Mass transfer stability, Lagrangian ejection, and gas fall-back in common envelope binaries

In this thesis we carry out a number of common envelope (CE) binary interaction simulations with the 3D, smooth particle hydrodynamics code Phantom, to study the dependence of the early, pre-CE mass transfer phase on the mass ratio between the two stars in a binary. We found that for larger mass ratios (𝑞 ≡ 𝑀₂/𝑀₁ ) the pre-CE Roche lobe overflow phase is more stable, and the mass transfer is longer lasting to the point that, should the mass ratio become such that the companion is more massive than the primary, the CE in-spiral is prevented (at least in the timescales accessible by the simulations). In studying this pre-CE phase, we have also found that binaries with large 𝑞 values eject more mass early, but unbind less of it when compared to lower 𝑞 counterparts. In both cases, any material that is likely to fall-back onto the system comes from the material which is ejected later on during the CE, and not beforehand, as much of the earlier ejecta’s behaviour is characteristic of homologous expansion, reaching velocities that render the material unbound by the CE’s conclusion. This also shows us that the pre-CE mass loss has a very small contribution, if any at all, to shaping the final CE ejection. We have also analysed the role of this gas fall-back on the properties of the post-CE objects, resolving that the final tightening of the orbit is unlikely to be possible solely at the hand of envelope return, given the amount of mass expected.