Atmospheric Signatures of Common Envelope Evolution in White Dwarf Planets

The majority of confirmed exoplanets orbit within 1 au of a main-sequence (MS) star. When their stellar hosts evolve off the MS, many of these planets will be engulfed and destroyed, creating empty "forbidden" zones around the stars as they evolve to their final state as a white dwarf (WD). However, several confirmed and candidate WD planets have been found within this forbidden zone. Two formation scenarios have been proposed to explain the existence of these close-in planets: high-eccentricity migration and common envelope evolution (CEE). There are currently few observational tests to distinguish between these pathways. In this study, we investigate whether CEE could leave a detectable atmospheric signature. Using Modules for Experiments in Stellar Astrophysics (MESA) models, we simulate an engulfed planet inspiraling into an AGB star, and allow the planet to accrete mass via Bondi-Hoyle-Lyttleton accretion. Assuming a range of planet masses (1 - 13 M Jup) and accretion efficiencies (0.01 - 1.0), we find that the planet can accrete up to 48% of its initial mass in the most extreme Eddington-limited scenario. Because this accreted material is enriched in hydrogen and helium, we expect it to decrease the planet's bulk metallicity. Using simulated emission spectra, we find that CEE can increase thermal emission by up to 9.0% for a cool planet such as WD 1856 b. For lower accretion efficiencies (0.01 - 0.5), thermal emission increases between 0.1 - 3.6%. This signature may be observable in the most favorable cases, providing a potential new probe for investigating the dynamical history of close-in planets around WDs.