Extending Common Envelope Simulations from Roche Lobe Overflow to the Nebular Phase

Abstract

We have simulated a common envelope interaction of a 0.88-M$_{\odot}$, 90-R$_{\odot}$, red giant branch star and a 0.6-M$_{\odot}$, compact companion with the smoothed particle hydrodynamics code, Phantom, from the beginning of the Roche lobe overflow phase to the beginning of the self-regulated inspiral, using three different resolutions. The duration of the Roche lobe overflow phase is resolution dependent and would lengthen with increased resolution beyond the $\sim$20 years observed, while the inspiral phase and the post-common envelope separation are largely independent of resolution. Mass transfer rates through the Lagrangian points drive the orbital evolution during the Roche lobe overflow phase, as predicted analytically. The absolute mass transfer rate is resolution dependent, but always within an order of magnitude of the analytical value. Similarly, the gravitational drag in the simulations is close to the analytical approximation. This gives us confidence that simulations approximate reality. The $L_2$ and $L_3$ outflow observed during Roche lobe overflow remains bound, forming a circumbinary disk that is largely disrupted by the common envelope ejection. However, a longer phase of Roche lobe overflow and weaker common envelope ejection typical of a more stable binary may result in a surviving circumbinary disk. Finally, we examine the density distribution resulting from the interaction for simulations that include or omit the phase of Roche lobe overflow. We conclude that the degree of stability of the Roche lobe phase may modulate the shape of the subsequent planetary nebula, explaining the wide range of post-common envelope planetary nebula shapes observed.