Higher mass ratios can widen common-envelope outcomes

What the study found: Higher companion-to-giant mass ratios can produce wider post-common-envelope orbits in these simulations, but the widest separations predicted are still smaller than the observed range. The authors also find that, for their parameters, circumbinary discs are more likely to form from fallback of leftover bound envelope material.
Why the authors say this matters: The authors say the systems they are trying to explain have orbital periods and eccentricities that are hard to reconcile with a standard common-envelope inspiral, so they test whether mass ratio can help produce wider outcomes. They also conclude that the resulting fallback discs have characteristics in line with those observed.
What the researchers tested: The researchers carried out 3D hydrodynamical common-envelope binary interaction simulations using the smoothed particle hydrodynamics code PHANTOM. They modeled a 0.88 solar-mass, 90-solar-radius red giant branch star with companions spanning mass ratios q = M2/M1 from 0.68 to 1.5.
What worked and what didn't: Larger q values led to wider post-common-envelope separations, and the pre-common-envelope mass transfer phase lasted longer for more massive companions. Around q ≳ 1, the inspiral became noticeably more stable, as predicted by analytical theory, but this phase was not converged with simulation resolution.
What to keep in mind: The upper limit they predict is only about 50 solar radii, which is still below the observed range. The authors also note that higher resolution would likely make the pre-inspiral phase even more stable and longer, and they present this as the first work in a series.

Key points

• The simulations found that higher mass ratios can widen post-common-envelope separations.
• The widest separation predicted was about 50 solar radii, below the observed range.
• For q ≳ 1, the inspiral became more stable and the pre-inspiral phase lasted longer.
• The authors say fallback of leftover bound envelope material is the more likely source of circumbinary discs in their models.
• The fallback discs were reported to have characteristics in line with observed discs.