A New Spin on Red Giant Rapid Rotators: Evidence for Chemical Exchange Between Planets and Evolved Stars

Abstract

Rapid rotation in red giant stars may be one signature of the past engulfment of a planetary companion. Models of the future tidal interaction of known exoplanet host stars with their planets show that many of these stars will accrete one or more of their planets, and the orbital angular momentum of these accreted planets is sometimes sufficient to spin up the host stars to a level commonly accepted as “rapid rotation” for giant stars. Planets accreted during the red giant phase should leave behind a chemical signature in the form of unusual abundance patterns in the host red giant's atmosphere. Proposed signatures of planet accretion include the enhancement of Li and ¹²C; both species are generally depleted in giant star atmospheres by convection but could be replenished by planet accretion. Moreover, accreted planets may preferentially enhance the stellar abundance of refractory elements assuming that the refractory nature of these elements leads to their relative enhancements in the planets themselves. Here we present preliminary results of a search for these predicted chemical signatures through high resolution spectroscopic abundance analysis of both rapidly rotating giant stars (i.e., stars with a higher probability of having experienced planet accretion) and normally rotating giant stars. We find that the rapid rotators are enhanced in Li relative to the slow rotators — a result consistent with Li replenishment through planet absorption.