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 (RIASS) coordinated pointings by the International Ultraviolet Explorer with the continuous X-ray/EUV mapping by the Rontgensatellit. The campaign 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 of the dwarfs show evidence for long-term variations in their Mg II lambda 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 decade timescales. Similarly, G8-K0 ''Clump'' giants show only modest long-term changes in their ultraviolet emissions. On the other hand, several of the ''hybrid chromosphere'' G and K supergiants show dramatic variability 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 profile 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 nonsimultaneous 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 that long-term cycles play any significant role in fostering the large dispersion in activity within any of the stellar classes. We correlated the X-ray and UV emissions, normalized to the stellar bolometric fluxes (e.g., R(x) drop fX/fbol). As found by Ayres, Marstad, & Linsky, MS stars of spectral types F-K obey a power-law relation between R(x) and 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 ''solar-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 essentially 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 supergiants-fall systematically to lower R(x) with respect to their R(CIV) and R(h) (as noted previously by Simon and Drake for the former two classes), and exhibit steeper power laws than the cooler stars. The MS and 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 EUV fluxes. Thus, the latter likely are dominated by T> 10(6) K emissions. There is no distortion in the X-ray/EUV diagram which would indicate that the ''X-ray deficiency'' of the F dwarfs or Hertzsprung gap giants is caused by local absorption at the source. All of the X-ray deficient 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. Many of the Clump giants, and G/K supergiants, show enhanced N V/C IV ratios, possibly associated with envelope enrichment of nitrogen at first dredge-up. C IV/C II ratios display a slight tilt toward larger values 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 lambda 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, particularly the X-ray deficient stars. Curiously, He II shows a 1:1 correlation 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 cooler MS stars. These behaviors are consistent with recombination domination in the dwarfs and Clump giants, but the situation among the warm luminous stars is ambiguous. The wide diversity of X-ray emission levels in the Clump-as exemplified by the Hyades K giants-emphasizes the sensitive role played by stellar evolution in controlling coronal activity. The wide diversity of X-ray emission in general points to a magnetic origin for the coronal heating.