The basal chromospheric Mg II h+k flux of evolved stars: probing the energy dissipation of giant chromospheres

Of a total of 177 cool G, K and M giants and supergiants, we measured the Mg II h + k line emission of extended chromospheres in high-resolution (LWR) International Ultraviolet Explorer (IUE) spectra by using the IUE final data archive at the Space Telescope Science Institute (STScI) and derived the respective stellar surface fluxes. They represent the chromospheric radiative energy losses presumably related to basal heating by the dissipation of acoustic waves, plus a highly variable contribution due to magnetic activity. Thanks to the large sample size, we find a very well defined lower limit, the basal chromospheric MgII h + k line flux of cool giant chromospheres, as a function of T-eff. A total of 16 giants were observed several times, over a period of up to 20 yr. Their respective minimal MgII h + k line fluxes confirm the basal flux limit very well because none of their emissions dips beneath the empirically deduced basal flux line representative for the overall sample. Based on a total of 15-22 objects with very low MgII h + k emission, we find as limit log F-Mg IIhk = 7.33 log T-eff - 21.75 (cgs units; based on the B - V relation). Within its uncertainties, this is almost the same relation as has been found in the past for the geometrically much thinner chromospheres of main-sequence stars. But any residual dependence of the basal flux on the surface gravity is difficult to determine, since especially among the G-type giants there is a large spread of the individual chromospheric MgII fluxes, apparently due to revived magnetic activity. However, it can be stated that over a gravity range of more than 4 orders of magnitude (main-sequence stars to supergiants), the basal flux does not appear to vary by more than a factor of 2. These findings are in good agreement with the predictions by previous hydrodynamic models of acoustic wave propagation and energy dissipation, as well as with earlier empirical determinations. Finally, we also discuss the idea that the ample energy flux of the chromospheric acoustic waves in a cool giant may yield, as a by-product, the energy flux required by its cool wind (i.e. non-dust-driven, 'Reimers-type' mass-loss), provided a dissipation mechanism of a sufficiently long range is operating.