Stellar activity and the Wilson-Bappu relation.

Abstract

The MgII h and k lines of 78 single stars observed with the International Ultraviolet Explorer (IUE) observatory have been analyzed. Stars of spectral classes F, G, K and M and luminosity classes I-V are represented in the sample. From these data it is shown that the Wilson-Bappu relation for the Mg II h and k lines is fulfilled over a range of 18 magnitudes, i.e. from M_v_=+12 to M_v_=-6. The sample contains slowly rotating stars of different activity levels and is suitable for investigations of a possible relation between line width and stellar activity. It is found that active stars have broader lines and show a larger variation in line widths than quiet stars. Observations of the active RS CVn binary σ Gem taken at epochs when it shows different levels of activity, clearly demonstrate line broadening following higher activity. Possible implications of the new results for the interpretation of the Wilson-Bappu relation are briefly discussed.