Issue |
A&A
Volume 381, Number 1, JanuaryI 2002
|
|
---|---|---|
Page(s) | 209 - 218 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20011074 | |
Published online | 15 January 2002 |
ISOCAM observations of the Ursa Major cirrus:
Evidence for large abundance variations of small dust grains
1
Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay, France
2
Département de Physique, Université Laval, Sainte-Foy, Québec, G1K 7P4, Canada
3
Laboratoire de radioastronomie, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
Corresponding author: M.-A. Miville-Deschênes, miville@lra.ens.fr
Received:
7
March
2001
Accepted:
24
July
2001
We present mid-IR imaging observations of a
high Galactic latitude cirrus obtained with the ISO camera ISOCAM at 6'' angular resolution.
The observations were done
with two filters LW2 (5–8.5 μm) and LW3 (12–18 μm)
that measure respectively the aromatic carbon bands and the underlying
continuum emission from small dust particles.
Three 0.05 square degree images sample atomic and molecular sections in
the Ursa Major cirrus. These images are compared with Hi, CO and IRAS observations.
In such a cloud transparent to stellar light ()
the mid-infrared to 100 μm and the mid-IR emissivity per hydrogen are
related to the abundance and the optical properties of
small dust particles independently of any modelling of the penetration of the radiation.
Within the atomic section of the cloud, the comparison
between ISOCAM images and 21 cm interferometric data
highlights an enhancement of the mid-IR emitters abundance by a factor ∼5
in an Hi filament characterized by a large transverse velocity gradient suggestive
of rotation. Furthermore, a drop in the abundance of the same mid-IR emitters is observed
at the interface between the atomic and molecular cirrus sections.
We propose that these abundance variations of the mid-IR emitters are related
to the production of small dust particles by grain shattering in energetic
grain-grain collisions generated by turbulent motions within the
cirrus and inversely by their disappearance due to coagulation on
large grains.
At the Hi-H2 interface we also observe a change in the
(LW2)/
(LW3) ratio by a factor 2.
This color variation indicates that the amplitude of the continuum near 15 μm, relative to the aromatic bands,
rises inside the molecular region. It could result from a
modification of the dust size distribution or of the intrinsic optical properties
of the small dust particles.
Key words: interstellar medium / infrared cirrus / dust-turbulence
© ESO, 2002
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