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Most previous efforts to calibrate how rotation and magnetic activity dependon stellar age and mass have relied on observations of clusters, whereisochrones from stellar evolution models are used to determine the propertiesof the ensemble. Asteroseismology employs similar models to measure theproperties of an individual star by matching its normal modes of oscillation,yielding the stellar age and mass with high precision. We use 27 days ofphotometry from the Transiting Exoplanet Survey Satellite to characterizesolar-like oscillations in the G8 subgiant of the 94 Aqr triple system. Theresulting stellar properties, when combined with a reanalysis of 35 yr ofactivity measurements from the Mount Wilson HK project, allow us to probe theevolution of rotation and magnetic activity in the system. The asteroseismicage of the subgiant agrees with a stellar isochrone fit, but the rotationperiod is much shorter than expected from standard models of angular momentumevolution. We conclude that weakened magnetic braking may be needed toreproduce the stellar properties, and that evolved subgiants in the hydrogenshell-burning phase can reinvigorate large-scale dynamo action and brieflysustain magnetic activity cycles before ascending the red giant branch.

The Evolution of Rotation and Magnetic Activity in 94 Aqr Aa from Asteroseismology with TESS