Anomalously fast core and envelope rotation in red giants

TL;DR

This study analyzes Kepler data to reveal unexpected rotation behaviors in red giants, including rapid core spins and faster envelope rotation, challenging existing stellar evolution theories.

Contribution

It uncovers new anomalies in red giant rotation rates and suggests binary interactions as a potential explanation, advancing understanding of stellar angular momentum transport.

Findings

Discovery of red giants with faster rotating envelopes than cores

Identification of rapidly rotating cores in some clump stars

Systematic changes in envelope-to-core rotation ratios

Abstract

Red giants undergo dramatic and complex structural transformations as they evolve. Angular momentum is transported between the core and envelope during this epoch, a poorly understood process. Here, we infer envelope and core rotation rates from Kepler observations of $\sim$1517 red giants. While many measurements are consistent with the existing studies, our investigation reveals systematic changes in the envelope-to-core rotation ratio and we report the discovery of anomalies such as clump stars with rapidly rotating cores, and red giants with envelopes rotating faster than their cores. We propose binary interactions as a possible mechanism by which some of these cores and envelopes are spun up. These results pose challenges to current theoretical expectations and can have major implications for compact remnants born from stellar cores.