ISOCAM observations of the Ursa Major cirrus:. Evidence for large abundance variations of small dust grains
M. A. Miville-Deschênes, F. Boulanger, G. Joncas, E. Falgarone;
AaA, 2002, 381, 209
ABSTRACT: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 mu m) and LW3 (12-18 mu m) 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 H_I, CO and IRAS observations.
In such a cloud transparent to
stellar light (AV < 0.5) the mid-infrared to 100 mu m 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
Inu(LW2)/Inu(LW3) ratio by a factor 2.
This color variation indicates that the amplitude
of the continuum near 15 mu m p, 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.
KEYWORDS: interstellar medium, infrared cirrus, dust-turbulence
CODE: miville-deschenes2002