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We report results of an Einstein Guest Observing program to map the occurrence of soft X-ray emission, which is a signature of hot stellar coronae (T > 10 6 K), in the cool half of the Hertzsprung Russell (H-R) diagram. We detect X-rays from F-M dwarfs and late F through early K giants, but not from the cooler giants, other than the spectroscopic binary e Car (KO II + B), or from any supergiants, other than Canopus (F0 Ib-II). The empirical separation of the cool half of the H-R diagram into a region where stellar soft X-ray emission is a common phenomenon, and a region where hot coronae are rare, if present at all among single stars, is similar to that found previously by Linsky and Haisch for C iv AA1548,1551 emission (T ~ 10 5 K) and by Stencel and Mullan for the onset of rapid mass loss in strong, cool (T < 10 4 K) stellar winds. We discuss the energy balance in the outer atmospheres of the coronal stars, the likely absorption of X-ray emission by cool winds in the " hybrid-spectrum " supergiants, a rotation-activity connection among the G dwarfs, and possible evolutionary origins of the structure seen in the cool half of the X-ray H-R diagram.

We report preliminary results of an ultraviolet survey of cool-star emission properties with IUE. We present 1150-2000 Á spectra of representative F-K dwarfs and giants and construct correlation diagrams that compare chromospheric (T<10 4 K) and transition-region (TaelO 5 K) emission line strengths, and broad-band coronal {T> 10 6 K) soft-X-ray fluxes. We find that the transition-region (TR) and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg ii \2800 doublet emission strength, which in turn is symptomatic of chromospheric energy losses. However, the power-law slopes are steeper than unity, particularly for soft X-rays. We conclude that while the distinct atmospheric layers very likely are physically associated, the internal heating mechanisms must be quite different. Despite the apparent chromosphere-TR-corona correlations among the G-K dwarfs and G giants, the earlier (F) and later (K) giants exhibit anomalous behavior. The former appear to have brighter TRs for a given chromospheric emission level, while the latter are systematically deficient in T>10 5 K material. In fact, not only are TRs very weak among the red giants, but chromospheres and coronae are systematically weak as well (at least compared with the mean activity levels of F-G giants). We propose that the weakness of hot outer atmospheres in the red giants compared with the yellow giants can be understood as a consequence of stellar evolution since stars of slightly different spectral type in the giant branch likely have very different main-sequence progenitors.

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