New theoretical predictions concerning Type II Cepheids: Towards a self-consistent Population II distance scale

Context. Type II Cepheids are pulsating stars that can be used as standard candles for old stellar populations due to their characteristic period-luminosity and period-luminosity-colour relations. They are traditionally divided into three sub-classes, namely BL Her, W Vir, and RV Tauri. Aims. In this paper, we focus on the first two sub-classes in order to provide a new theoretical scenario and develop tools and relations to be adopted in distance-scale and old stellar population studies. Methods. We built new non-linear convective pulsation models of Type II Cepheids, computed along selected stellar evolution tracks and spanning a wide range of pulsation period and stellar parameters. Three chemical compositions have been taken into account, namely Z = 0.0001 Y = 0.245, Z = 0.001 Y = 0.245, and Z = 0.01 Y = 0.26. For each assumed Z and Y, models were computed following stellar evolution predictions for of-zero-age horizontal-branch evolution of stellar masses lower than typical RR Lyrae stars, crossing the classical instability strip as BL Her or W Vir pulsating stars. Results. A new theoretical prediction for the instability strip boundaries of these classes of variable stars has been obtained together with their dependence on metal abundance. The predicted light and radial-velocity curves were computed along the evolution inside the strip, showing how the amplitude and the morphology are affected by the position relative to the edges and by the luminosity and mass values. The transformation of bolometric light curves into various photometric systems allowed us to provide new theoretical period-luminosity and period-Wesenheit relations for BL Her and W Vir. Conclusions. These relations are found to be consistent with previously published RR Lyrae model results but with a smaller metallicity dependence. Moreover, the application of the inferred theoretical relations to Magellanic and Galactic Type II Cepheid data provides results in good agreement with some independent distance estimates in the literature.