Evolution of the chromospheres and winds of low- and intermediate-mass giant stars

We analyze empirically the global thermodynamical properties of the outer atmospheres of giant stars. The paper differs from earlier work in two ways: (1) we address mass-loss energetics and the heating of the outer layers together, using the theoretical framework of Holzer and MacGregor; (2) our stellar sample represents most phases of giant-branch evolution without restrictions imposed by using individual observational techniques. The sample contains stars of representative variability classes and infrared properties. Ultraviolet radiation losses from giants between spectral types K1 III and M5 III (mostly first giant branch [FGB] stars) follow trends found for earlier spectral types by previous authors. Later spectral types (mostly second ascent or asymptotic giant branch [AGB] stars) show a larger scatter, probably related to the increased variability of these objects. Wind energy fluxes are remarkably similar, for both FGB and AGB stars, and they may be independent of dust-shell optical depths. The wind energy requirements of infrared carbon stars as a group are slightly higher than those of the other stars, and they also have larger wind velocities. We conclude that mass-loss rates are not strongly dependent on the actual physical processes driving the winds, and suggest that nonlinear processes act to regulate wind energy fluxes. As a corollary, correlations of mass, energy, and momentum fluxes with any other observed property cannot be used to examine the actual processes leading to mass loss for most red giants. This brings into question the validity of some earlier studies. We suggest that dust formation is not a primary instigator of mass loss from most cool giants. We discuss possible mechanisms responsible for chromospheric heating and mass loss, pointing to the need for theories to account for the relative invariance of the energy fluxes. We argue that sufficient energy is available in long-period photospheric variations in all cool giants to account for the wind losses, and suggest directions for future studies.