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

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 waves' 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 waves' 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) WD 1856+534 (TIC 267574918) and for the WD-brown dwarf binary system ZTFJ003855.0+203025.5.

Automated summary

This study investigates the impact of strong convection in low mass red giant branch stars during core helium flash, which excites waves to the envelope, causing envelope expansion and brightening. The energy deposited by these waves results in expansion by tens to hundreds solar radii in a few years, enhancing mass-loss rate and dust formation. The red transient event may lead to stars engulfing exoplanets and potentially allowing planets and brown dwarfs to survive during common envelope evolution.