Super-Eddington luminosities in hydrostatic model atmospheres manifest
themselves by the presence of gas pressure inversions. Such inversions are not
an artifact of the assumption of hydrostatic equilibrium but can also be
present in hydrodynamical model atmospheres. Only for very large mass loss
rates hardly realized in supergiants will the inversions be removed.
Instabilities may, however, still be present in such inversions, which is
investigated for both H-rich and H-deficient late-type supergiant model
atmospheres. A local, non-adiabatic, linear stability analysis reveals that
sound waves can be amplified due to the strong radiative forces. However,
despite the super-Eddington luminosities, the efficiency of the radiative
instabilities is fairly low compared to for early-type stars with growth rates
of 10−5s−1.Comment: 11 pages; accepted for publication in Astronomy & Astrophysic
