ISOCAM observations of the Ursa Major cirrus: - Evidence for large abundance variations of small dust grains

We present mid-IR imaging observations of a high Galactic latitude cirrus obtained with the ISO camera ISOCAM at 6 00 angular resolution. The observations were done with two lters LW2 (5{8.5 m) and LW3 (12{18 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 Hi, CO and IRAS observations. In such a cloud transparent to stellar light (AV < 0:5) the mid-infrared to 100 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 5i n anHi lament 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 I(LW2)/I(LW3) ratio by a factor 2. This color variation indicates that the amplitude of the continuum near 15 m, relative to the aromatic bands, rises inside the molecular region. It could result from a modication of the dust size distribution or of the intrinsic optical properties of the small dust particles.