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Direct detection of pulsations of the Cepheid star ζ Gem and an independent calibration of the period–luminosity relation


Cepheids are a class of variable (pulsating) stars whose absolute luminosities are related in a simple manner to their pulsational periods. By measuring the period and using the ‘period–luminosity’ relationship, astronomers can use the observed visual brightness to determine the distance to the star. Because these stars are very luminous, they can be observed in other galaxies, and therefore can be used to help determine the expansion rate of the Universe1 (the Hubble constant). Calibration of the period–luminosity relation is a necessary first step, but the small number of sufficiently nearby Cepheids has forced the use of a number of indirect means, with associated systematic uncertainties. Here we present a distance to the Cepheid ζ Geminorum, determined using a direct measurement (by an optical interferometer) of its changes in diameter as it pulsates. Within our uncertainty of 15 per cent, our distance is in agreement with previous indirect determinations. Planned improvements to the instrument will allow us to calibrate directly the period–luminosity relation to better than a few per cent.

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Figure 1: The uniform-disk (UD) angular diameter of ζ Gem as measured by PTI.
Figure 2: The UD angular diameter of ζ Gem as a function of pulsational phase with a model based on radial velocity data.

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We thank R. Akeson, T. Armstrong, A. Bouchez, M. Colavita, T. Nordgren, M. Nunez and D. Sasselov for valuable comments. Part of the work described in this paper was performed at the Jet Propulsion Laboratory under contract with the National Aeronautics and Space Administration. This research has made use of the Simbad database, operated at Centre de Données astronomiques de Strasbourg, Strasbourg, France. B.F.L. gratefully acknowledges the support of NASA through the Michelson fellowship programme. B.F.L. acknowledges the support of NASA and the NSF.

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Correspondence to S. R. Kulkarni.

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Lane, B., Kuchner, M., Boden, A. et al. Direct detection of pulsations of the Cepheid star ζ Gem and an independent calibration of the period–luminosity relation. Nature 407, 485–487 (2000).

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