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Fig. 10.12 X-ray "bubblegram" of a complete sample of giants within 25 pc around the Sun and hybrid stars. Plotted are the color-magnitude diagram positions of nearby giants detected as X-ray sources (black circles), the positions of nearby giants not detected as X-ray sources (dark circles), and the positions of hybrid stars detected as X-ray sources (grey circles)

extensive ROSAT observations, hybrid stars are now known to possess hot coronal plasma, presumably quite similar to cool main-sequence stars.

The current observational situation is summarized in Fig. 10.12, where giant stars within 25 pc around the Sun and the hybrid stars are shown in a color-magnitude diagram. As is obvious from Fig. 10.12, the XDL shows very clearly up for giants of luminosity class III, while most of the hybrid stars have been detected as X-ray sources, some of them having a spectral type that puts them well beyond the XDL for luminosity class II giants. The general concept of a dividing line has been questioned when it was recognized that all X-ray detections lie to the left of an evolutionary track with M = 1.25 MQ in the HR diagram. Low-mass stars ascending the giant branch are restricted to a rather narrow mass range and must therefore be rare. In this light the XDL may be interpreted as an effect of stellar evolution.

X-ray emission from M giants is extremely rare. Only about a dozen candidates for X-ray emitting M giants have been identified with ROSAT. Utilizing the high angular resolution of the Chandra telescope, M. Hunsch and collaborators [20] confirmed the previously detected X-ray emission from these objects, and in particular showed that the X-ray emission must indeed be attributed to the M giant stars, rather than to a coincidental nearby object. Since most of these M giants must be old, their extremely large X-ray luminosities in excess of 1030 erg s-1 are difficult to explain.

This - admittedly small - class of X-ray emitters represents a true puzzle for our understanding of the evolution of stellar activity.

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