Continuous mass ablation of planets engulfed in stellar envelopes

Original Abstract

Most stars host short-period planets that are expected to be engulfed during post-main-sequence expansion. The dissolution of engulfed planets has been proposed as a possible mechanism for producing stars enriched in lithium and refractory elements. We perform three-dimensional hydrodynamical simulations of a Jupiter-like planet engulfed within a stellar envelope using the Seven-League Hydro code. Unlike previous studies that represent the planet as a point mass or rigid sphere, we adopt a wind-tunnel setup that resolves the planet's gaseous structure. We find that a continuous mass-ablation process operates during planetary engulfment, contrary to the common assumption that destruction occurs at a specific depth due to ram pressure, tidal forces, or thermal evaporation. The ablation rate scales nearly linearly with the wind momentum flux and is largely insensitive to the Mach number, consistent with an analytical model based on Kelvin-Helmholtz instability developing at the planetary surface. We define efficiency coefficients for drag and ablation, finding pressure-drag coefficients of 0.44-0.56 and smaller ablation efficiencies of 0.054-0.11. Applying these coefficients to a numerically integrated inspiral through a stellar profile, we find that continuous ablation could lead to complete dissolution of the planet within the convective envelope, producing observable lithium enrichment at the stellar surface. Our results provide prescriptions for drag and mass loss that enable large parameter-space studies of planetary engulfment and suggest that chemical enrichment may occur over a broader range of stellar parameters than previously thought.