Transition region structure in F dwarfs.

Abstract

Strong transition region line emission detected in the IUE spectra of two F dwarfs is discussed. The corresponding surface fluxes are found to be comparable to those of active binaries, such as RS CVn-systems. Line flux ratios formed from the multiplets Si IV 1394 A + 1403 A, C IV 1549 A, and N V 1240 A are used in deriving the structure of the transition region in the F dwarfs and other active stars. It is suggested that the shape of the T(z)-relation indicates conversion of conductive flux into turbulent motions in the transition region at temperatures of about 100,000 K. Assuming that conductive heating balances the radiative losses between 100,000 and 200,000 K, pressures are derived for the F dwarfs transition regions that are one order of magnitude larger than that of the quiet sun. It is proposed that the large surface fluxes and pressures are consequences of the rapid rotation of these stars.