Type II Cepheids Pulsating in the First Overtone from the OGLE Survey

Abstract

We report the discovery of the first type II Cepheids (BL Herculis stars) pulsating solely in the first overtone. We found two such objects among tens of millions of stars regularly observed by the OGLE survey in the Large Magellanic Cloud. Our classification and the pulsation mode identification is based on the position of these stars on the period-luminosity and color-magnitude diagrams and on the light curve analysis. We discuss why single-mode first-overtone BL Her pulsators must be very rare. For the two discovered stars we present non-linear models that successfully reproduce their light variation. These models indicate that both first-overtone pulsators should be more massive than it is typically assumed for BL Her stars, i.e. their masses should be above 0.75 Solar masses. However, the higher mass requires higher luminosity to match the observed periods of the stars, which is inconsistent with observations.

arXiv comment16 pages, 9 figures, accepted for publication in ApJ

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• The stellar photosphere-hydrogen ionization front interaction in Classical Pulsators: a theoretical explanation for observed period-colour relations
• Type II and anomalous Cepheids in the Kepler K2 mission
• A joint 1% calibration of the RR Lyrae & type-II Cepheid Leavitt laws yields homogeneous distances to 93 Galactic globular clusters
• Period-Luminosity Relations for Galactic Type II Cepheids in the Sloan bands
• The Variable Sky Through the OGLE Eye
• Variable Stars in M31 Stellar Clusters from the Panchromatic Hubble Andromeda Treasury