Research Group Seminar
Observing magnetic activity with giant stars: between magnetic braking and revival

Abstract

By contrast to coronal X-ray detections, chromospheric emission
 measures seem to be a less biased indicator for magnetic activity
 among cool giant stars. We review the legacy of Mount Wilson
 "S-index" observations, and together with our own chromospheric
 activity monitoring data of bright, cool giants (obtained by the robotic
 telescope project TIGRE in Guanajuato, MEX) we put magnetic activity
 among giant stars into context with stellar evolution. We show that
 (1) despite huge -compered to the Sun- convective envelopes, activity
 is a common phenomenon among cool giant stars.
 (2) After magnetic braking on the main sequence and a first revival
 in the Hertzsprung  gap, giant activity suffers from magnetic braking
 again during central Helium burning (as the 4 Hyades K giants
demonstrate)...
 (3) ...until higher up on the AGB then chromospheric emission is,
 surprisingly, again  on the rise. Coincidently, each activity revival
 coincides in the HRD with a phase of fast core contraction. Is this a hint
 on the origin of the dynamo, by a spinning up of the contracting core?