Onset of common envelope evolution during a core helium flash by rapid envelope expansion

Stanislav Fainer, Ealeal Bear, Noam Soker

Published 2022-04-26Version 1

Applying the suggestion that vigorous core convection during core helium flash on the tip of the red giant branch (RGB) of low mass stars excites waves that carry energy to the envelope and inflate it for few years, we find that in binary systems this process leads to a sharp peak of extreme horizontal branch (EHB; sdB and sdO) stars with masses of ~0.47Mo and ease the formation of elliptical planetary nebulae (PNe) by such low mass stars. Using the open-source mesa-binary we follow the evolution of a number of eccentric binary systems with an initial primary stellar mass of 1.6Mo. The energy that the waves carry to the envelope leads to envelope expansion at the tip of the RGB. The inflated RGB star engulfs many secondary stars to start a common envelope evolution (CEE) that otherwise would not occur. If the secondary star manages to remove most of the RGB envelope the primary evolves to become an EHB star with a mass of ~0.47Mo. In cases where the secondary star does not have time to spiral-in to close orbits, it ends at a large orbit and leaves a massive enough envelope for the primary star to later evolve along the asymptotic giant branch and to engulf the secondary star, therefore forming a non-spherical planetary nebula.