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

Abstract

About ten percent of Sun-like ($1$-$2M_{\odot }$) stars will engulf a $1$-$10M_{\mathrm{J}}$ 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. For these mass ratios, the inspiral takes $\sim 10$-$10^2$ years ($\sim 10^2$-$10^3$ orbits); the planet undergoes tidal disruption at a radius of $\sim R_{\odot }$. We use 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. For RGB stars, as well as AGB stars with $M_{\mathrm{p}}\lesssim 5M_{\mathrm{J}}$ planets, the star responds quasistatically but still brightens measurably on a timescale of years. In addition, asteroseismic indicators, such as the frequency spacing or rotational splitting, differ before and after engulfment. For AGB stars, engulfment of a $M_{\mathrm{p}}\gtrsim 5M_{\mathrm{J}}$ planet drives supersonic expansion of the envelope, causing a bright, red, dusty eruption similar to a "luminous red nova." Based on the peak luminosity, color, duration, and expected rate of these events, we suggest that engulfment events on the AGB could be a significant fraction of low-luminosity red novae in the Galaxy. We 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.

24 pages, 11 figures, 1 table. Accepted to ApJ

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• Aerosols and hydrocarbons in the atmosphere of a white dwarf planet
• Engulfment of a hot Jupiter as a possible origin of the rapid spin and internal spin misalignment of the planet-hosting red giant Kepler-56
• Second-generation planet formation after tidal disruption from common envelope evolution
• Mass Ejection Driven by Sudden Energy Deposition in Stellar Envelopes
• The Tale of a Hungry Subgiant and Its Brown Dwarf: Interior Radiative Damping Dominates the Tidal Evolution of TOI-5882
• VVV-WIT-13: an eruptive young star with cool molecular features
• Atmospheric Signatures of Common Envelope Evolution in White Dwarf Planets
• Evolution of the ZTF SLRN-2020 star-planet merger