  We present a structural analysis of nearby galaxies in spiral-spiral pairs in
optical BVRI bands and compare with the structures of isolated spiral galaxies
and galaxies in ongoing mergers. We use these comparisons to determine how
galaxy structure changes during galaxy interactions and mergers. We analyze
light concentration (C), asymmetry (A), and clumpiness (S) parameters, and use
the projections of CAS parameter space to compare these samples. We find that
the CAS parameters of paired galaxies are correlated with the projected
separations of the pair. For the widest and closest pairs, the CAS parameters
tend to be similar to those of isolated and ongoing major mergers (ULIRGs),
respectively. Our results imply that galaxy CAS morphology is a robust quantity
that only changes significantly during a strong interaction or major merger.
The typical time-scale for this change in our paired sample, based on dynamical
friction arguments, is short, t ~ 0.1 - 0.5 Gyr. We find average enhancement
factors for the spiral pair asymmetries and clumpiness values of ~2.2 and 1.5.
The S parameter, which is related to star formation activity, has a moderate
level of enhancement suggesting that this activity in modern spirals depends
more on internal processes than on external conditions. We furthermore test the
statistical criterion for picking up interacting galaxies in an automated way
by using the A-S projection plane. The diversity of our spiral pair sample in
the CAS space suggests that structural/SF/morphological properties of
interacting galaxies change abruptly only when the interaction becomes very
strong and that the criteria for finding galaxies involved in major mergers
from Conselice (2003) is effective.
