  Mid-infrared (MIR) imaging and spectroscopic observations are presented for a
well defined sample of eight closely interacting (CLO) pairs of spiral galaxies
that have overlapping disks and show enhanced far-infrared (FIR) emission. The
goal is to study the star formation distribution in CLO pairs, with special
emphasis on the role of 'overlap starbursts'. Observations were made with the
Infrared Space Observatory (ISO) using the CAM and SWS instruments. The ISO~CAM
maps, tracing the MIR emission of warm dust heated by young massive stars, are
compared to new ground based H$\alpha$ and R-band images. We identify three
possible subgroups in the sample, classified according to the star formation
morphology: (1) advanced mergers (Arp~157, Arp~244 and Arp~299), (2) severely
disturbed systems (Arp~81 and Arp~278), and (3) less disturbed systems
(Arp~276, KPG 347 and KPG 426). Localized starbursts are detected in the
overlap regions in all five pairs of subgroups (1) and (2), suggesting that
they are a common property in colliding systems. Except for Arp~244, the
'overlap starburst' is usually fainter than the major nuclear starburst in CLO
pairs. Star formation in 'less disturbed systems' is often distributed
throughout the disks of both galaxies with no 'overlap starburst' detected in
any of them. These systems also show less enhanced FIR emission, suggesting
that they are in an earlier interaction stage than pairs of the other two
subgroups where the direct disk collisions have probably not yet occurred.

Astrophysics