The shock strength in super- and hypergiant atmospheres derived from microturbulence data

This study is based on evidence that small-scale motions, observed as microturbulent line broadening, in the atmospheres of super- and hypergiants can possibly be interpreted as fields of shock waves moving outwards through the atmosphere. We derive values for the average shock strength M 1 for a number of well-observed stars. We found that the value of M 1 increases strongly with stellar luminosity, suggesting that shock wave pressure is relevant for the stability limit of most evolved stars (Humphreys-Davidson Limit) and for stellar evolution calculations. Our results however apply only to those stars in which photospheric shock wave patterns exist. For some stars we studied the dependence of M' 1 on the Rosseland optical depths τ R and we found that M' 1 (τ R ) does not change very much