Circumstellar Environments, Post-Red Supergiant Evolution, and Blue Loops

摘要

The evolved yellow and red hypergiants near the empirical upper luminosity boundary all show evidence for instabilities and high mass loss, but a few are distinguished by extensive circumstellar ejecta and evidence for episodic mass-loss events and eruptions. What distinguishes these stars and what are the implications for their evolutionary state? I will also present some new results for lower-mass massive stars.