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., Garc铆a, Rafael A., Huber, Daniel, Lu, Yuxi (Lucy), & See, Victor. (2025). *Astrophysical Journal, 986*(2), 120.
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鈥攕treams of charged particles flowing from the star. This magnetic activity is driven by a process called a聽stellar dynamo, which depends on the聽Coriolis force聽(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鈥攁nd also to lose angular momentum effectively through聽magnetic braking聽(the process by which stellar winds carry away a star鈥檚 spin).
In this study, scientists looked at聽six early K-type stars聽(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聽spectropolarimetry), their聽mass-loss rates聽(from either Ly伪 or X-ray measurements), and estimates of their聽rotation periods, masses, and sizes.
They found a sharp drop鈥攎ore than tenfold鈥攊n the braking strength of the stellar winds when the聽Rossby number聽(a value that relates a star鈥檚 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聽magnetic activity cycles, the findings suggest that this drop in magnetic braking may happen when the star’s dynamo becomes聽subcritical, meaning it鈥檚 still active but operating in a weaker, less organized mode.
Figure 1.听厂迟辞办别蝉听V聽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.
