OGLE-GAL-ACEP-091: The First Known Multi-mode Anomalous Cepheid
Soszyński, I.; Smolec, R.; Udalski, A.; Szymański, M. K.; Pietrukowicz, P.; Skowron, D. M.; Skowron, J.; Mróz, P.; Poleski, R.; Kozłowski, S.; Iwanek, P.; Wrona, M.; Gromadzki, M.; Ulaczyk, K.; Rybicki, K.
Poland, United States, United Kingdom
Abstract
Anomalous Cepheids (ACs) are metal-deficient, core-helium-burning pulsating stars with masses in the range 1.2-2.2 ${M}_{\odot }$ . Until recently, all known ACs were pure single-mode pulsators. The first candidate for an AC pulsating in more than one radial mode—OGLE-GAL-ACEP-091—was recently identified in the Milky Way based on the photometric database of the Optical Gravitational Lensing Experiment (OGLE) survey. We analyze this object showing that it is actually a triple-mode pulsator. Its position in the Petersen diagram, the light-curve morphology quantified by Fourier coefficients, and absolute magnitudes derived from the Gaia parallax are consistent with the assumption that OGLE-GAL-ACEP-091 is an AC. Our grid of linear pulsation models indicates that OGLE-GAL-ACEP-091 is a 1.8 ${M}_{\odot }$ star with a metallicity of about [Fe/H] = -0.5 dex.