# Research

### 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 $a = 0.41 R_a$, where $R_a= 2GM / v_\infty^{2}$ is the gravitational focusing impact parameter of the center of mass of the binary, whose total mass is $M$. The right side of the figure shows density slices through the orbital plane. A wind of initial density $\rho_\infty$ flows from left to right at speed $v_\infty$. 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 $m_1$ and $m_2$ 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 $0.19 \pm 0.01$ 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.

### Common Envelope Evolution

CE simulation of a secondary moving through the envelope of a giant. The density gradient breaks the symmetry of the gas. Dense material (bottom of the figure) is swept up into regions of less density. Streamlines entering in the upper part of the figure face a barrier and cannot be focused into a downstream wake. The fluid never fully circularizes into a disk around the accretor, but the flow (and the accompanying drag forces) are highly variable at very small radii. Things smooth out at larger radii.
Common envelope (CE) is an essential phase in the formation of many types of close binary systems. A CE phase occurs when one star in the binary becomes embedded in the expanding envelope of its giant companion. The relative motion between the embedded star and the envelope gas results in drag forces, which deposit orbital kinetic energy into the envelope material. As drag forces strip energy and angular momentum from the orbit, the embedded star spirals deeper within the envelope of the giant. Whether or not the envelope can be completely unbound and the binary survives the encounter depends on the amount of energy the drag forces deposit into the envelope and over what timescale this energy transfer occurs. Our results suggest that the inclusion of realistic density gradients in the calculation of drag forces could result in a shorter CE interaction and a more rapid inspiral than analytical estimates suggest.