Giant Planet Engulfment by Evolved Giant Stars: Light Curves, Asteroseismology, and Survivability

About ten percent of Sun-like (1-2 M (circle dot)) stars will engulf a 1-10 M (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 similar to 10-10(2) yr (similar to 10(2)-10(3) orbits); the planet undergoes tidal disruption at a radius of similar to 1 R (circle dot). 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 (p) less than or similar to 5 M (J) planets, the star responds quasi-statically 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 an M (p) greater than or similar to 5 M (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.