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
Orbit-averaged total accretion rate, , in units of
versus initial semi-major axis,
, in units of
. The black points show the mean over an integer number of orbits between
and
while the error bars show the time variation of
over the same time period (to
). The solid blue line is just
, while the solid gray line is
. The mass dependence of
and
is proportional to the total mass squared,
. The two dashed lines show the same formulas, but with
replaced with
. At smaller separations, the accretion rate approaches that of a single particle with
, while at larger separations, the accretion rate approaches that of two separate particles with
.
Common Envelope Evolution

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.