BL HERCULIS MODEL PULSATIONS .3. LIVERMORE OPACITIES

We revisit the nonlinear pulsational properties of BL Her stars with t he new Livermore opacities. In contrast to classical Cepheid and RR Ly rae models, the Petersen diagrams for the BL Her models show very litt le sensitivity to the switch from the now obsolete Los Alamos opacitie s to the new Livermore opacities. In addition, the light curves and ra dial velocity curves are very similar, as are therefore their Fourier decomposition coefficients. This work confirms the major results of a previous survey, viz. that the 2:1 resonance between the fundamental m ode and the second overtone, which, in classical Cepheids, shapes the 'Hertzsprung progression' of the Fourier decomposition parameters, is also visible here, but much more in the radial velocity than in the ma gnitude coefficients. Because of the large sensitivity of the BL Her l ight and radial velocity curves to mass and luminosity (and to a lesse r extent metallicity) the BL Her stars form a rather inhomogeneous gro up compared to the classical Cepheids. Consequently the resonance stru cture is washed out in a global sample of BL Her models. We confirm as well the presence of another interesting feature of the BL Her models , also related to a resonance, viz. the existence of a narrow, almost- equal-to 0.5d wide window of RV Tau-like alternations, that are strict ly periodic, however. The BL Her first overtone blue edge is close, to within almost-equal-to -100 to +50K of the fundamental blue edge, and all other overtones are stable. Their radial velocity Fourier coeffic ients merge with those of the fundamental pulsators making a discrimin ation difficult. Their magnitude Fourier coefficients, on the other ha nd, are perhaps slightly better separated. The few observational data on almost-equal-to 1d period BL Her stars that are currently available do not allow a clear-cut dichotomy into a first overtone and a fundam ental group. However, the two types of pulsators may be distinguished on the basis of their amplitudes which are very small for the first ov ertones.