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Stellar rotation is a strong function of both mass and evolutionary state.Missions such as Kepler and CoRoT provide tens of thousands of rotationperiods, drawn from stellar populations that contain objects at a range ofmasses, ages, and evolutionary states. Given a set of reasonable startingconditions and a prescription for angular momentum loss, we address theexpected range of rotation periods for cool field stellar populations. We findthat cool stars fall into three distinct regimes in rotation. Rapid rotatorswith surface periods less than 10 days are either young low-mass main sequence(MS) stars, or higher mass subgiants which leave the MS with high rotationrates. Intermediate rotators (10-40 days) can be either cool MS dwarfs,suitable for gyrochronology, or crossing subgiants at a range of masses.Gyrochronology relations must therefore be applied cautiously, since there isan abundant population of subgiant contaminants. The slowest rotators, atperiods greater than 40 days, are lower mass subgiants undergoing envelopeexpansion. We identify additional diagnostic uses of rotation periods. Thereexists a period-age relation for subgiants distinct from the MS period-agerelations. There is also a period-radius relation that can be used as aconstraint on the stellar radius, particularly in the interesting case ofplanet host stars. The high-mass/low-mass break in the rotation distribution onthe MS persists onto the subgiant branch, and has potential as a diagnostic ofstellar mass. Finally, this set of theoretical predictions can be compared toextensive datasets to motivate improved modeling.

Fast Star, Slow Star; Old Star, Young Star: Subgiant Rotation as a Population and Stellar Physics Diagnostic