Strange Cepheids and RR Lyrae stars

Strange modes can occur in radiative classical Cepheids and RR Lyrae models. These are vibrational modes that are trapped near the surface as a result of a "potential barrier" caused by the sharp hydrogen partial ionization region. Typically, the modal number of the strange mode falls between the seventh and 12th overtone, depending on the astrophysical parameters of the equilibrium stellar models (L, M, (T-eff, X, and Z). Interestingly, these modes can be linearly unstable outside the usual instability strip, in which case they should be observable as new kinds of variable stars : "strange Cepheids" or "strange RR Lyrae" stars. The present paper reexamines the linear stability properties of the strange modes by taking into account the effects of a quasi-isothermal atmosphere and turbulent convection, both of which could render these modes moot. It is found that the linear vibrational instability of the strange modes is resistant to both of these effects. Nonlinear hydrodynamic calculations indicate that the pulsation amplitude of these modes is likely to saturate at the millimagnitude level. These modes should therefore be detectable, but albeit not without effort.