We have systematically searched for X-ray emission from late-type giants and supergiants in the Bright Star Catalogue. Specifically, we looked for X-ray sources detected in the ROSAT all-sky survey lying close to all A, F, G, K, and M-type stars of luminosity classes I, II, or III. The resulting catalogue (Hunsch et al. 1998a) contains 450 entries (out of 3839 input stars). This catalogue forms a flux-limited but otherwise unbiased sample of X-ray emitting evolved stars. The detection rate is low among the A-type giants; it steeply rises to more than 40% at early F-types and gradually decreases towards later types. However, X-ray emission can be detected at nearly all types of giants and supergiants. By means of the just recently available Hipparcos parallaxes, we construct HR diagrams of the X-ray detected giants and discuss the observed picture of X-ray emission from evolved stars. We also address the questions of the onset of convection, the existence of the X-ray dividing line, and the connection between stellar activity and stellar evolution off the main-sequence.
Cool Stars, Stellar Systems, and the Sun
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