2004ApJ...606.1174B

We continue our investigations into the mechanisms heating the outer layers of cool dwarf stars. In this study we specifically seek to determine whether in the layers with temperatures around 250,000-300,000 K, in which the O VI lines are emitted, the temperatures are determined by heat conduction from the coronae or by the same processes that heat the lower temperature regions. To study this we discuss here 22 spectra of Hyades F stars taken by the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite to study the O VI lines at 1032 Å and the C III lines at 977 Å and compare them with other lower transition layer lines, observed with HST and IUE, and with existing X-ray data. For our targets with B-V>0.4, the X-ray fluxes of single F stars increase, on average, slowly with increasing B-V, while the O VI line fluxes show the same steep decrease around B-V=0.43 as previously found for the lower temperature transition layer lines. For single stars the X-ray fluxes decrease with increasing vsini, except for the stars with B-V between 0.418 and 0.455, while for the O VI lines, as for the other transition layer lines, fluxes increase with increasing vsini, if vsini is larger than 30 km/s. For smaller vsini, line fluxes are independent of vsini. The B-V and vsini dependences of the O VI line fluxes are then very different from those of the X-ray fluxes. We thus conclude that for electron temperature T_ebelow 300,000 K, the transition layers for Hyades F stars are not mainly heated by heat conduction from their coronae.