Magnetic structure in cool stars. VIII. The MG II H and K surface fluxes in relation to the MT Wilson photometric CA II H and K measurements.

Data from IUE observations of 14 F, G, and K stars of luminosity II-V in the Mg II h + k lines are presented in tables and compared with published Mt. Wilson photometry of the Ca II H + K lines, and empirical relations are derived to facilitate the use of Ca II H + K data in investigating the chromospheric structure of cool stars. The results are presented in graphs, and consideration is given to the Vaughan-Preston gap for main-sequence stars; the (B-V)-dependent flux minima for main-sequence (LC V) stars, LC IV subgiants, and LC III giants (consistent with a dynamo model with magnetic braking and tidal synchronization); the very large fluxes of FK Com stars; and the positions of the LC II bright giants and LC I supergiants on the chromospheric-flux/color diagram.