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.

16 pages, 9 figures, accepted for publication in ApJ

References in corpus (2)

• The Dartmouth Stellar Evolution Database
• The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables

Cited by in corpus (6)

• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• TESS observations of Cepheid stars: first light results
• High-precision distance measurements with classical pulsating stars
• An absolute calibration of the near-infrared Period-Luminosity Relations of Type II Cepheids in the Milky Way and in the Large Magellanic Cloud
• 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