Updated pulsation models for BL Herculis stars
Population II pulsatingvariables play a relevant role both as distance indicators and as tracers of the properties of old stellar populations. In this paper we present an update and homogeneous pulsational scenario for a wide range of stellar parameters typical of BL Her stars i.e., Population II Cepheids with periods shorter than 8 days. To this purpose, we adopt a nonlinear convective hydrodinamical code evaluate the stability and full amplitude behaviour of an extensive set of BL Her pulsation models. Various assumptions of mass, luminosity and metallicity consistent with the most recent evolutionary prescriptions, are adopted. We obtain the theoretical instability strip for both fundamental and first overtone pulsators and present a detailed atlas of light/radial-velocity curves. Some relations for the boundaries of the instability strip and for the dependence of the absolute magnitude on period, mass and color, as well as the fundamental period-amplitude relations are derived.Finally, we provide the theoretical period-radius relation for BL Her and find that it is in excellent agreement with the empirical relation by Burki & Meylan and consistent with the one holding for shorter periods for RR Lyrae stars.