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Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system

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Abstract

Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars1 and other exotic binary star systems2,3,4. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers5. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars6,7 or as probes of the extreme environments in which low-mass white dwarfs form8,9,10 cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions11,12 or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.

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Figure 1: Observations of J0247-25.
Figure 2: Positions of J0247-25A and J0247-25B in the Hertzsprung–Russell diagram.
Figure 3: Adiabatic pulsation frequencies for models of J0247-25B.

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Acknowledgements

We thank the ESO staff who obtained our UVES data for carefully scheduling the observations at the correct orbital phases. We thank A. Cherman and D. Kurtz for comments on a draft version of the paper. This work is based on observations collected at the ESO, Chile (program ID: 086.D-0194). A.M.S. is partially supported by a Re-integration Grant (PIRG-GA-2009-247732; FP7-People), and a MICINN grant (AYA2011-24704). V.S. acknowledges funding by the Deutsches Zentrum für Luft- und Raumfahrt (grant 50 OR 1110) and by the Erika-Giehrl-Stiftung. T.R.M. acknowledges funding from the UK Science and Technology Facilities Council (ST/I001719/1).

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Contributions

P.F.L.M. analysed the light curves and spectroscopy and wrote the paper. A.M.S. calculated the models of the formation and evolution of J0247-25B. A.M. conducted the investigation into the pulsation properties of J0247-25B. T.R.M. and P.F.L.M. produced the light curves from the Ultracam images. U.H. calculated the synthetic stellar spectra used to check our effective temperature estimates for J0247-25B. T.R.M., V.S.D., S.L. and C.C. are responsible for the operation and maintenance of Ultracam and contributed to the planning and execution of the observations. B.S. calculated the synthetic stellar spectra and performed the comparison with the observed spectra for J0247-25A. V.S. and E.B. contributed to the execution of the observations.

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Correspondence to Pierre F. L. Maxted.

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The authors declare no competing financial interests.

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Maxted, P., Serenelli, A., Miglio, A. et al. Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system. Nature 498, 463–465 (2013). https://doi.org/10.1038/nature12192

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