Instabilities in the Yellow Hypergiant domain

Yellow Hypergiants (YHGs) are massive stars that are commonly interpreted to be in a post-red supergiant evolutionary state. These objects can undergo outbursts on timescales of decades, which are suspected to be due to instabilities in the envelope. To test this conjecture, the stability of envelope models for YHGs with respect to infinitesimal, radial perturbations is investigated. Violent strange mode instabilities with growth rates in the dynamical regime are identified if the luminosity to mass ratio exceeds $\approx 10^4$ in solar units. For the observed parameters of YHGs we thus predict instability. The strange mode instabilities persist over the entire range of effective temperatures from red to blue supergiants. Due to short thermal timescales and dominant radiation pressure in the envelopes of YHGs, a nonadiabatic stability analysis is mandatory and an adiabatic analysis being the basis of the common perception is irrelevant. Contrary to the prevailing opinion, the models considered here do not exhibit any adiabatic instability.