Description
Post-red giant branch (post-RGB) and post-asymptotic giant branch (post-AGB) binaries consist of a primary star that has recently evolved off either the RGB or AGB, having lost the majority of its envelope, and a main-sequence companion. These systems are characterised by the presence of a stable circumbinary disc. Observed Galactic post-AGB and post-RGB binaries exhibit eccentricities between $0-0.63$. This contradicts canonical binary evolution, which predicts that such post-mass-transfer systems should have circularised.
One proposed mechanism for pumping the eccentricity of these binaries is interaction with their circumbinary discs via gravitational and accretion torques. Although previous studies showed that this process is too weak to reproduce the observed eccentricities of post-AGB and post-RGB binaries, this study employs a new disc-binary interaction formalism, based on detailed hydrodynamic simulations, to investigate this hypothesis.
Our disc-binary interaction model can reproduce the observed eccentricity distribution of post-RGB and post-AGB binaries. However, the assumptions required for this model challenge our current understanding of such binaries.
Firstly, initial eccentricities as large as $0.05-0.1$ are required, which means that another eccentricity-pumping mechanism is needed before the onset of disc-binary interaction.
Secondly, the amount of mass that needs to be accreted onto the binary is between $0.1-1$ $M_\odot$, which is greater than the observed disc masses of post-RGB and post-AGB binaries ($10^{-3}-10^{-2}$ $M_\odot$). This would suggest the existence of a progenitor population of post-mass-transfer systems surrounded by substantial reservoirs of material.
Thirdly, the accretion efficiency of disc-binary interaction onto the binary components themselves must be negligible. Otherwise, the primary star would enter another giant phase, as the expected accretion rates and amounts of accreted mass are sufficient for it to regain an extended envelope. This would cause the primary star to begin refilling its Roche lobe, resulting in another phase of mass transfer.
| Talk category | NOVA Network 3 |
|---|---|
| Second preference | Plenary |
