Updated pulsation models for BL Herculis stars

摘要

Population II pulsatingvariables play a relevant role both as distanceindicators and as tracers of the properties of old stellar populations. In thispaper we present an update and homogeneous pulsational scenario for a widerange of stellar parameters typical of BL Her stars i.e., Population IICepheids with periods shorter than 8 days. To this purpose, we adopt anonlinear convective hydrodinamical code evaluate the stability and fullamplitude behaviour of an extensive set of BL Her pulsation models. Variousassumptions of mass, luminosity and metallicity consistent with the most recentevolutionary prescriptions, are adopted. We obtain the theoretical instabilitystrip for both fundamental and first overtone pulsators and present a detailedatlas of light/radial-velocity curves. Some relations for the boundaries of theinstability strip and for the dependence of the absolute magnitude on period,mass and color, as well as the fundamental period-amplitude relations arederived.Finally, we provide the theoretical period-radius relation for BL Herand 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 Lyraestars.