MgII emission lines of Hyades F stars

With the Hubble Space Telescope (HST) we have observed Hyades F stars, usin g the Space Telescope Imaging Spectrograph (STIS), in order to get more inf ormation about the heating mechanism(s) for the chromospheres and transitio n layers and their dependence on rotation and age. In this paper we study t he Mg II lines at 2800 Angstrom. We include earlier observations with the I nternational Ultraviolet Explorer (IUE) satellite. The Mg II emission lines become observable for B-V >0.3. The emission line fluxes increase steeply until B-V similar to 0.40. For single stars there is a steep decrease in fl ux between B-V = 0.41 and B-V = 0.44, similar to the behavior of the Ca II emission line cores. For larger B-V the Mg II emission line fluxes again in crease, but much more slowly than for the Ca II lines. Generally, the low p oint of the emission is reached between B-V = 0.43 and B-V = 0.45, i.e., si milar to the Ca II emission cores. For the Hyades F stars there appears to be a difference between the emissions for single stars and those for binari es. We find that for Hyades stars with surface line fluxes larger than 10(6 ) ergs cm(-2) s(-1) the emission line fluxes decrease with increasing v sin i. For smaller fluxes they may increase with increasing v sin i. We have o nly three stars that perhaps show this. We study the flux ratios of the Mg II k and h lines at 2795.7 and 2802.5 Angstrom in order to determine where the lines fall on the AZ curve of growth. For the earliest F stars studied here the ratio is close to 2, as expected for optically thin lines. General ly, it seems that the optical depths in the line centers are less than 10. There remain problems in understanding the size of the line widths. We disc uss the interpretation of the Wilson-Bappu effect. For the Hyades F stars t here is a strong dependence of the line width on the effective temperature.