Insights into the Rotational Evolution of Old Sun-like Stars Beyond Standard Gyrochronology

Observations of open clusters reveal well-defined rotational sequences of coeval stars as a function of stellar mass; that is, the rotation period is largely a deterministic function of stellar age and mass. This empirical relation forms the basis of gyrochronology and is generally interpreted as a consequence of magnetic braking with a strong dependence on spin period, which erases the dispersion in initial rotation periods. However, Kepler asteroseismology has shown that standard gyrochronology relations calibrated on young open clusters fail when extrapolated to older field stars, which rotate more rapidly than predicted. This discrepancy may reflect a fundamental change in magnetic-field generation and morphology in slowly rotating stars, often referred to as weakened magnetic braking (WMB). Motivated by this discrepancy, we have been investigating the rotation of old Sun-like stars from multiple perspectives, using both space-based photometry and high-resolution spectroscopy. Our main findings so far are as follows. (i) Kepler photometry provides only limited constraints on old Sun-like stars through rotational modulation because of strong detection biases, and any interpretation requires explicit modeling of these biases. (ii) Projected rotational velocities (vsini) measured from high-resolution spectra support WMB independently of asteroseismology. (iii) vsin⁡i measurements of field visual binaries suggest that a deterministic relationship between rotation and age may still persist even among old Sun-like stars.