Spicular downflows and the transition to high mass-loss rates in G and K giants

Abstract

The mass-loss rates and coronal base pressures of stars that have evolved up to and beyond the so-called high mass-loss rate transition locus have been examined, using an energy-balance model in which the coronal energy losses due to downflowing spicular material are included. The minimum flux assumption of A.G. Hearn is used to evaluate the jump in mass-loss rate expected when the sonic point of the stellar wind reaches the coronal base. An order-of-magnitude increase in mass-loss rate is predicted. In addition, this jump in mass-loss rate is estimated, using a temperature-height profile for a transition region dominated by spicular downflows to estimate tha amount of downflowing material. Both methods give similar results, leading to the conclusion that as a star evolves across the transition locus, an order-of-magnitude increase in stellar mass-loss rate is to be expected.