THE RIASS CORONATHON - JOINT X-RAY AND ULTRAVIOLET OBSERVATIONS OF NORMAL F-K STARS

Between 1990 August and 1991 January the ROSAT/IUE All Sky Survey (RIA SS) coordinated pointings by the International Ultraviolet Explorer wi th the continuous X-ray/EUV mapping by the Rontgensatellit. The campai gn provided an unprecedented multiwavelength view of a wide variety of cosmic sources. We report findings for F-K stars, a large proportion of the RIASS targets. Forty-eight of our 91 ''Coronathon'' candidates were observed by the IUE during the campaign. For stars missed by the IUE, we supplemented the ROSAT survey fluxes with archival UV spectra and/or follow-on observations. In addition to the coordinated work, we examined the UV emission histories of the Coronathon stars. Several o f the dwarfs show evidence for long-term variations in their Mg II lam bda 2802[h] emissions, and C IV lambda 1549 in a few cases. alpha Cen B (K0 V) and 0(2) Eri (K1 V) are noteworthy examples. Hertzsprung gap giants like 31 Com (G0 III) and the secondary of Capella (alpha Aur Ab : G0 III) are relatively constant in C IV and Mg II, at least over dec ade timescales. Similarly, G8-K0 ''Clump'' giants show only modest lon g-term changes in their ultraviolet emissions. On the other hand, seve ral of the ''hybrid chromosphere'' G and K supergiants show dramatic v ariability in the wind absorption components of their Mg II h profiles . The closely related but more active G supergiants like beta Cam and beta Dra show smaller changes in their Mg II fluxes, and symmetric pro file variations. Despite clear indications of secular variability, we find little support for the suspicion that previous broad statistical X-ray/UV studies of normal stars have been compromised by nonsimultane ous data sets. For well-studied stars, the temporal standard deviation rarely exceeds 20% in C IV, and 10% in Mg II h. We find no evidence t hat long-term cycles play any significant role in fostering the large dispersion in activity within any of the stellar classes. We correlate d the X-ray and UV emissions, normalized to the stellar bolometric flu xes (e.g., R(x) drop fX/fbol). As found by Ayres, Marstad, & Linsky, M S stars of spectral types F-K obey a power-law relation between R(x) a nd R(h) with a slope of approximate to 3. Here, the dwarf stars follow a R(CIV) versus R(h), power law of slope approximate to 2, and a R(x) versus R(CIV) power law of slope approximate to 1.5. The F9-G2 ''sola r-type'' MS stars have a significantly steeper slope in X-rays versus C IV than the cooler G8-K5 dwarfs. The populous Clump giants follow es sentially the same behavior as the cooler MS stars, showing a nearly 1 :1 correlation between X-rays and C IV. However, other key groups-some F dwarfs, the Hertzsprung gap giants, and virtually all of the superg iants-fall systematically to lower R(x) with respect to their R(CIV) a nd R(h) (as noted previously by Simon and Drake for the former two cla sses), and exhibit steeper power laws than the cooler stars. The MS an d evolved stars detected in both X-rays and the WFC S1 and S2 filters show close to a 1:1 connection between their normalized coronal and EU V fluxes. Thus, the latter likely are dominated by T> 10(6) K emission s. There is no distortion in the X-ray/EUV diagram which would indicat e that the ''X-ray deficiency'' of the F dwarfs or Hertzsprung gap gia nts is caused by local absorption at the source. All of the X-ray defi cient stars follow a normal (i.e., MS) correlation between R(CIV) and R(h), suggesting that the X-ray deficit truly is a coronal anomaly. Ma ny of the Clump giants, and G/K supergiants, show enhanced N V/C IV ra tios, possibly associated with envelope enrichment of nitrogen at firs t dredge-up. C IV/C II ratios display a slight tilt toward larger valu es with increasing R(x) in the dwarf stars, but a nearly 1:1 relation in the Clump giants. The origin of the effect is uncertain. O I/Mg II ratios exhibit a systematic increase in the more luminous objects. The origin likely is Bowen fluorescence. The ''coronal proxy'' He II lamb da 1640 shows a near 1:1 correlation with X-rays in the MS stars as a function of R(CIV), but a more complex behavior among the giants, part icularly the X-ray deficient stars. Curiously, He II shows a 1:1 corre lation with Si IV among the X-ray deficient giants, displaced a factor of similar to 2 below the relation obeyed by the Clump giants and coo ler MS stars. These behaviors are consistent with recombination domina tion in the dwarfs and Clump giants, but the situation among the warm luminous stars is ambiguous. The wide diversity of X-ray emission leve ls in the Clump-as exemplified by the Hyades K giants-emphasizes the s ensitive role played by stellar evolution in controlling coronal activ ity. The wide diversity of X-ray emission in general points to a magne tic origin for the coronal heating.