Stellar Rotation, Mixing, and the Chronology of the Galaxy

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

As Sun-like stars age, their initial rapid rotation slows through the braking action of magnetized winds. Stellar evolution theory incorporating angular momentum loss and internal angular momentum transport strongly predicts that at late times all stars of a particular mass should spin at the same rate, regardless of initial conditions. This convergence of rates and subsequent decline could, in principle, be used as a clock for determining the ages of field main-sequence stars. There are, however, two fundamental problems with this picture. First, there are simply not enough data to tell whether the convergence takes place as predicted and how it depends on mass. Second, there are many different possible processes for angular momentum transport. There are even some indications from both theory and observation that the convergence might not happen, particularly if the loss of angular momentum stops below a threshold rotation rate. To address these issues, Dr. Pinsonneault and collaborators will undertake a combined observational and theoretical investigation of the rotational evolution of stars in selected open clusters. Time-series photometry at the MDM observatory will pick out cluster members with high accuracy and will find all stars with rotational brightness modulation above 0.3% and periods less than 30 days. Targeted clusters will be selected to fill important gaps in age and mass from previous studies. The physical ingredients in stellar rotation codes will be extended to include wave- or magnetic-driven angular momentum transport processes. The various transport phenomena will be distinguished by their effect on the distribution of rotation and on the rate of destruction of light elements such as lithium, beryllium, or boron in young stars. The stellar evolution codes incorporating new treatments of angular momentum transfer from magnetic instabilities or waves will be made available to other researchers. The project will establish the range of mass and time over which a rotation clock can be calibrated, and an early application of the new clock will be to analyze the vast number of rotation periods to be found by NASA's Kepler mission. Along the way, new distances and metallicities for the targeted clusters will refine the extragalactic distance scale through recalibration of the Cepheid period-luminosity relation. Light element abundances in existing studies will be placed on a common temperature and abundance scale, and the new theoretical models will be used to create better tests of nucleosynthesis in the Big Bang through studies of lithium abundances in old halo stars. This project will also be used to create new content on stellar evolution for the middle school program at the COSI Science Center in Columbus. Dr. Pinsonneault and collaborators will give approximately five lectures per year as part of the field trip experience and help create teacher professional development materials, pre- and post-experience testing, web and CD-ROM materials for students, etc., in collaboration with the COSI team.