The Chandra Deep Field–North Survey. XVII. Evolution of Magnetic Activity in Old Late‐Type Stars

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 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 L X ' 27 ergs s À1 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 pre-sumably 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 À2 ergs s À1 , which is faster than the t À1 decay rate predicted from widely accepted rotational spin-down rates and X-ray–activity relations.