PEPSI

Antisolar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae

by Silva Järvinen on 2026-10-14

Recent observations have shown that sufficiently slow rotation disrupts the organization of large-scale magnetic field in older main-sequence stars, leading to weakened magnetic braking (WMB) and a collapse in the efficiency of the global stellar dynamo. Recent simulations predict a shift from solar-like to antisolar differential rotation (DR) at slower rotation rates, which typically do not occur on the main sequence due to WMB. However, physical expansion on the subgiant branch can eventually slow the stellar rotation beyond this threshold, yielding a noncycling large-scale field that revives magnetic braking. We combine asteroseismology from the Transiting Exoplanet Survey Satellite (TESS) with spectropolarimetry from the Large Binocular Telescope (LBT) to test these predictions in the old metal-rich subgiant 31 Aql.

Ratio of the observationally estimated and model wind-braking torque as a function of the Rossby number normalized to the solar value. The dotted line and gray shaded region illustrate the fit and 95% confidence interval described by T. S. Metcalfe et al. (2025). The revived magnetic braking in 31 Aql at higher Ro is evident, and quadratic growth is illustrated schematically with a dashed line.