The PTF Open Cluster Survey: Tracking the Evolution of Rotation and Activity on the Lower Main Sequence

A star's age is one of its most fundamental parameters. It is also, for isolated field stars, notoriously difficult to measure. Skumanich (1972) first proposed the existence of a relation between a star's age, rotation, and magnetic activity, but in the intervening 40 years, observational limitations have hampered the assembly of complete and uniform samples of rotation and activity measurements for stars spanning a wide range of ages and masses. We are still far from being able to describe fully the evolution of either rotation or activity for stars of a given mass, or from being able to use rotation or activity measurements to determine precisely the ages of isolated field stars. Fortunately, recent advances have transformed our ability to characterize open clusters, which provide the optimal environment for obtaining robust age estimates for individual stars. Robotic surveys such as the Palomar Transient Factory (PTF) now enable high cadence, long baseline monitoring of large-angular-sized clusters. And multi-object spectrographs and sensitive X-ray detectors make it possible to extend activity measurements to fainter stars in distant, older, and more populous clusters. Here we present recent results from our efforts to assemble a complete, uniform sample of rotation and activity measurements for sub-solar stars in six open clusters ranging in age from 35 Myr to 2.5 Gyr (Alpha Per, the Pleiades, Praesepe, M37, NGC 752, and Ruprecht 147). This sample will be used to derive a description of the age-rotation-activity relation for sub-solar stars from their arrival on the zero-age main-sequence to when they are a significant fraction of the age of the Sun.