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

Abstract

ABSTRACT 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.