2001ApJ...550..457B

With the Hubble Space Telescope (HST) we have observed Hyades F stars, using the Space Telescope Imaging Spectrograph (STIS), in order to get more information about the heating mechanism(s) for the chromospheres and transition layers and their dependence on rotation and age. In this paper we study the Mg II lines at 2800 Å. We include earlier observations with the International 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∼0.40. For single stars there is a steep decrease in flux 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 increase, but much more slowly than for the Ca II lines. Generally, the low point of the emission is reached between B-V=0.43 and B-V=0.45, i.e., similar 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 binaries. We find that for Hyades stars with surface line fluxes larger than 10⁶ ergs.cm^–2.s^–1 the emission line fluxes decrease with increasing vsini. For smaller fluxes they may increase with increasing vsini. We have only 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 Å in order to determine where the lines fall on the curve of growth. For the earliest F stars studied here the ratio is close to 2, as expected for optically thin lines. Generally, 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 discuss the interpretation of the Wilson-Bappu effect. For the Hyades F stars there is a strong dependence of the line width on the effective temperature.