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Binary stars are common. While only those with small separations may exchangegas with one another, even the widest binaries interact with their gaseoussurroundings. Drag forces and accretion rates dictate how these systems aretransformed by these interactions. We perform three-dimensional hydrodynamicsimulations of Bondi-Hoyle-Lyttleton flows, in which a binary movessupersonically relative to a homogeneous medium, using the adaptive meshrefinement code FLASH. We simulate a range of values of the initial semi-majoraxis of the orbit relative to the gravitational focusing impact parameter ofthe pair. When the binary separation is less than the gravitational focusingimpact parameter, the pair orbits within a shared bow shock. When the pair iswider, each object has an individual bow-shock structure. The long-termevolution of the binary is determined by the timescales for accretion, slowingof the center of mass, and orbital inspiral. We find a clear hierarchy of thesetimescales; a binary's center-of-mass motion is slowed over a shorter timescalethan the pair inspirals or accretes. In contrast to previous analyticpredictions, which assume an unperturbed background medium, we find that thetimescale 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 forcescan drive binaries either to coalescence or to the critical separation at whichgravitational radiation dominates their further evolution. We discuss theimplications of our results for binaries embedded in the interstellar medium,active galactic nuclei disks, and common envelope phases.

The Evolution of Binaries in a Gaseous Medium: Three-Dimensional Simulations of Binary Bondi-Hoyle-Lyttleton Accretion