Metcalfe, Travis S., Petit, Pascal, van Saders, Jennifer L., Ayres, Thomas R., Buzasi, Derek, Kochukhov, Oleg, Stassun, Keivan G., Pinsonneault, Marc H., Ilyin, Ilya V., Strassmeier, Klaus G., Finley, Adam J., García, Rafael A., Huber, Daniel, Lu, Yuxi (Lucy), & See, Victor. (2025). Testing the Rossby paradigm: Weakened magnetic braking in early K-type stars. *Astrophysical Journal, 986*(2), 120. https://doi.org/10.3847/1538-4357/add40a
The way a star spins down over time is closely linked to how it generates and organizes its magnetic field and how that field interacts with stellar winds—streams of charged particles flowing from the star. This magnetic activity is driven by a process called a stellar dynamo, which depends on the Coriolis force (caused by the star’s rotation). The Coriolis force helps organize magnetic regions and prevents them from canceling each other out entirely. Because of this, it’s expected that a star needs to spin fast enough to support this large-scale magnetic activity—and also to lose angular momentum effectively through magnetic braking (the process by which stellar winds carry away a star’s spin).
In this study, scientists looked at six early K-type stars (cool, orange stars slightly smaller than the Sun) at different stages in their evolution. To understand how fast these stars are losing angular momentum, the researchers used data on their large-scale magnetic fields (from spectropolarimetry), their mass-loss rates (from either Lyα or X-ray measurements), and estimates of their rotation periods, masses, and sizes.
They found a sharp drop—more than tenfold—in the braking strength of the stellar winds when the Rossby number (a value that relates a star’s rotation to its internal convection) reached a certain threshold. This means that beyond a certain point, stars stop slowing down as efficiently. Since all the stars studied still show magnetic activity cycles, the findings suggest that this drop in magnetic braking may happen when the star’s dynamo becomes subcritical, meaning it’s still active but operating in a weaker, less organized mode.
Figure 1. Stokes V polarization profiles for HD 103095 (top) and HD 166620 (bottom) from LBT observations on 2023 December 6 and 2024 July 5, respectively. The mean LSD profile is shown as a black line with uncertainties indicated by the gray shaded area. The dashed blue line is an axisymmetric model profile assuming dipole morphology with a fixed inclination.
