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RE: Cepheids
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Title: Square root two period ratios in Cepheid and RR Lyrae variable stars
Author: Michael Hippke, John G. Learned, A. Zee

We document the presence of nine Cepheid and RR Lyrae variable stars with previously unrecognized characteristics. These stars exhibit the statistically unlikely property of a period ratio of main pulsation divided by secondary pulsation (P1/P2) very close to sqrt(2). Other stars of these types have period ratios which show clustering not with a close association with a single remarkable and nonharmonic number. In the way of explanation, we suggest that this indicates a previously unknown resonance of pulsations. Close examination reveals a deviation of multiples of a few times 0.06% for these stars. This deviation seems to be present in discrete steps on the order of about 0.000390(2), indicating the possible presence of a sort of fine structure in this oscillation. Physical explanation of the source of these regularities remains for 3D simulations of variable stars, and we only claim to make note of the regularities which are suggestive of physical principles.

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Title: Cepheids in Open Clusters: An 8-D All-sky Census
Authors: Richard I. Anderson, Laurent Eyer, Nami Mowlavi

Cepheids in open clusters (cluster Cepheids: CCs) are of great importance as zero- point calibrators of the Galactic Cepheid period-luminosity relationship (PLR). We perform an 8-dimensional all-sky census that aims to identify new bona- fide CCs and provide a ranking of membership confidence for known CC candidates through membership probabilities. The probabilities are computed for combinations of known Galactic open clusters and classical Cepheid candidates, based on spatial, kinematic, and population-specific membership constraints. Data employed in this analysis are taken largely from published literature and supplemented by a year-round observing program on both hemispheres dedicated to determining systemic radial velocities of Cepheids.
In total, we find 13 bona-fide CCs, 3 of which are identified for the first time, including an overtone-Cepheid member in NGC 129. Inconclusive cases are discussed in detail, some of which have been previously mentioned in the literature. Our results are inconsistent with membership for 7 candidates that have been studied previously. We employ our bona-fide CC sample to revisit the Galactic PLR and obtain results consistent with most other calibrations, being limited by cluster uncertainties.
In the near future, Gaia will enable our study to be carried out in much greater detail and accuracy, thanks to data homogeneity and greater levels of completeness.

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X Puppis and XX Sagittarii
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Title: Discovery of the spectroscopic binary nature of the Cepheids X Puppis and XX Sagittarii
Authors: László Szabados, Aliz Derekas, Csaba Kiss, Péter Klagyivik

We present the analysis of photometric and spectroscopic data of two bright Galactic Cepheids, X Puppis and XX Sagittarii. Based on the available data in the literature as well as own observations spanning 75 years, we conclude that both Cepheids belong to spectroscopic binary systems. However, the data are not sufficient to determine the orbital periods nor other elements for the orbit. This discovery corroborates the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. The photometric data revealed that the pulsation period of X Pup is continuously increasing with Delta P=0.007559 d/century likely caused by stellar evolution. The pulsation period of XX Sgr turned out to be very stable in the last ~100 years.

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Title: X-Ray, UV and Optical Observations of Classical Cepheids: New Insights into Cepheid Evolution, and the Heating and Dynamics of Their Atmospheres
Authors: Scott G. Engle, Edward F. Guinan

To broaden the understanding of classical Cepheid structure, evolution and atmospheres, we have extended our continuing secret lives of Cepheids program by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) / cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids Polaris, {\delta} Cep and {\beta} Dor. Previous studies made with the international ultraviolet explorer (IUE) showed a limited number of UV emission lines in Cepheids. The well-known problem presented by scattered light contamination in IUE spectra for bright stars, along with the excellent sensitivity & resolution combination offered by HST/COS, motivated this study, and the spectra obtained were much more rich and complex than we had ever anticipated. Numerous emission lines, indicating 10^4 K up to ~3 x 10^5 K plasmas, have been observed, showing Cepheids to have complex, dynamic outer atmospheres that also vary with the photospheric pulsation period. The FUV line emissions peak in the phase range {\phi} ~ 0.8-1.0 and vary by factors as large as 10x. A more complete picture of Cepheid outer atmospheres is accomplished when the HST/COS results are combined with X-ray observations that we have obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to date have X-ray luminosities of log Lx ~ 28.5-29.1 ergs/sec, and plasma temperatures in the 2-8 x 10^6 K range. Given the phase-timing of the enhanced emissions, the most plausible explanation is the formation of a pulsation-induced shocks that excite (and heat) the atmospheric plasmas surrounding the photosphere. A pulsation-driven {\alpha}^2 equivalent dynamo mechanism is also a viable and interesting alternative. However, the tight phase-space of enhanced emission (peaking near 0.8-1.0 {\phi}) favour the shock heating mechanism hypothesis.

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Cepheid distance scale
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Title: Insights into the Cepheid distance scale
Authors: G. Bono, F. Caputo, M. Marconi, I. Musella

We present an investigation of Cepheid distances using theory and observations. Cepheid models indicate that the slope of the Period-Luminosity (P-L) relation covering the entire period range (0.40<= logP <=2.0) becomes steeper when moving from optical to NIR bands, and that the metallicity dependence of the slope decreases from the B- to the K-band. We estimated V- and I-band slopes for 87 Cepheid data sets belonging to 48 external galaxies with nebular oxygen abundance 7.5<= 12+log (O/H) <=8.9. By using Cepheid samples including more than 20 Cepheids, the chi^2 test indicates that the hypothesis of a steepening of the P-L_{V,I} relations with increased metallicity can be discarded at the 99% level. On the contrary, the observed slopes agree with the metallicity trend predicted by pulsation models, i.e. the slope is roughly constant for galaxies with 12+log (O/H) < 8.17 and becomes shallower in the metal-rich regime, with a confidence level of 62% and 92%, respectively. The chi˛ test concerning the hypothesis that the slope does not depend on metallicity gives confidence levels either similar (P-L_V, 62%) or smaller (P-L_I, 67%). We found that the slopes of optical and NIR Period-Wesenheit (P-W) relations in external galaxies are similar to the slopes of LMC. On this ground, the P-W relations provide robust distances relative to the LMC, but theory and observations indicate that the metallicity dependence of the zero-point in the different passbands has to be taken into account. We compared the galaxy distances provided by Rizzi et al. (2007) using the TRGB with our set of Cepheid distances based on the P-W relations. We found that the metallicity correction on distances based on the P-WBV relation is gamma_(B,V)=-0.52 mag dex^-1, whereas it is vanishing for the distances based on the P-WVI and on the P-WJK relations. These findings fully support Cepheid theoretical predictions.

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Cosmic Distance Scale
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The Cosmic Distance Scale

In 1908, Harvard astronomer Henrietta Swan Leavitt discovered that a class of stars called Cepheids have brightnesses that vary regularly with periods that are directly related to their intrinsic luminosities. She calibrated this effect so that a comparison of the calculated and observed luminosity of a Cepheid can yield its distance. When Edwin Hubble began his study of galaxy velocities, he used Cepheid stars and Leavitt's relationship to determine their distances. This is how he discovered that galaxies are moving away from us, and that more distant galaxies are receding faster.
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The universe, astronomers have just discovered, is younger, spottier and even more repellent than we thought it was. That's the latest result from the Hubble Space Telescope, the astonishingly successful satellite which, after some teething trouble at its launch two decades ago, has been sending back wonderful pictures of the edges of space.
One of the ancient paradoxes about the night sky is that it is spotty: it has stars, rather than the universal blaze of light that might be expected if there were infinite numbers of fixed and immobile suns out there.

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Known to all astronomers, the Cepheids, of variable stars, are used as benchmarks to measure distances and made it possible for Hubble to discover its famous law. But they become unusable beyond a hundred million light years. However, we can go much further with a particular class of Cepheids from a group of American astronomers.

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Title: The Cepheid Period-Luminosity Relation (The Leavitt Law) at Mid-Infrared Wavelengths: III. Cepheids in NGC 6822
Authors: Barry F. Madore, Jane Rigby, Wendy L. Freedman, S. E. Persson, Laura Sturch, Violet Mager

We present the first application of mid-infrared Period-Luminosity relations to the determination of a Cepheid distance beyond the Magellanic Clouds. Using archival IRAC imaging data on NGC 6822 from Spitzer we were able to measure single-epoch magnitudes for sixteen long-period (10 to 100-day) Cepheids at 3.6um, fourteen at 4.5um, ten at 5.8um and four at 8.0um. The measured slopes and the observed scatter both conform to the relations previously measured for the Large Magellanic Cloud Cepheids, and fitting to those relations gives apparent distance moduli of mod{3.6} = 23.57 ±0.06, mod{4.5} = 23.55 ±0.07, mod{5.8} = 23.60 ±0.09 and mod{8.0} = 23.51 ±0.08 mag. A multi-wavelength fit to the new IRAC moduli, and previously published BVRIJHK moduli, allows for a final correction for interstellar reddening and gives a true distance modulus of 23.49 ±0.03 mag with E(B-V) = 0.26 mag, corresponding to a metric distance of 500 ±8 kpc.

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