Onset of Mass Loss in Red Giants: Association with an Evolutionary Event

Stencel & Mullan used asymmetries in the Mg II k emission line profile to determine the location of a "velocity dividing line" (VDL) in the H-R diagram. Stars to the right of (and above) the VDL were observed to have asymmetries that are consistent with the presence of cool massive winds. Stars to the left of (and below) the VDL showed no evidence for winds. We show that the VDL lies close to a certain event of stellar evolution on the red giant branch (RGB). The event occurs when the hydrogen-burning shell evolves outward through a discontinuity in molecular weight. In some low-mass stars, this event causes a kink in the evolutionary track of an individual low-mass star. In a cluster, the combined effects of such kinks create a "bump" in the luminosity function. Our result suggests that evolution through the kink (or bump) on the RGB is associated with the onset of a cool massive wind. Theoretical possibilities to explain this association will be explored in a subsequent paper.