Evolution of dust properties in an interstellar filament

We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 mum with an angular resolution of 2-3.5', of a quiescent dense filament (typically A(V) similar to 4) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS I-60 mum/I-100 mum 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 mum 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(-0.1)(+0.2) K with beta = 1.9 +/- 0.2 (or 12.0(-0.1)(+0.2) K using beta = 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 Desert 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(-0.7)(+0.3) (for typically n(H) > 3 +/- 1 x 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.