The Chandra Deep Field-North Survey: XVII. Evolution of magnetic activity in old late-type stars
E. D. Feigelson, A. E. Hornschemeier, G. Micela, F. E. Bauer, D. M. Alexander, W. N. Brandt, F. Favata, S. Sciortino, G. P. Garmire, Penn State, Johns Hopkins, Palermo, Cambridge, ESTEC
Abstract
The extremely sensitive Chandra Deep Field-North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spindown of stellar rotation from a magnetized stellar wind. Eleven X-ray sources constitute a well-defined sample of 2 G, 2 K0-K4, and 7 M2-M5 stars with median distance around 300 pc. X-ray luminosities are typically log Lx ~ 27 erg/s and is dominated by flares rather than quiescent coronal emission. Models of the population indicates that the CDF-N stars are the most magnetically active old disk stars. A substantial decline in X-ray luminosities over the 1<t<11 Gyr age interval is required. This is the first demonstration that the coronal and flaring components of stellar magnetic activity -- and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface -- exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types and X-ray luminosities of the sample has Lx ~ 1/t^2 erg/s which is faster than the 1/t decay rate predicted from widely accepted rotational spindown rates and X-ray-activity relations.