Disruption of a planet spiralling into its host star

Abstract

The processes leading deformation and destruction of planets spiraling into the convective envelope of their host stars are described. The planet is compressed by the ram pressure, and deformed into a flattened shape for which a quantitative model is developed. Compression increases the planet's density contrast with the envelope and its gravitational binding energy. This increases the survivability especially of gas planets. An estimate is given for the depth of disruption by ram pressure, and for the subsequent fragmentation of the remnants. We show how the debris of rocky or iron planets, instead of mixing through the convection zone, sinks below the base of the convection zone. The time scale of the entire sequence of events is of the order of a few orbital times of the planet. If spiral-in of (partly) icy, rocky or iron planets has happened to the pre-main sequence Sun, it could account for the higher opacity below the base of the convection zone as inferred from helioseismology.