The Evolution of Binaries in a Gaseous Medium: Three-Dimensional Simulations of Binary Bondi-Hoyle-Lyttleton Accretion
Time series movie of the binary system with semi-major axis
, where
is the gravitational focusing impact parameter of the center of mass of the binary, whose total mass is
. The right side of the figure shows density slices through the orbital plane. A wind of initial density
flows from left to right at speed
. The large-scale structure of the primary shock inherits the shape of canonical BHL flow. Inside the primary shock, however, the orbital motion is supersonic and the binary excites spiral shock waves which propagate down stream. The left side of the figure shows, from top to bottom,
forces along the center-of-mass velocity, forces along the barycentric velocities of the two particles,
and the instantaneous accretion rates. In each case, the instantaneous values for
and
are shown in dark and light grey (respectively), while the black line shows the sum of the two. Each panel is normalized to the BHL value.
Abstract: 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 (BHL) 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 Common Envelope Evolution
