Evolutionary Scenario for Metal-Poor Pulsating Stars.II.Anomalous Cepheids

Pulsating convective models and evolutionary tracks are used, together with stellar atmosphere models, to constrain the blue magnitude MB and period P of metal-poor high-mass variables (HMV) in the horizontal branch (HB) phase. The first straightforward result is that the lowest total mass for the occurrence of HMV is ∼1.3 with Z=0.0001 and ∼1.8 Msun with Z=0.0004. By scaling the calculated MB to the magnitude of the zero-age horizontal branch (ZAHB) model at the mean effective temperature (log Te=3.83) of the RR Lyrae strip, we derive the predicted distribution of theoretical fundamental (F) and first-overtone (H) pulsators in the ΔMB vs P plane. The theoretical limits of this distribution conform well to the observed data of most known Anomalous Cepheids (AC), and confirm that these variables are HB pulsators with mass M≥1.5Msun. We also present evidence that some AC candidates (for example, those observed in the dwarf spheroidal galaxy Ursa Minor) might be low-mass variables, instead. Based on their position in the period-luminosity and period-amplitude diagrams, observed AC turn out to be a mix of F and H pulsators. Two distinct (not parallel) period-luminosity relations are derived for the two pulsation modes, namely MB(F)=-0.31-2.56 log P(F) and MB(H)=-1.37-4.43 log P(H). Concerning the origin of AC (single young stars or old binary systems), the results as a whole do not support a single interpretation. For example, the variables in Carina are fully consistent with a very young stellar population, whereas those in Leo II and NGC 5466 agree with mass transfer in old binaries.