1994A&A...285..595N

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 M₁ for a number of well-observed stars. We found that the value of M₁ 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. We compared the energy fluxes associated with the shocks with acoustic energy fluxes given by Bohn and with radiative energy losses from the chromospheres (only possible for cool giants and supergiants). We found that these fluxes are compatible for cool stars. The energy fluxes of Bohn cannot be used for super- and hypergiants with T_eff > 6500 K as all these stars are lacking convection zones.