The Outer Atmospheres of Cool, Low Gravity Stars as Revealed by HST

Abstract

We have used the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) to measure the macroturbulent and flow velocities, the acceleration of winds, and the amount of hot (transition-region) plasma in the outer atmospheres of Carbon and K - M Giant/Supergiant stars. We see the acceleration of the stellar winds in the chromospheres of several of these stars from initial velocities of 3 - 9 km/s to upper velocities of 15 - 25 km/s and measured chromospheric macroturbulences ranging from approximately 25 to 35 km/s. We have found in the non-coronal giant alfa Tau weak C IV emission indicative of hot transition-region plasma, many new fluorescent lines of Fe II, and fluorescent molecular hydrogen emission and Ca II recombination lines seen for the first time in a giant star.