Wind mass transfer in a binary star

Wind mass transfer occurs when one star accretes part of a companion's stellar wind without the donor necessarily filling its Roche lobe.

For a fast isotropic cold wind, the mass density at a companion at separation $a$ is $(-{\dot{M}}_d)/(4\pi a^2{v}_w)$. The ballistic capture model gives ${\dot{M}}_a/(-{\dot{M}}_d)=G^2{M}_a^2/(a^2{v}_w^4)$. Fast relative to orbital motion also makes the capture radius small compared with the separation.

Wind Roche-lobe overflow occurs when a slow dense wind nearly fills the donor's Roche lobe and is gravitationally focused through the inner Lagrange region. It can transfer a much larger fraction of the outflow than a uniform fast-wind Bondi--Hoyle estimate.

A tidally enhanced stellar wind is a model in which a companion increases a giant's wind mass loss as the giant approaches its Roche lobe. Removing envelope mass before contact can reduce or reverse the mass ratio and make subsequent Roche-lobe overflow more stable. This is one proposed route to wide systems with Algol-like reversed evolutionary ordering. Van der Sluys's compact-binary study, section 5.7.1 discusses the enhanced-wind scenario and its effect on mass-transfer stability.

In Jeans-mode mass loss, an isotropic wind escapes from one binary component carrying that star's specific orbital angular momentum. If the companion accretes negligibly, a circular binary obeys $aM=\text{constant}$ and $P{M}^2=\text{constant}$.

A barium star is a main-sequence or giant star enriched in carbon and slow-neutron-capture elements despite being too unevolved to have made them itself. It was polluted by an asymptotic-giant-branch companion that is now a white dwarf.

An S-type star has strong zirconium-oxide bands and enhanced slow-neutron-capture elements. Intrinsic S-type stars are thermally pulsing asymptotic giants, while extrinsic S-type stars were polluted by a former asymptotic-giant-branch companion.