Stellar Evidence that the Solar Dynamo may be in Transition

TL;DR

This paper suggests that the Sun may be in a transitional phase of its magnetic and rotational evolution, with a significant change in magnetic braking efficiency occurring in middle-aged stars, based on Kepler and chromospheric activity data.

Contribution

It provides new insights into stellar magnetic evolution by linking chromospheric activity, differential rotation, and magnetic braking in solar-type stars, proposing the Sun's unique evolutionary status.

Findings

Magnetic braking efficiency drops beyond a critical Rossby number.

A fundamental shift in differential rotation occurs in middle-aged stars.

The Sun may be in a transitional evolutionary phase.

Abstract

Precise photometry from the Kepler space telescope allows not only the measurement of rotation in solar-type field stars, but also the determination of reliable masses and ages from asteroseismology. These critical data have recently provided the first opportunity to calibrate rotation-age relations for stars older than the Sun. The evolutionary picture that emerges is surprising: beyond middle-age the efficiency of magnetic braking is dramatically reduced, implying a fundamental change in angular momentum loss beyond a critical Rossby number (Ro~2). We compile published chromospheric activity measurements for the sample of Kepler asteroseismic targets that were used to establish the new rotation-age relations. We use these data along with a sample of well characterized solar analogs from the Mount Wilson HK survey to develop a qualitative scenario connecting the evolution of chromospheric activity to a fundamental shift in the character of differential rotation. We conclude that the Sun may be in a transitional evolutionary phase, and that its magnetic cycle might represent a special case of stellar dynamo theory.