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.

Aims. We develop a simple model of coagulated dust particles of two sizes (3.5 and 60 nm radius) to understand the nature and the effects of coagulation, which could explain the evolution of the far-infrared (FIR) dust opacity observed in the transition between the diffuse and the dense interstellar medium (ISM) (n H > 10 3 cm −3 ). Methods. Using the discrete-dipole approximation (DDA) method, we have calculated the absorption coefficient, directly proportional to the opacity, of coagulated grains with varying numbers of sub-grains and of different grain composition. Results. We show that, in the transition from diffuse to dense clouds, an increase in the FIR opacity by a factor of about 2.7 is possible and a decrease in the grain temperature by up to 3−4 K can be explained by the presence of coagulated aggregates composed of four big grains and 4000 very small grains (40% of the volume of the BGs). The coagulation of very small grains into the aggregates leads to a decrease in the 60 μm emission. Conclusions. This model can explain the observed increase in opacity at long wavelengths, the decrease in temperature from the diffuse ISM to denser regions with the coagulation of grains into aggregates and the absence of the 60 μm emission with the coagulation of very small grains onto the surface of the big grains.