Anti-Solar Differential Rotation May Have Revived Magnetic Braking in the Subgiant 31 Aquilae

TL;DR

This study combines asteroseismology and spectropolarimetry to investigate magnetic braking in the old subgiant 31 Aql, providing evidence that anti-solar differential rotation may revive magnetic braking after stellar expansion.

Contribution

It presents the first combined observational and modeling evidence supporting revived magnetic braking due to anti-solar differential rotation in a subgiant star.

Findings

Strong large-scale magnetic field detected in 31 Aql.

Archival data confirms the star is non-cycling over 50 years.

Rotation periods vary, indicating differential rotation.

Abstract

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 anti-solar 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 non-cycling 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. The LBT observations reveal a strong large-scale magnetic field in this star, and archival measurements of its chromospheric emission over 50 years confirm that it is non-cycling, as predicted. The star exhibits a variety of rotation periods during different observing seasons, consistent with DR but with no means of distinguishing between solar-like and anti-solar patterns. We incorporate the TESS observations to estimate the current wind braking torque of 31 Aql, demonstrating that it supports revived magnetic braking in this old subgiant. We also use rotational evolution modeling to place a preliminary constraint on the stellar Rossby number for the transition to anti-solar DR. Future refinements in both asteroseismic observations and rotational modeling may yield improvements to this initial analysis.