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

We suggest that the 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 to increase the number of extreme horizontal branch (EHB; sdB and sdO) stars with masses of $\simeq 0.47 \, \mathrm{M}_\odot$ with respect to canonical binary evolution. Using the open-source mesa-binary, we follow the evolution of a number of eccentric binary systems with an initial primary stellar mass of $1.6 \, \mathrm{M}_\odot$. 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 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 $\simeq 0.47 \, \mathrm{M}_\odot$. However, we expect that in most cases the secondary star does not have time to spiral-in to close orbits. It rather 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.