Bright metal-poor variables: Why “Anomalous” Cepheids?

Abstract

We investigate the theoretical scenario concerning the large sample of variables recently discovered in the dwarf, metal-poor irregular galaxy Leo A, focusing the attention on the “Anomalous” Cepheid phenomenon and its correlation with RR Lyrae stars, Classical and Population II Cepheids. To this purpose, we make use of suitable stellar and pulsation models to depict the pulsational history of evolutionary structures with metallicity . We find that He-burning pulsators are expected only outside the mass interval ~0.8–1.7 . Stars from ~1.8 to 4 , a mass range including both Anomalous and Classical Cepheids, populate to good approximation a common MV-logPF instability strip, independently of the previous occurrence of a He flash event. Their periods and luminosities increase with the stellar mass and they are at a lower luminosity level mag, as observed in Leo A. The class of less massive pulsators ( , namely RR Lyrae stars and Population II Cepheids) populate a distinct instability strip, where the magnitudes become brighter and the periods longer when the pulsator mass decreases. The dependence on metal content in this scenario has been investigated over the range to 0.008. The edges of the pulsational strip for the more massive class of pulsators appear to be independent of metallicity, but with a minimum mass of these bright pulsators which decreases with decreasing metallicity, thus decreasing the predicted minimum luminosity and period. Comparison with data for Cepheids in Leo A and in the moderately metal-rich extragalactic stellar system Sextans A discloses an encouraging agreement with the predicted pulsational scenario. On this basis, we predict that in a stellar system where both RR Lyrae stars and Cepheids are observed their magnitude difference may help in constraining both the metal content and the distance. The current classification of metal-poor Cepheids is briefly discussed and suggestion is advanced for an updated terminology abreast with the current knowledge of stellar evolution.