MAGNETIC CONFINEMENT, ALFVEN-WAVE REFLECTION, AND THE ORIGINS OF X-RAY AND MASS-LOSS DIVIDING LINES FOR LATE-TYPE GIANTS AND SUPERGIANTS

We discuss a simple qualitative model for the origin of the coronal and mass-loss dividing lines separating late-type giants and supergiants with and without hot, X-ray-emitting corona, and with and without significant mass loss. The basic physical effects we appeal to are the necessity of magnetic confinement for hot coronal material on the surface of such stars-which turns out to imply that any matter in "open" regions must be relatively cool-and the large reflection efficiency for Alfven waves in cool exponential atmospheres. Our model assumes that the magnetic field geometry of these stars changes across the observed "dividing lines" from being mostly closed on the high effective temperature side to being mostly open on the low effective temperature side.