Metcalfe, Travis S.; Van Saders, Jennifer L.; Ayres, Thomas R.; Buzasi, Derek.; Drake, Jeremy J.; Egeland, Ricky.; Garc铆a, Rafael A.; Kochukhov, Oleg.; Saar, Steven H.; Stassun, Keivan G.; Basu, Sarbani.; Ong, J. M. Joel.; Stokholm, Amalie.; Bedding, Timothy R.; Breton, Sylvain N.; Ilyin, Ilya V.; Petit, Pascal.; Pinsonneault, Marc H.; Strassmeier, Klaus G. (2026).听.听Astronomical Journal, 171(5), 287.听
This study looks at how aging stars generate magnetic fields and how that affects the way they lose rotation over time. Previous observations suggested that when stars rotate very slowly, their large-scale magnetic fields can become disorganized, which weakens magnetic braking, the process by which a star slows down as it loses angular momentum through its magnetic wind. Computer simulations also predict that at slow enough rotation rates, a star鈥檚 surface can switch from 鈥渟olar-like鈥� differential rotation, where the equator spins faster than the poles, to 鈥渁ntisolar鈥� differential rotation, where the poles spin faster than the equator. These conditions usually are not reached on the main sequence, the long stable part of a star鈥檚 life, because magnetic braking prevents stars from slowing that much. But when a star evolves into a subgiant and expands, its rotation can slow further and possibly cross that threshold. To test these ideas, the researchers combined asteroseismology, which uses tiny oscillations inside a star to probe its structure, from NASA鈥檚 TESS mission with spectropolarimetry, a technique that measures magnetic fields using polarized light, from the Large Binocular Telescope, focusing on the old metal-rich subgiant 31 Aql. They found that the star has a strong large-scale magnetic field and that 50 years of chromospheric emission data show it does not cycle the way the Sun does, matching the predicted behavior. The star also shows different rotation periods at different times, consistent with differential rotation, although the data do not reveal whether the pattern is solar-like or antisolar. By combining the TESS data with rotational modeling, the team estimated the current wind-braking torque and found evidence that magnetic braking has become active again in this evolved star. They also used the results to place an initial estimate on the Rossby number, a quantity that relates rotation to convection, for the transition to antisolar differential rotation.
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Figure 1.听Top: power spectral density (PSD) in the frequency range of the fitted modes. In gray is the raw spectrum and in black a smoothed PSD. The blue line represents the fitted spectrum. The vertical red, yellow, and magenta bars indicate the central frequencies of the听鈩�听=听0, 1, and 2 modes, respectively. Bottom: 茅chelle diagram with 螖谓听=听88.40渭Hz. The fitted modes and their associated uncertainties are shown as circles, diamonds, and triangles with the same color-coding as in the top panel.