# Giant planet engulfment by evolved giant stars: light curves, asteroseismology, and survivability

**Authors:** Christopher E. O'Connor, Lars Bildsten, Matteo Cantiello, Dong Lai  
**arXiv:** [2304.09882](https://arxiv.org/abs/2304.09882)  
**Date:** 2023-04-19  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.09882v1/

## Key points

- About 10% of Sun-like stars will engulf a 1-10 Mj planet during the red giant branch (RGB) or asymptotic giant branch (AGB) phase of their evolution.
- Engulfed planets spiral inward, depositing energy and angular momentum over a period of approximately 10-100 years.
- The star responds quasistatically but still brightens measurably on a timescale of years for RGB stars as well as AGB stars with Mp ≤ 5 Mj planets.
- Asteroseismic indicators such as frequency spacing or rotational splitting differ before and after engulfment.
- Engulfment events on AGB could be a significant fraction of low-luminosity red novae in the Galaxy based on peak luminosity, color, duration, and expected rate of these events.
- There are no conditions where the envelope is ejected prior to the planet's tidal disruption, complicating the interpretation of short-period giant planets orbiting white dwarfs as survivors of common-envelope evolution.

## Summary

In their paper "Giant planet engulfment by evolved giant stars: light curves, asteroseismology, and survivability," Christopher E. O'Connor, Lars Bildsten, Matteo Cantiello, and Dong Lai explore the phenomenon of giant planet engulfment by evolved giant stars. They find that about ten percent of Sun-like ($1$-$2 M_\odot$) stars will engulf a $1$-$10 M_{\rm J}$ planet as they expand during the red giant branch (RGB) or asymptotic giant branch (AGB) phase of their evolution. Once engulfed, these planets experience a strong drag force in the star's convective envelope and spiral inward, depositing energy and angular momentum. The inspiral takes $\sim 10$-$10^{2}$ years ($\sim 10^{2}$-$10^{3}$ orbits), with the planet undergoing tidal disruption at a radius of $\sim R_\odot$. Using the Modules for Experiments in Stellar Astrophysics (MESA) software instrument to track the stellar response to the energy deposition while simultaneously evolving the planetary orbit, they find that for RGB stars as well as AGB stars with $M_{\rm p} \lesssim 5 M_{\rm J}$ planets, the star responds quasistatically but still brightens measurably on a timescale of years. In addition, asteroseismic indicators such as frequency spacing or rotational splitting differ before and after engulfment. Based on peak luminosity, color, duration and expected rate of these events they suggest that engulfment events on AGB could be a significant fraction of low-luminosity red novae in the Galaxy. They do not find conditions where the envelope is ejected prior to the planet's tidal disruption, complicating the interpretation of short-period giant planets orbiting white dwarfs as survivors of common-envelope evolution. Overall, this study sheds light on an intriguing phenomenon that occurs during stellar evolution and provides insights into how it affects both the star and engulfed planet.