Magnetic structure in cool stars. IX. Ultraviolet emission lines from chromospheres and transition regions.

A compilation of ultraviolet fluxes from 132 stars in the IUE catalog is analyzed in order to study the power-law relations for chromospheric and transition region lines. Flux-flux plots covering 5 decades for transition-region lines and 3.5 decades for chromospheric lines indicate tight power-law relations in the case of single stars, particularly FK Comae stars and most binary stars. The relations are found to be independent of spectral type or luminosity class. Departures from the obtained flux-flux relations are found in the case of emission line dwarfs, and F-type and G-type contact binaries, respectively. The tightness of the relations between the emission fluxes from different temperature regimes suggests that the overall structure and heating of the outer atmospheres of cool stars are determined by a single parameter. A summary of the measured IUE images is given.