Seismic evidence for a rapidly rotating core in a lower-giant-branch star observed with Kepler

TL;DR

This study uses Kepler data to detect mixed oscillation modes in a red giant star, revealing a rapidly rotating core at least five times faster than its envelope, providing new observational constraints on stellar angular momentum transport.

Contribution

It presents the first detailed internal rotation profile of an early red giant, showing a rapidly rotating core, based on seismic data and inversion techniques.

Findings

Core rotates at least five times faster than the envelope

Detected and analyzed 18 rotationally split modes

Provided observational constraints on angular momentum transport theories

Abstract

Rotation is expected to have an important influence on the structure and the evolution of stars. However, the mechanisms of angular momentum transport in stars remain theoretically uncertain and very complex to take into account in stellar models. To achieve a better understanding of these processes, we desperately need observational constraints on the internal rotation of stars, which until very recently were restricted to the Sun. In this paper, we report the detection of mixed modes - i.e. modes that behave both as g modes in the core and as p modes in the envelope - in the spectrum of the early red giant KIC7341231, which was observed during one year with the Kepler spacecraft. By performing an analysis of the oscillation spectrum of the star, we show that its non-radial modes are clearly split by stellar rotation and we are able to determine precisely the rotational splittings of 18 modes. We then find a stellar model that reproduces very well the observed atmospheric and seismic properties of the star. We use this model to perform inversions of the internal rotation profile of the star, which enables us to show that the core of the star is rotating at least five times faster than the envelope. This will shed new light on the processes of transport of angular momentum in stars. In particular, this result can be used to place constraints on the angular momentum coupling between the core and the envelope of early red giants, which could help us discriminate between the theories that have been proposed over the last decades.