A New Asteroseismic Kepler Benchmark Constrains the Onset of Weakened Magnetic Braking in Mature Sun-like Stars

Stellar spin down is a critical yet poorly understood component of stellar evolution. In particular, results from the Kepler Mission imply that mature age, solar-type stars have inefficient magnetic braking, resulting in a stalled spin-down rate. However, a large number of precise asteroseismic ages are needed for mature (>= 3 Gyr) stars in order to probe the regime where traditional and stalled spin-down models differ. In this paper, we present a new asteroseismic benchmark star for gyrochronology discovered using reprocessed Kepler short cadence data. KIC 11029516 (Papayu) is a bright (Kp = 9.6 mag) solar-type star with a well-measured rotation period (21.1 +/- 0.8 days) from spot modulation using 4 yr of Kepler long-cadence data. We combine asteroseismology and spectroscopy to obtain T eff = 5888 +/- 100 K, [Fe/H] = 0.30 +/- 0.06 dex, M = 1.24 +/- 0.05 M circle dot, R = 1.34 +/- 0.02 R circle dot, and age of 4.0 +/- 0.4 Gyr, making Papayu one of the most similar stars to the Sun in terms of temperature and radius with an asteroseismic age and a rotation period measured from spot modulation. We find that Papayu sits at the transition of where traditional and weakened spin-down models diverge. A comparison with stars of similar zero-age main-sequence temperatures supports previous findings that weakened spin-down models are required to explain the ages and rotation periods of old solar-type stars.