The concept of Magnetic Aging
Magnetosphere of the dM4.5 star V374 Peg
from Donati et al. 2007
Observing and calibrating the magnetic evolution of M stars is arguably the best method for reliably determining their ages. This is most easily accomplished by way of "magnetic tracers" such as X-ray to UV (XUV) activity measures and (due to the a-w interface dynamo of early to mid M stars) rotation periods. It is well known that F, G, K and M stars "spin down" with age - i.e. they rotate slower over time due to the loss of angular momentum via magnetic-threaded winds. Since time-series ground-based photometry is easier to obtain than space-based measures of XUV activity, if we can measure the rotation rates of enough M stars of known ages, then we can calibrate an Age-Rotation Relationship which will be a very powerful tool in, for example, determining the ages of the thousands of M stars currently be observed by the Kepler satellite. The ultra-precise photometry of Kepler will easily measure rotation rates for the observed M stars, and the Age-Rotation relationship being constructed by this program will become an essential astrophysical aid.