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RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system

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Abstract

RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies1,2,3. There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system4, because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses () and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with and stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung–Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.

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Figure 1: Orbital motion of the two binary components, and the pulsational motion of the pulsating component of the RRLYR-02792 system.
Figure 2: Change of brightness of the binary system caused by the mutual eclipses, and the intrinsic change of the brightness of the primary component caused by its pulsations.
Figure 3: Positions of the two stars in the RRLYR-02792 binary system on the Hertzsprung–Russell diagram.

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Acknowledgements

We acknowledge financial support from the Chilean Center for Astrophysics FONDAP, the BASAL Centro de Astrofisica y Tecnologias Afines (CATA), NSF, the Polish Ministry of Science (Ideas Plus), the Foundation for Polish Science (FOCUS, TEAM), and the GEMINI-CONICYT found. The OGLE project received funding from the European Research Council ‘Advanced Grant’ Program. We thank the staff astronomers at Las Campanas and ESO Paranal (program 287.D-5022(A)) who provided support in data acquisition.

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G.P., photometric and spectroscopic observations and reductions, and data analysis. I.B.T., spectroscopic observations and reductions, radial velocity measurements, and data analysis. W.G., spectroscopic observations and data analysis. D.G., modelling and data analysis. K.S., theoretical models and analysis. G.B., theoretical models. A.U., photometric observations and reductions, and data analysis. I.S., photometric observations and reductions. B.P. and P.G.P.M., modelling. G.W.P., N.N., M.G., J.S. and P.K., analysis of the data. A.M. and I.U.R., spectroscopic observations. G.P. and W.G. worked jointly to draft the manuscript, with all authors reviewing and contributing to its final form.

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Correspondence to G. Pietrzyński.

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The authors declare no competing financial interests.

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This file contains Supplementary Text, Supplementary Figures 1-2 and Supplementary Table 1. This file was replaced on 03 May 2012 to correct an error in the legend to Supplementary Table 1. (PDF 194 kb)

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Pietrzyński, G., Thompson, I., Gieren, W. et al. RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system. Nature 484, 75–77 (2012). https://doi.org/10.1038/nature10966

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