Coronal and chromospheric emission from cool stars in near-simultaneous ROSAT all-sky survey and Mount Wilson data.

Mt. Wilson Ca II H&K line-core emission fluxes for 215 F-, G- and K-type stars were obtained within at most a few days of the corresponding ROSAT All-Sky Survey observations. These stars cover wide ranges of stellar activity, spectral type and luminosity class. In this paper we study the well-known relationship between the Ca II H&K line-core emission in excess of the minimum emission and the soft X-ray emission. We find that flux densities normalised with the bolometric flux densities are the best quantity in which to express activity when comparing radiative emission in different temperature regimes. We find a power-law relationship, in which the X-ray normalised emission varies approximately quadratically with the normalised excess Ca II H&K line-core emission. This relationship does not depend on luminosity class at least up to luminosity class III, and it does not depend on effective temperature. The scatter around this relationship is consistent with the measurement errors. The X-ray spectral hardness ratios of main-sequence stars increase with the X-ray flux densities; a similar trend, but with substantially larger scatter, is also present for evolved stars. A comparison between values from different passbands of the Mt. Wilson HK spectrophotometer shows that relatively hot stars ((B-V)<=0.50) appear to have a Ca II line core emission peak about a factor 2 to 3 wider than cooler stars.