  The dynamics of the wind-wind collision in massive stellar binaries is
investigated using three-dimensional hydrodynamical models which incorporate
gravity, the driving of the winds, the orbital motion of the stars, and
radiative cooling of the shocked plasma. In this first paper we restrict our
study to main-sequence O+O binaries. The nature of the wind-wind collision
region is highly dependent on the degree of cooling of the shocked plasma, and
the ratio of the flow timescale of the shocked plasma to the orbital timescale.
The pre-shock wind speeds are lower in close systems as the winds collide prior
to their acceleration to terminal speeds. Radiative inhibition may also reduce
the pre-shock wind speeds. Together, these effects can lead to rapid cooling of
the post-shock gas. Radiative inhibition is less important in wider systems,
where the winds are accelerated to higher speeds before they collide, and the
resulting collision region can be largely adiabatic. In systems with eccentric
orbits, cold gas formed during periastron passage can persist even at apastron,
before being ablated and mixed into its surroundings and/or accelerated out of
the system.
