2004ApJ...611.1107F -
Astrophys. J., 611, 1107-1120 (2004/August-3)
The Chandra deep field-north survey. XVII. Evolution of magnetic activity in old late-type stars.
FEIGELSON E.D., HORNSCHEMEIER A.E., MICELA G., BAUER F.E., ALEXANDER D.M., BRANDT W.N., FAVATA F., SCIORTINO S. and GARMIRE G.P.
Abstract (from CDS):
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 logLX≃27 ergs/s 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 1Gyr<t<11Gyr 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 LX∝t–2 ergs/s, which is faster than the t–1 decay rate predicted from widely accepted rotational spin-down rates and X-ray-activity relations.
Abstract Copyright:
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Journal keyword(s):
Stars: Activity - Stars: Coronae - Stars: Late-Type - Stars: Magnetic Fields - X-Rays: Stars
Simbad objects:
16
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