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T.R. Ayres, A. Brown, G.M. Harper (CASA)
\noindent We have used the highly sensitive ``solar-blind'' {\em Chandra}\/ High Resolution Camera (HRC-I) to search for 0.2--10~keV coronal X-ray emission from the key ``noncoronal'' red giants Arcturus (\alpha~Boo: K1~III) and Aldebaran (\alpha~Tauri: K5~III). Our program follows up previous detections of subcoronal (T~105~K) emission lines, such as \ion{C}{4} \lambda1548, by {\em HST}\/ STIS, and its predecessor GHRS. The two deep (19~ks) HRC-I pointings failed to detect either red giant, however, with 3\,\sigma upper limits of 1\times 10-4 cnts~s-1 and 2\times 10-4 cnts~s-1 for Arcturus and Aldebaran, respectively. The corresponding 0.2--2.0~keV L\rm X/L\rm bol levels are a factor of a thousand lower than the Sun (itself already an inconspicuous coronal object), establishing new limits of coronal futility among late-type stars. At the same time, STIS far-ultraviolet spectra suggest the presence of a ``cool absorber'' in the red giant atmosphere capable of selectively extinguishing the subcoronal spectrum shortward of ~1500~Å. The cool absorber must lie {\em beneath}\/ the extensive chromospheric (T~7000~K) envelope, because the chromospheric lines lack absorption signatures from the cool layer. As a result, the hot-line structures must be doubly buried under a large column of neutral hydrogen, undoubtedly smothering any soft X-ray emission that might be present. If small-scale magnetic active regions indeed exist in the lower atmospheres of red giants like Arcturus and Aldebaran, they might in some way be responsible for initiating and sustaining the cool outflows of such stars. The source of the near surface magnetism could be analogous to that of the small-scale ephemeral bipolar regions seen ubiquitously on the Sun throughout the sunspot cycle, and thought to be of direct convective origin.\\[-3mm]
\noindent This work was supported by {\em Chandra}\/ grant G02-3014X and {\em HST}\/ grant GO-09273.01--A to the University of Colorado.
The author(s) of this abstract have provided an email address for comments about the abstract: ayres@casa.colorado.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.