Spinning up the Surface: Evidence for Planetary Engulfment or Unexpected Angular Momentum Transport?

TL;DR

This study reports a giant star with an unusual surface rotation faster than its envelope, challenging existing models and suggesting possible planetary engulfment or complex angular momentum transport mechanisms.

Contribution

It provides observational evidence of a nonmonotonic rotation profile in a giant star, exploring potential causes like planetary ingestion or tidal wave-driven angular momentum transfer.

Findings

Star shows faster surface rotation than envelope

Spectroscopic data supports unusual rotation profile

No strong chemical peculiarities detected

Abstract

In this paper, we report the potential detection of a nonmonotonic radial rotation profile in a low-mass lower-luminosity giant star. For most low- and intermediate-mass stars, the rotation on the main sequence seems to be close to rigid. As these stars evolve into giants, the core contracts and the envelope expands, which should suggest a radial rotation profile with a fast core and a slower envelope and surface. KIC 9267654, however, seems to show a surface rotation rate that is faster than its bulk envelope rotation rate, in conflict with this simple angular momentum conservation argument. We improve the spectroscopic surface constraint, show that the pulsation frequencies are consistent with the previously published core and envelope rotation rates, and demonstrate that the star does not show strong chemical peculiarities. We discuss the evidence against any tidally interacting stellar companion. Finally, we discuss the possible origin of this unusual rotation profile, including the potential ingestion of a giant planet or unusual angular momentum transport by tidal inertial waves triggered by a close substellar companion, and encourage further observational and theoretical efforts.