A survey of MG II H and K emission in near-solar-type stars.

Abstract

IUE measurements of Mg II h and k emission fluxes are presented for 30 F and G stars that are on or near the main sequence and compared with Wilson's measurements of the Ca II H and K fluxes in these stars. The survey includes a large proportion of stars with very low chromospheric activity as well as 111 Tau, χ¹Ori and other examples of strong chromospheric emission. Emission cores are present in all of the stars observed. A sharp lower limit to the flux in the cores of the k lines implies the existence of a minimum level of chromospheric activity in which the k line flux is a constant fraction of stellar luminosity. Reduction of Wilson's values to absolute fluxes produces a close correlation between Mg and Ca strength with possibly some dependence on colour. For the most active stars, the Mg k and Ca fluxes are consistent with the presence of solar plages covering up to one half of the stellar surface.