Evolution of dust properties in an interstellar filament
B. Stepnik, A. Abergel, J. P. Bernard, F. Boulanger, L. Cambrésy, M. Giard, A. P. Jones, G. Lagache, J. M. Lamarre, C. Meny, F. Pajot, F. Le Peintre, I. Ristorcelli, G. Serra, J. P. Torre;
AaA, 2003, 398, 551
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', of a quiescent dense filament (typically
AV ~ 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 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+/- 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.
(\cite{Desert1990}).
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 nH> 3 +/- 1x 103
cm-3, AV > 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.
KEYWORDS: ism: clouds, ism: dust, extinction, infrared: ism, radiative transfer, radio continuum: ism
PERSOKEY:dust, ,
CODE: stepnik2003