Observable Signatures of Planet Accretion in Red Giant Stars I: Rapid Rotation and Light Element Replenishment

TL;DR

This study investigates how planet accretion affects red giant stars by examining rapid rotation and light element abundances, finding evidence of planet ingestion through lithium enrichment in rapid rotators.

Contribution

It provides observational evidence linking rapid rotation in red giants to planet accretion, highlighting lithium enrichment as a signature of this process.

Findings

Rapid rotators show higher lithium levels than slow rotators.

Both groups have similar 12C/13C ratios, indicating limited mixing effects.

Li enrichment suggests accretion of a few Jupiter-mass planets.

Abstract

The orbital angular momentum of a close-orbiting giant planet can be sufficiently large that, if transferred to the envelope of the host star during the red giant branch (RGB) evolution, it can spin-up the star's rotation to unusually large speeds. This spin-up mechanism is one possible explanation for the rapid rotators detected among the population of generally slow-rotating red giant stars. These rapid rotators thus comprise a unique stellar sample suitable for searching for signatures of planet accretion in the form of unusual stellar abundances due to the dissemination of the accreted planet in the stellar envelope. In this study, we look for signatures of replenishment in the Li abundances and (to a lesser extent) 12C/13C, which are both normally lowered during RGB evolution. Accurate abundances were measured from high signal-to-noise echelle spectra for samples of both slow and rapid rotator red giant stars. We find that the rapid rotators are on average enriched in lithium compared to the slow rotators, but both groups of stars have identical distributions of 12C/13C within our measurement precision. Both of these abundance results are consistent with the accretion of planets of only a few Jupiter masses. We also explore alternative scenarios for understanding the most Li-rich stars in our sample---particularly Li regeneration during various stages of stellar evolution. Finally, we find that our stellar samples show non-standard abundances even at early RGB stages, suggesting that initial protostellar Li abundances and 12C/13C may be more variable than originally thought.