For single stars and single-lined spectroscopic binaries a unique relation exists (Table 4 and Fig. 5) between the chromospheric Ca II H&K line-core emission in excess of the minimum emission and the coronal soft X-ray emission, both normalised to the bolometric emission. This relation is approximately quadratic, with power-law exponent , and does not depend on luminosity class or effective temperature within the extremes contained in our present sample. The reduced for this relation is 1.04.
The surface flux densities in X-ray and Ca II define also a unique relation with a reduced of 1.1 (Fig. 4), but only if stars with are excluded. We conclude therefore that the normalised emission is a more appropriate unit in which to express magnetic activity, when only comparing radiative emission measures, because the relationship between these units is independent of effective temperature.
The reduced for the relationship between normalised emission units is virtually equal to unity, meaning that the scatter about this relationship is dominated by measurement uncertainties. This is also the case if we select stars for which the observational data are most accurately determined. We cannot, therefore, make any statement about the possibility that deviations from this relationship significantly depend on any other parameter, like rotation, effective temperature, or luminosity class.
A strong correlation is seen between the ROSAT hardness ratio and the X-ray surface flux density of main-sequence stars. This suggests that as a star becomes more active, it will either raise the temperature of the coronal material as a whole or produce more high-temperature plasma. Both options have the effect of increasing the hardness ratio of the spectrum. For giants this correlation is less pronounced.
© European Southern Observatory (ESO) 1997
Online publication: April 28, 1998