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Post Info TOPIC: OGLE-LMC-CEP0227


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OGLE-LMC-CEP-0227
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Title: The Eclipsing Binary Cepheid OGLE-LMC-CEP-0227 in the Large Magellanic Cloud: pulsation modelling of light and radial velocity curves
Authors: M. Marconi, R. Molinaro, G. Bono, G. Pietrzynski, W. Gieren, B. Pilecki, R. F. Stellingwerf, D. Graczyk, R. Smolec, P. Konorski, K. Suchomska, M. Gorski, P. Karczmarek

We performed a new and accurate fit of light and radial velocity curves of the Large Magellanic Cloud (LMC) Cepheid --OGLE-LMC-CEP-0227-- belonging to a detached double-lined eclipsing binary system. We computed several sets of nonlinear, convective models covering a broad range in stellar mass, effective temperature and in chemical composition. The comparison between theory and observations indicates that current theoretical framework accounts for luminosity --V and I band-- and radial velocity variations over the entire pulsation cycle. Predicted pulsation mass --M=4.14±0.06 solar masses-- and mean effective temperature --Te=6100±50 K-- do agree with observed estimates with an accuracy better than 1 sigma. The same outcome applies, on average, to the luminosity amplitudes and to the mean radius. We find that the best fit solution requires a chemical composition that is more metal--poor than typical LMC Cepheids (Z=0.004 vs 0.008) and slightly helium enhanced (Y=0.27 vs 0.25), but the sensitivity to He abundance is quite limited. Finally, the best fit model reddening --E(V-I)=0.171±0.015 mag-- and the true distance modulus corrected for the barycenter of the LMC --µ_{0,LMC}=18.50±0.02±0.10 (syst) mag--, agree quite well with similar estimates in the recent literature.

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RE: OGLE-LMC-CEP0227
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Title: Is there a mass discrepancy in the Cepheid binary OGLE-LMC-CEP0227?
Authors: Hilding R. Neilson, Norbert Langer (AIfA)

Context. The Cepheid mass discrepancy, the difference between masses predicted from stellar evolution and stellar pulsation calculations, is a challenge for the understanding of stellar astrophysics. Recent models of the eclipsing binary Cepheid OGLE-LMC-CEP-0227 have suggested that the discrepancy may be resolved.
Aims. We explore for what physical parameters do stellar evolution models agree with the measured properties of OGLE-LMC-CEP0227 and compare to canonical stellar evolution models assuming no convective core overshooting.
Methods. We construct state-of-the-art stellar evolution models for varying mass, metallicity, and convective core overshooting and compare the stellar evolution predictions with the observed properties.
Results. The observed mass, effective temperature, and radius of the two stars in the binary system are well fit by numerous combinations of physical parameters, suggesting a Cepheid mass discrepancy of 10-20% relative to canonical stellar evolution models.
Conclusions. The properties of the observed binary Cepheid suggest that the Cepheid mass discrepancy is still a challenge and requires more specific observations, such as the rate of period change, to better constrain and understand the necessary physics for stellar evolution models to resolve the discrepancy.

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OGLE-LMC-CEP-0227: the First Precisely Weighed Classical Cepheid

OGLE-LMC-CEP-0227 is a classical Cepheid in a well detached, double-lined eclipsing binary in the Large Magellanic Cloud, discovered by the OGLE survey (Soszynski et al., 2008). Precise OGLE-III photometry and spectroscopic observations collected with the Magellan 6.5-m telescope and MIKE spectrograph at Las Campanas Observatory, Chile and HARPS spectrograph attached to the 3.6-m ESO telescope at La Silla, Chile allowed the mass determination of the Cepheid component in the the system with accuracy of about 1%.
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By discovering the first double star where a pulsating Cepheid variable and another star pass in front of one another, an international team of astronomers has solved a decades-old mystery. The rare alignment of the orbits of the two stars in the double star system has allowed a measurement of the Cepheid mass with unprecedented accuracy. Up to now astronomers had two incompatible theoretical predictions of Cepheid masses. The new result shows that the prediction from stellar pulsation theory is spot on, while the prediction from stellar evolution theory is at odds with the new observations.
The new results, from a team led by Grzegorz Pietrzyski (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), appear in the 25 November 2010 edition of the journal Nature.

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