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This study uses 3D hydrodynamical simulations to investigate common envelope interactions between red giant stars and compact companions, focusing on systems with high mass ratios. The researchers found that when the companion star is similar in mass or more massive than the giant star, the orbital inspiral becomes significantly more stable, resulting in wider final separations (up to 40 solar radii) and the formation of circumbinary discs from fallback material. While these wider separations still cannot fully explain the extremely wide post-interaction binaries observed (100-800 solar radii), the simulated circumbinary disc properties match observational data.
Why it matters
These findings challenge the standard picture of common envelope evolution, which is crucial for understanding the formation of close binary systems, including mergers that produce gravitational wave sources and various exotic stellar remnants. The discovery that high mass ratio systems can avoid complete merger while forming circumbinary discs helps explain puzzling observations of wide post-giant binaries and provides new constraints for binary evolution models.
arXiv:2512.16225v3 Announce Type: replace
Abstract: The common envelope (CE) interaction between an expanding giant star and a compact companion typically leads to a rapid orbital decay, ending in either a merger or the formation of a close binary. However, the existence of post-red giant and post-asymptotic giant branch binaries with separations of 100 to 800 Rsun challenges this standard picture, as these systems appear to have experienced strong interactions without undergoing a classic CE inspiral. In this work, we investigate the effect of high mass ratio, q = M2/M1, on the CE inspiral using three-dimensional hydrodynamical simulations performed with the smoothed particle hydrodynamics code PHANTOM. The primary is a 0.88 Msun, 90 Rsun red giant branch star, while the companion masses span q = 0.68 to 1.5. Higher mass ratios lead to wider post-CE separations, with a maximum of approximately 40 Rsun. The pre-CE mass transfer phase is longer for larger companion masses, and for q greater than or equal to 1 the inspiral becomes significantly more stable, broadly consistent with analytical expectations. This phase is not fully converged with respect to numerical resolution, and higher resolution simulations are expected to further increase its duration and stability. Although higher q systems show enhanced mass loss through the L2 and L3 Lagrange points, we find that circumbinary discs are more likely to form from fallback of bound envelope material. Fallback times are short, of order a few hundred years, and fallback radii lie well outside the binary, between 0.5 and 5 au, where discs are expected to spread efficiently through viscous torques. While high mass ratio systems produce wider post-interaction separations, these remain smaller than those observed. In contrast, fallback-formed discs have properties consistent with observed circumbinary discs.
Source: Weakened Inspirals I: High Mass Ratio Common Envelope Interactions in RGB Stars