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

TL;DR

This paper proposes that core helium flash-induced wave energy can cause envelope expansion in low-mass red giants, triggering common envelope evolution and leading to the formation of extreme horizontal branch stars and complex planetary nebulae.

Contribution

It introduces a novel mechanism where core convection during helium flash triggers envelope expansion, influencing binary evolution and stellar remnants.

Findings

Wave energy from core convection can inflate the stellar envelope.

Inflated envelopes lead to common envelope events in low-mass binaries.

Resulting stellar evolution produces extreme horizontal branch stars and non-spherical planetary nebulae.

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 ~0.47Mo 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 1.6Mo. 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 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. 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.