  We carried out a statistical analysis of galaxy pairs in hydrodynamical
Lambda-CDM simulations. We focused on the triggering of star formation by
interactions and analysed the enhancement of star formation activity in terms
of orbital parameters. By comparing to a suitable sample of simulated galaxies
without a nearby companion, we find that close encounters (r<30 kpc/h) may
effectively induce star formation. However, our results suggest that the
stability properties of systems and the spatial proximity are both relevant
factors in the process of triggering star formation by tidal interactions. In
order to assess the effects of projection and spurious pairs in observational
samples, we also constructed and analysed samples of pairs of galaxies in the
simulations obtained in projection. We found a good agreement with
observational results with a threshold at rp ~ 25 kpc/h for interactions to
effectively enhance star formation activity.
  For pairs within rp < 100 kpc/h, we estimated a ~27% contamination by
spurious pairs, reduced to ~19% for close systems. We also found that spurious
pairs affect more strongly high density regions with 17% of spurious pairs
detected for low density regions compared to 33% found in high density ones.
Also, we analysed the dependence of star formation on environment by defining
the usual projected density parameter for both pairs and isolated galaxies in
the simulations. We find the expected star formation-local density relation for
both galaxies in pairs and without a close companion, with a stronger density
dependence for close pairs which suggests a relevant role for interactions in
driving this relation.
