What Can Observations of Giants and Supergiant Stars Tell Us About Chromospheric and Coronal Heating?

Abstract

We discuss chromospheric and coronal activities in specific yellow and red giant and supergiant stars. Implications for heating mechanisms are described. Our list includes 31 Comae, α Aquarii, α Bootis, ζ Aurigae, α Ononis, and ο Ceti. We note that coronal yellow giant stars, which are rapid rotators, are most likely dominated by heating due to dynamo generated magnetic fields. The heating in noncoronal yellow giant stars, on the other hand, is probably dominated by the dissipation of acoustic waves. These stars have slow rotation rates and show no major chromospheric activities with the remarkable exception of the highly variable He I λ10830 line. However, short-period acoustic waves cannot explain the observed mass loss in these stars. Other types of outer atmospheric activity include pulsation and semiregular variabilities, present in Mira-type stars and red supergiants, respectively. We discuss potential observations available with the Hubble Space Telescope and with ROSAT and their relevance for our topic. We also emphasize the need for precise magnetic field measurements and new ab-initio models for stellar photospheres, chromospheres and coronae.