A 3D radiation-hydrodynamic AGB binary model (1910.08027v2)

Abstract: The origin of chemically peculiar stars and non-zero eccentricity in evolved close binaries have been long-standing problems in binary stellar evolution. Answers to these questions may trace back to an intense mass transfer during AGB binary phase. In this work, we use AstroBEAR to solve the 3D radiation-hydrodynamic equations and calculate the mass transfer rate in asymptotic-giant-branch (AGB) binaries that undergo the wind-Roche-lobe-overflow or Bondi-Hoyle-Lyttleton (BHL) accretion. MESA produces the density and temperature of the boundary condition of the AGB star. To improve the resolution of the dynamics of a circumbinary disk, we implement an azimuthal angle-dependent 3D radiation transfer. We consider optically thin cooling and obtain the number density of the coolants by solving Saha equations. One of the goals of this work is to illustrate the transition from the wind-Roche-lobe-overflow to BHL accretion. Both circumbinary disks and spiral structure outflows can appear in the simulations. Circumbinary disks may form when the optical thickness in the equatorial region increases. The increase of the optical thickness is due to the deflected wind. The resulting mass transfer efficiency in our models is up to a factor of 8 times higher than what the standard BHL accretion scenario predicts, and the outflow gains up to 91% of its initial angular momentum when it reaches 1.3 binary separations. Consequently, some AGB binaries may undergo orbit shrinkage, and some will expand. The high mass transfer efficiency is closely related to the presence of the circumbinary disks.