The relationship between stellar rotation and age, foundational to the method of gyrochronology, has been well-established primarily for young stars in open clusters where ages are accurately determined via Hertzsprung-Russell diagrams. However, for older field stars, the evolution of stellar rotation remains poorly constrained. Traditional methods relying on time-series photometry from missions like Kepler and TESS struggle to detect rotation periods in older stars due to their slow rotation and diminished starspot activity. Consequently, it remains unclear whether a unique rotation-age relationship persists into late evolutionary stages. In this study, we investigate the uniqueness and dispersion of rotation as a function of mass and age by focusing on visual twin binaries identified from Gaia data. Twin binaries, characterized by nearly identical colors and magnitudes, are assumed to possess near-identical masses and ages. By comparing the rotation rates within these pairs, we can isolate the intrinsic stochasticity of stellar braking. Using the high-dispersion spectrograph GAOES-RV on the Seimei Telescope, we obtained high-resolution spectra for 20 pairs of FG-type visual twin binaries. We measured the projected rotational velocity, v i, from the broadening of absorption lines. By assuming an isotropic distribution of stellar rotation axes, we estimated the variance of the rotation velocities between the binary components. Our analysis reveals a rotational velocity dispersion of approximately 2515%. This result suggests that a deterministic relationship between rotation and age exists even for older field stars, albeit with measurable intrinsic scatter. These findings provide critical empirical constraints for extending gyrochronology to evolved stellar populations and refining models of angular momentum evolution.
Ogawa et al. (Mon,) studied this question.
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