LBT/PEPSI Spectropolarimetry of a Magnetic Morphology Shift in Old Solar-type Stars

Abstract

Solar-type stars are born with relatively rapid rotation and strong magnetic fields. Through a process known as magnetic braking, the rotation slows over time as stellar winds gradually remove angular momentum from the system. The rate of angular momentum loss depends sensitively on the magnetic morphology, with the dipole field exerting the largest torque on the star. Recent observations suggest that the efficiency of magnetic braking may decrease dramatically in stars near the middle of their main-sequence lifetimes. One hypothesis to explain this reduction in efficiency is a shift in magnetic morphology from predominantly larger to smaller spatial scales. We aim to test this hypothesis with spectropolarimetric measurements of two stars that sample chromospheric activity levels on opposite sides of the proposed magnetic transition. As predicted, the more active star (HD 100180) exhibits a significant circular polarization signature due to a non-axisymmetric large-scale magnetic field, while the less active star (HD 143761) shows no significant signal. We identify analogs of the two stars among a sample of well-characterized Kepler targets, and we predict that the asteroseismic age of HD 143761 from future TESS observations will substantially exceed the age expected from gyrochronology. We conclude that a shift in magnetic morphology likely contributes to the loss of magnetic braking in middle-aged stars, which appears to coincide with the shutdown of their global dynamos.

7 pages, 3 figures, 2 tables, ApJ Letters (accepted)

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• Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and $\tau$ Cet
• Weakened Magnetic Braking in the Exoplanet Host Star 51 Peg
• Asteroseismology and Spectropolarimetry of the Exoplanet Host Star $\lambda$ Serpentis
• Hunting for anti-solar differentially rotating stars using the Rossby number -- An application to the Kepler field