPM and IRAS data, that, inside the filament, there is no evidence for emission from the transiently heated small particles responsible for the 60 µm emission, and that the temperature of large grains in thermal equilibrium with the radiation field is reduced in the inner parts of the filament. The temperature is as low as 12.1 +0.2 −0.1 K with β = 1.9 ± 0.2 (or 12.0 +0.2 −0.1 K using β = 2) toward the filament centre. These phenomena are responsible for the IRAS colour ratio observed toward the filament. In order to explain this cold temperature, we have developed a model for the emission from the filament using star counts from the 2MASS catalog as an independent tracer of the total column density and a radiative transfer code. We first use the optical properties of the dust from the standard model of Désert et al. (1990). The computed brightness profiles fail to reproduce the data inside the filament, showing that the dust properties change inside the filament. An agreement between data and model can be found by removing all the transiently heated particles from the densest parts of the filament, and multiplying the submillimetre emissivity by a significant factor, 3.4 +0.3 −0.7 (for typically n H > 3 ± 1 × 10 3 cm −3 , A V > 2.1 ± 0.5). We show that grain-grain coagulation into fluffy aggregates may occur inside the filament, explaining both the deficit of small grain abundance and the submillimetre emissivity enhancement of the large grains.

Abstract. We present a compilation of PRONAOS-based results concerning the temperature dependence of the dust submillimeter spectral index, including data from Galactic cirrus, star-forming regions, dust associated to a young stellar object, and a spiral galaxy. We observe large variations of the spectral index (from 0.8 to 2.4) in a wide range of temperatures (11 to 80 K). These spectral index variations follow a hyperbolic-shaped function of the temperature, high spectral indices (1.6-2.4) being observed in cold regions (11-20 K) while low indices (0.8-1.6) are observed in warm regions (35-80 K). Three distinct effects may play a role in this temperature dependence: one is that the grain sizes change in dense environments, another is that the chemical composition of the grains is not the same in different environments, a third one is that there is an intrinsic dependence of the dust spectral index on the temperature due to quantum processes. This last effect is backed up by laboratory measurements and could be the dominant one.

Abstract. The wavelength and flux calibration, and the inorbit performance of the Infrared Space Observatory LongWavelength Spectrometer (LWS) are described. The LWS calibration is mostly complete and the instrument's performance in orbit is largely as expected before launch. The effects of ionising radiation on the detectors, and the techniques used to minimise them are outlined. The overall sensitivity figures achieved in practice are summarised. The standard processing of LWS data is described.Send offprint requests to: B.M. Swinyard

Abstract. The Long-Wavelength Spectrometer (LWS) is one of two complementary spectrometers aboard the European Space Agency's Infrared Space Observatory 1 (ISO) (Kessler et al., 1996). It operates over the wavelength range 43 196:9 m at either medium (about 150 to 200) or high (6800 to 9700) spectral resolving power. This Letter describes the instrument and its modes of operation; a companion paper describes its performance and calibration.Send offprint requests to: P.E. Clegg (p.e.clegg@qmw.ac.uk) ? ISO is an ESA project with instruments funded by ESA Member States (especially the PI countries: France Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Context. Molecular oxygen, O 2 , has been expected historically to be an abundant component of the chemical species in molecular clouds and, as such, an important coolant of the dense interstellar medium. However, a number of attempts from both ground and from space have failed to detect O 2 emission. Aims. The work described here uses heterodyne spectroscopy from space to search for molecular oxygen in the interstellar medium. Methods. The Odin satellite carries a 1.1 m sub-millimeter dish and a dedicated 119 GHz receiver for the ground state line of O 2 . Starting in 2002, the star forming molecular cloud core ρ Oph A was observed with Odin for 34 days during several observing runs. Results. We detect a spectral line at v LSR = +3.5 km s −1 with ∆v FWHM = 1.5 km s −1 , parameters which are also common to other species associated with ρ Oph A. This feature is identified as the O 2 (N J = 1 1 −1 0 ) transition at 118 750.343 MHz. Conclusions. The abundance of molecular oxygen, relative to H 2 , is 5 × 10 −8 averaged over the Odin beam. This abundance is consistently lower than previously reported upper limits.