Stars spin down over time. Magnetic braking — the coupling of stellar winds to magnetic field lines — extracts angular momentum and slows the rotation. This is well-established for main sequence stars like the Sun. But evolved stars, subgiants that have exhausted their core hydrogen, sometimes rotate faster than expected. The magnetic brake appears to have weakened or failed.
31 Aquilae, a subgiant, shows the opposite anomaly: it rotates slower than evolved stars typically do at its stage, as if the brake was re-engaged. The proposed mechanism is anti-solar differential rotation — the equator rotating slower than the poles, the reverse of the Sun's pattern.
On the Sun, the equator spins faster than the poles. This differential rotation pattern is thought to be efficient at generating magnetic fields through the solar dynamo. As stars evolve off the main sequence, their convective zones deepen, and the differential rotation pattern can change. If it weakens or becomes uniform, the dynamo weakens, and magnetic braking stalls — explaining the anomalously fast-rotating subgiants.
But if the differential rotation doesn't just weaken but reverses — poles spinning faster than the equator — it might re-energize the dynamo through a different pathway. Anti-solar differential rotation could sustain or even strengthen magnetic field generation, re-engaging the magnetic brake that was supposed to have failed. The star slows down again, not because the original brake persisted, but because a new brake — driven by the reversed rotation pattern — took over.