The Origin of Weakened Magnetic Braking in Old Solar Analogs

TL;DR

This study investigates the decline in magnetic braking efficiency in aging solar analogs, revealing a magnetic transition that significantly reduces angular momentum loss after about 3 billion years, likely due to changes in stellar dynamo processes.

Contribution

It identifies a magnetic transition in solar analogs aged 2-7 Gyr that causes a drop in angular momentum loss, providing new insights into stellar magnetic evolution and rotational slowdown.

Findings

Angular momentum loss drops by over an order of magnitude between 2.6-3.7 Gyr.

A magnetic transition correlates with reduced differential rotation.

The transition likely disrupts the global dynamo mechanism.

Abstract

The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations suggested a shift in magnetic morphology between two F-type stars with similar rotation rates but very different ages (88 Leo and rho CrB). In this Letter, we identify a comparable transition in an evolutionary sequence of solar analogs with ages between 2-7 Gyr. We present new spectropolarimetry of 18 Sco and 16 Cyg A & B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD 76151 and 18 Sco, providing additional constraints on the nature and timing of this transition. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, demonstrating that the rate of angular momentum loss drops by more than an order of magnitude between the ages of HD 76151 and 18 Sco (2.6-3.7 Gyr) and continues to decrease modestly to the age of 16 Cyg A & B (7 Gyr). We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline plans to probe this phenomenon in additional stars spanning a wide range of spectral types.