The dynamical mass of a classical Cepheid variable star in an eclipsing binary system

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Stellar pulsation theory provides a means of determining the masses of pulsating classical Cepheid supergiants—it is the pulsation that causes their luminosity to vary. Such pulsational masses are found to be smaller than the masses derived from stellar evolution theory: this is the Cepheid mass discrepancy problem1,2, for which a solution is missing3,4,5. An independent, accurate dynamical mass determination for a classical Cepheid variable star (as opposed to type-II Cepheids, low-mass stars with a very different evolutionary history) in a binary system is needed in order to determine which is correct. The accuracy of previous efforts to establish a dynamical Cepheid mass from Galactic single-lined non-eclipsing binaries was typically about 15–30% (refs 6, 7), which is not good enough to resolve the mass discrepancy problem. In spite of many observational efforts8,9, no firm detection of a classical Cepheid in an eclipsing double-lined binary has hitherto been reported. Here we report the discovery of a classical Cepheid in a well detached, double-lined eclipsing binary in the Large Magellanic Cloud. We determine the mass to a precision of 1% and show that it agrees with its pulsation mass, providing strong evidence that pulsation theory correctly and precisely predicts the masses of classical Cepheids.

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Figure 1: The procedure adopted to separate pulsational and orbital motion of the Cepheid.
Figure 2: Orbital motion of the two binary components, and the pulsational motion of the Cepheid variable in the binary system.
Figure 3: Change of brightness of the binary system caused by the mutual eclipses, and the intrinsic change of the brightness of the Cepheid component caused by its pulsations.
Figure 4: The period and brightness of the Cepheid component of our system confirm that it is a classical Cepheid.


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This work was supported by the Chilean Center for Astrophysics FONDAP, the BASAL Centro de Astrofisica y Tecnologias Afines (CATA), NSF, the Polish Ministry of Science, the Foundation for Polish Science (FOCUS, TEAM), and the GEMINI-CONICYT Fund. The OGLE project has received funding from the European Research Council (‘Advanced Grants’). We thank the staff astronomers at Las Campanas and ESO La Silla who provided support in the data acquisition. We also thank D. Queloz, S. Udry and C. Lovis for their help in reducing and analysing the radial-velocity data obtained with the HARPS instrument.

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

Correspondence to G. Pietrzyński.

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Pietrzyński, G., Thompson, I., Gieren, W. et al. The dynamical mass of a classical Cepheid variable star in an eclipsing binary system. Nature 468, 542–544 (2010) doi:10.1038/nature09598

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