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 logLX~=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 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-1, which is faster than the t-1 decay rate predicted from widely accepted rotational spin-down rates and X-ray-activity relations.
Based in part on observations obtained with the Hobby-Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.- Publication:
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The Astrophysical Journal
- Pub Date:
- August 2004
- DOI:
- 10.1086/422248
- arXiv:
- arXiv:astro-ph/0405026
- Bibcode:
- 2004ApJ...611.1107F
- Keywords:
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- Stars: Activity;
- Stars: Coronae;
- Stars: Late-Type;
- Stars: Magnetic Fields;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 22 pages, 5 figures, accepted for publication in the Astrophysical Journal