  Binary stars are common. While only those with small separations may exchange
gas with one another, even the widest binaries interact with their gaseous
surroundings. Drag forces and accretion rates dictate how these systems are
transformed by these interactions. We perform three-dimensional hydrodynamic
simulations of Bondi-Hoyle-Lyttleton flows, in which a binary moves
supersonically relative to a homogeneous medium, using the adaptive mesh
refinement code FLASH. We simulate a range of values of the initial semi-major
axis of the orbit relative to the gravitational focusing impact parameter of
the pair. When the binary separation is less than the gravitational focusing
impact parameter, the pair orbits within a shared bow shock. When the pair is
wider, each object has an individual bow-shock structure. The long-term
evolution of the binary is determined by the timescales for accretion, slowing
of the center of mass, and orbital inspiral. We find a clear hierarchy of these
timescales; a binary's center-of-mass motion is slowed over a shorter timescale
than the pair inspirals or accretes. In contrast to previous analytic
predictions, which assume an unperturbed background medium, we find that the
timescale for orbital inspiral is proportional to the semi-major axis to the
$0.19 \pm 0.01$ power. This positive scaling indicates that gaseous drag forces
can drive binaries either to coalescence or to the critical separation at which
gravitational radiation dominates their further evolution. We discuss the
implications of our results for binaries embedded in the interstellar medium,
active galactic nuclei disks, and common envelope phases.
