HEATING MECHANISM(S) FOR TRANSITION LAYERS IN GIANTS

We study the emission-line fluxes of lines originating in the lower parts of the transition layers between stellar chromospheres and coronae. Simon and Drake suspect different heating mechanisms for "hot" and cool stars. Changes in the flux ratios for the C IV to C II emission lines support this suspicion. Large C IV/C II line flux ratios appear to be indicative of magnetically controlled heating. We confirm for the cooler giants (late F and cooler) a correlation between excess continuum flux around 1950 angstrom and C II emission-line fluxes. Excess continuum flux correlates positively with large C IV/C II line flux ratio. The excess continuum flux corresponds to an increase in temperature by several hundred degrees in layers with mean optical depth tau-BAR approximately 0.03. For chromospherically active stars these layers experience a mechanical flux deposition of the order of 1% of the total radiative flux. We tentatively identify this flux as an MHD wave flux similar to Alfven waves.