Mass Loss from Red Giants: Results from Ultraviolet Spectroscopy

New instrumentation in space, primarily the IUE spacecraft, has enabled the application of ultraviolet spectroscopic techniques to the determination of physical properties and reliable mass loss rates for red giant winds. One important result is the determination of where in the H-R diagram are found stars with hot outer atmospheres and with cool winds. So far it appears that single cool stars, except perhaps for the so-called hybrid stars, have either hot outer atmospheres or cool winds but not both. The C II resonance (1335 AA) and intersystem (2325 AA) mutliplets have been used to derive temperatures, densities, and geometrical extents for the chromospheric portions of red giant winds, with the result that the red giants and the earlier giants with hot coronae have qualitatively different chromospheres. Mass loss rates can now be derived accurately from the analysis of asymmetric emission lines, such as the Mg II resonance lines, and from $\Rho$ Cygni profile lines of atoms in the dominant ionization stage when a hot star is available to probe the wind of a red giant. The $\zeta$ Aur systems, consisting of a K-M supergiant and a main sequence $\Beta$ star are important systems for which reliable mass loss rates for the red supergiant components are becoming available.