Stellar Rotation and Planet Ingestion in Giants

We investigate the expected increase in the rotation rate of post-main-sequence stars as they expand and ingest orbiting planets. This phenomenon is expected to occur when the stellar radius becomes larger than the planet’s periastron distance. We calculate the expected frequency of planet ingestion during the red giant, horizontal branch (HB), and early asymptotic giant branch phases for planets of mass mp 1MJ. We also calculate the probability of observing anomalous rotation rates in a population of solar metallicity giants as a function of stellar mass and evolutionary stage. Planet ingestion is most easily detectable in a solar mass HB star, with a probability of about 1% for solar-neighborhood metallicity. Our analysis is based on the observed distribution of mass, eccentricity, semimajor axis for extrasolar planets around solar-type main-sequence stars, on stellar evolution models, and on the typical observed rotation rates observed in a sample of solar-neighborhood giants.