The Origin of Weakened Magnetic Braking in Old Solar Analogs

• Published in: Astrophysical journal. Letters Vol. 933; no. 1; p. L17
• Main Authors: Metcalfe, Travis S., Finley, Adam J., Kochukhov, Oleg, See, Victor, Ayres, Thomas R., Stassun, Keivan G., van Saders, Jennifer L., Clark, Catherine A., Godoy-Rivera, Diego, Ilyin, Ilya V., Pinsonneault, Marc H., Strassmeier, Klaus G., Petit, Pascal
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01-07-2022; IOP Publishing; Bristol : IOP Publishing
• Subjects: Age; Analogs; Angular momentum; Astrophysics; Braking; Differential rotation; F stars; Magnetic fields; Magnetic transitions; Main sequence stars; Morphology; Sciences of the Universe; Solar analogs; Spectropolarimetry; Stellar evolution; Stellar magnetic fields; Stellar rotation; Stellar seismology; Stellar winds; X-ray astronomy; Stellar winds; Stellar magnetic fields; Stellar evolution; Solar analogs; Spectropolarimetry
• ISSN: 2041-8205; 2041-8213; 2041-8213
• DOI: 10.3847/2041-8213/ac794d

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
ρ
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 and 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 and 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.