The coronal dividing line in the Rosat X-ray All-Sky Survey

Rosat All-Sky Survey soft X-ray observations of nearly 1000 bright single evolved stars of spectral types G, K, and M in the vicinity of the dividing line proposed by Linsky and Haisch (1979) are reported. Most observations consist of upper limits in the 0.1-2.0-keV band distributed between 604 stellar targes of spectral type K3 or earlier and 264 stellar targets of spectral type K4 or later. Of the 65 Rosat detections, only one involves an apparently single star of spectral type later than K3: HR 4289 (K5 III). A clear dichotomy exists between coronal and noncoronal stars of luminosity classes II, III, and IV at approximately spectral type Ke. The extremely low upper limit for the archetypal 'noncoronal' red giant, Arcturus, less than 3 x 24 exp 25 ergs/s achieved by Rosat during an 18.6-ks targeted observations by Ayres et al. (1991) indicates a very steep decline at the coronal dividing line.