First observations of a corona of a cool bright giant with height resolution.

Abstract

Two time series of observations of the corona of HR 6902, as seen in the CIV and SiIV absorption lines during the 1988 and 1989 eclipses, are used to examine the density distribution of the expanding corona as a function of height. It is shown that the scaling laws for thermally driven winds predict coronal densities and mass-loss rates too low by factors of 1000 and 10,000, respectively. This is attributed to the fact that the density scale height by far exceeds the thermal hydrostatic equilibrium value and keeps coronal densities high far outwards up to the critical point of the flow.