2020ApJ...898...76S

We use 5337 spectroscopic vsini measurements of Kepler dwarfs and subgiants from the APOGEE survey to study stellar rotation trends. We find a detection threshold of 10 km s^–1, which allows us to explore the spindown of intermediate-mass stars leaving the main sequence, merger products, young stars, and tidally synchronized binaries. We see a clear distinction between blue stragglers and the field turnoff in α-rich stars, with a sharp rapid rotation cutoff for blue stragglers consistent with the Kraft break. We also find rapid rotation and radial velocity variability in a sample of red straggler stars, considerably cooler than the giant branch, lending credence to the hypothesis that these are active, tidally synchronized binaries. We see clear evidence for a transition between rapid and slow rotation on the subgiant branch in the domain predicted by modern angular momentum evolution models. We find substantial agreement between the spectroscopic and photometric properties of KIC targets added by Huber et al. based on Two Micron All Sky Survey photometry. For the unevolved lower main sequence, we see the same concentration toward rapid rotation in photometric binaries as that observed in rotation period data, but at an enhanced rate. We attribute this difference to unresolved near-equal-luminosity spectroscopic binaries with velocity displacements on the order of the APOGEE resolution. Among cool unevolved stars we find an excess rapid rotator fraction of 4% caused by pipeline issues with photometric binaries.