The Chandra Deep Field-North survey. XVII. Evolution of magnetic activity in old late-type stars

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 spin-down of stellar rotation from a magnetized stellar wind. Thirteen X-ray sources are associated with late-type stars; 11 of these constitute a well-defined sample for statistical analysis. This sample consists of two G, two K0-K4, and seven M2-M5 stars with median V-band magnitude around 19 and median distance around 300 pc. X-ray luminosities are typically log LX ≃ 27 ergs s^-1 but are substantially higher in two cases. The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5-30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission. These X-ray properties are quantitatively similar to those seen in the active contemporary Sun. The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions (XLFs) with the known spatial distribution of old-disk stars. The model indicates that the CDF-N stars are the most magnetically active old-disk stars. A substantial decline in X-ray luminosities over the 1 Gyr < t < 11 Gyr age interval is required: 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval. This is a clear 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 L _x ∞ t^-2 ergs s^-1, which is faster than the t^-1 decay rate predicted from widely accepted rotational spin-down rates and X-ray-activity relations.