The Evolution of Binaries in a Gaseous Medium: Three-Dimensional Simulations of Binary Bondi-Hoyle-Lyttleton Accretion
Physical and Biological Sciences
Physics (Astrophysics) Senior Thesis
Binary (two-star) systems are common. While only those with small separations may exchange gas with one another, even the widest binaries interact with their gaseous surroundings. A gravitating object is transformed through accretion (gravitational capture of the surrounding gas) and through drag forces as the fluid tries to oppose the motions of the object. For binaries, the drag forces and rates of accretion realized in these interactions dictate how the translational and orbital motion of the binary is transformed by the gas. We perform three-dimensional hydrodynamic simulations of binary systems traveling supersonically relative to a uniform gas. We simulate a range of values of the initial separation of the binary relative to the length-scale that determines the binary's gravitational influence on the gas. When this ratio is much less than one, 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±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 wave radiation dominates their further evolution. We discuss the implications of our results for binaries embedded in the interstellar medium, in the gaseous discs at the centers of active galaxies and in evolving triple-star systems.
