Common envelope simulations between a thermally pulsating AGB star and a low mass companion

At least one in five of all planetary nebulae are the product of a common envelope (CE) interaction, where the companion in-spirals into the envelope of an asymptotic giant branch (AGB) star ejecting the nebula and leaving behind a compact binary. In this work we carry out 3D smoothed particle hydrodynamics simulations of the CE interaction between a 1.7 solar mass thermally pulsating AGB star and a 0.6 solar mass companion. The interaction takes place when the giant is on the expanding phase of the seventh thermal pulse and has a radius of 250 R . We compare the outcome of three simulations using different equations of state: an ideal gas, ideal gas plus radiation, and a tabulated equation of state that includes the delivery of recombination energy. The final separations are in the range 20-31 R, depending on the equation of state adopted, with the tabulated equation of state resulting in the wider separation. We determine that the timescale between Roche lobe overflow and the end of the in-spiral is shorter than the natural duration of the thermal pulse. If so this implies that thermal pulses can trigger CEs thus extending the ability of AGB stars to capture companions into CEs, leading to the prediction of a larger population of post-CE binaries. This conclusion is based on short in-spiral times (~3-5 years) combined with the fact that the natural expansion of the stellar radius during the pulse speeds up the Roche lobe overflow phase such that CE interactions can take place during the pulse. Simulations that include a tabulated equation of state unbind a great deal more gas, and, although we do not observe the entire envelope being unbound in our simulations, we predict that the entire envelope would be unbound in due course. The shape of the expanded CE is less equatorially concentrated for the simulation with a tabulated equation of state and in the innermost region does not present the evacuated funnels seen in other simulations. Finally, we make some considerations on the value of the CE efficiency implied by these simulations.