Evolution of dust properties in an interstellar filament

Free access article

Issue		A&A Volume 398, Number 2, February I 2003


Page(s)		551 - 563
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20021309

A&A 398, 551-563 (2003)
DOI: 10.1051/0004-6361:20021309

Evolution of dust properties in an interstellar filament

B. Stepnik^{1, 2}, A. Abergel¹, J.-P. Bernard¹, F. Boulanger¹, L. Cambrésy³, M. Giard², A. P. Jones¹, G. Lagache¹, J.-M. Lamarre¹, C. Meny², F. Pajot¹, F. Le Peintre¹, I. Ristorcelli², G. Serra² and J.-P. Torre⁴

¹ Institut d'Astrophysique Spatiale (IAS), Bât. 121, Université Paris XI, 91405 Orsay Cedex, France
² Centre d'Étude Spatiale des Rayonnements (CESR), 9 Av. Colonel Roche, BP 4346, 31028 Toulouse, France
³ Observatoire Astronomique de Strasbourg, UMR 7550, Université Louis Pasteur, 67000 Strasbourg, France
⁴ Service d'Aéronomie du CNRS (SA), BP 3, 91371 Verrières Le Buisson, France

(Received 16 January 2001 / Accepted 8 March 2002 )

Abstract
We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 $\mu$ m with an angular resolution of 2-3.5 $^{\prime}$ , of a quiescent dense filament (typically $A_{V} \sim 4$ ) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS $I_{\rm 60 \,\mu m}/I_{\rm 100 \,\mu m}$ 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 $\mu$ m 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 $\beta=1.9\pm 0.2$ (or 12.0 ^+0.2_-0.1 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 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 $n_{H}> 3 \pm 1\times 10^3$ cm ^-3, $A_{V} > 2.1\pm 0.5$ ). 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.

Key words: ISM: clouds -- ISM: dust, extinction -- infrared: ISM -- radiative transfer -- radio continuum: ISM

Offprint request: A. Abergel, alain.abergel@ias.fr

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