Rapid expansion of red giant stars during core helium flash by waves propagation to the envelope and implications to exoplanets

TL;DR

This study models how waves generated during the core helium flash in low-mass red giant stars can cause rapid envelope expansion, brightening, and increased mass loss, potentially leading to planet engulfment and observable transient events.

Contribution

It introduces a novel mechanism where core convection-driven waves induce envelope expansion during the helium flash, affecting stellar evolution and planet survival.

Findings

Envelope expands by tens to hundreds of solar radii within years.

Star's brightness increases but may appear fainter in visible light due to dust.

Possible explanation for planet engulfment around white dwarfs.

Abstract

We assume that the strong convection during core helium flash of low mass red giant branch (RBG) stars excite waves that propagate to the envelope, and find that the energy that these waves deposit in the envelope causes envelope expansion and brightening. We base our assumption and the estimate of the wave energy on studies that explored such a process due to the vigorous core convection of massive stars just before they experience a core collapse supernova explosion. Using the stellar evolutionary code MESA we find that the wave energy causes an expansion within few years by tens to hundreds solar radii. Despite the large brightening, we expect the increase in radius and luminosity to substantially enhance mass loss rate and dust formation. The dust shifts the star to become much redder (to the infrared), and the star might actually become fainter in the visible. The overall appearance is of a faint red transient event that lasts for months to few years. We suggest that in some cases envelope expansion might lead stars that are about to leave the RGB to engulf exoplanets. The extended envelope has a smaller binding energy to a degree that allows planets of several Jupiter masses or more and brown dwarfs to survive the common envelope evolution. We suggest this scenario to account for the planet orbiting the white dwarf (WD) WD1856+534 (TIC 267574918) and for the WD - brown dwarf binary system ZTFJ003855.0+203025.5.