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RE: Cepheid Envelopes
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A Canadian-led study of the distinctive, pulsating stars that astronomers use to calculate vast distances across galaxies is forcing a rethink of these "standard candles" and should lead to more accurate measurements of the key attributes of the universe -- including its size, age and rate of expansion.
University of Western Ontario astronomer Pauline Barmby told Postmedia News that her team's analysis of new images from NASA's Spitzer space telescope show that the ebb-and-flow glow of so-called "Cepheid" stars -- deemed indispensable in understanding the scale of the cosmos -- can be altered through the erosion of their constituent plasma, gas and dust as the great balls of energy hurtle through space.

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Cepheid AW Per
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Title: The Angular Separation of the Components of the Cepheid AW Per
Authors: D. Massa, N. R. Evans

The 6.4 day classical Cepheid AW Per is a spectroscopic binary with a period of 40 years. Analysing the centroids of HST/STIS spectra obtained in November 2001, we have determined the angular separation of the binary system. Although we currently have spatially resolved data for a single epoch in the orbit, the success of our approach opens the possibility of determining the inclination, sini, for the system if the measurements are repeated at additional epochs. Since the system is potentially a double lined spectroscopic binary, the combination of spectroscopic orbits for both components and the visual orbit would give the distance to the system and the masses of its components, thereby providing a direct measurement of a Cepheid mass.

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Cepheid XZ Ceti
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Title: The anomalous Cepheid XZ Ceti
Authors: Laszlo Szabados, Laszlo L. Kiss, Aliz Derekas

XZ Ceti is the only known anomalous Cepheid in the Galactic field. Being the nearest and brightest such variable star, a detailed study of XZ Ceti may shed light on the behaviour of anomalous Cepheids whose representatives have been mostly detected in external galaxies. CCD photometric and radial velocity observations have been obtained. The actual period and amplitude of pulsation were determined by Fourier analysis. The long time scale behaviour of the pulsation period was studied by the method of the O-C diagram using the archival Harvard photographic plates and published photometric data. XZ Ceti differs from the ordinary classical Cepheids in several respects. Its most peculiar feature is cycle-to-cycle variability of the light curve. The radial velocity phase curve is not stable either. The pulsation period is subjected to strong changes on various time scales including a very short one. The ratio of amplitudes determined from the photometric and radial velocity observations indicates that this Cepheid performs an overtone pulsation, in accord with the other known anomalous Cepheid in our Galaxy, BL Boo (V19 in the globular cluster NGC 5466). Continued observations are necessary to study the deviations from regularity, to determine their time scale, as well as to confirm binarity of XZ Ceti and to study its role in the observed peculiar behaviour.

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Cepheid Envelopes
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Using ESO's Very Large Telescope Interferometer (VLTI) at Cerro Paranal, Chile, and the CHARA Interferometer at Mount Wilson, California, a team of French and North American astronomers has discovered envelopes around three Cepheids, including the Pole star.

This is the first time that matter is found surrounding members of this important class of rare and very luminous stars whose luminosity varies in a very regular way. Cepheids play a crucial role in cosmology, being one of the first "steps" on the cosmic distance ladder.
The southern Cepheid L Carinae was observed with the VINCI and MIDI instrument at the VLTI, while Polaris (the Pole Star) and Delta Cephei (the prototype of its class) were scrutinised with FLUOR on CHARA, located on the other side of the equator. FLUOR is the prototype instrument of VINCI. Both were built by the Paris Observatory (France).
For most stars, the observations made with the interferometers follow very tightly the theoretical stellar models. However, for these three stars, a tiny deviation was detected, revealing the presence of an envelope.

"The fact that such deviations were found for all three stars, which however have very different properties, seems to imply that envelopes surrounding Cepheids are a widespread phenomenon" - Pierre Kervella, one of the lead authors.



The envelopes were found to be 2 to 3 times as large as the star itself. Although such stars are rather large - about fifty to several hundreds of solar radii - they are so far away that they can't be resolved by single telescopes. Indeed, even the largest Cepheids in the sky subtend an angle of only 0.003 arc second. To observe this is similar to viewing a two-storey house on the Moon.
Astronomers have thus to rely on the interferometric technique, which combines the light of two or more distant telescopes, thereby providing the angular resolution of a unique telescope as large as the separation between them. With the VLTI, it is possible to achieve a resolution of 0.001 arc second or less.

"The physical processes that have created these envelopes are still uncertain, but, in analogy to what happens around other classes of stars, it is most probable that the environments were created by matter ejected by the star itself" - Antoine Mérand, lead-author of the second paper describing the results.

Cepheids pulsate with periods of a few days. As a consequence, they go regularly through large amplitude oscillations that create very rapid motions of its apparent surface (the photosphere) with velocities up to 30 km/s, or 108 000 km/h. While this remains to be established, there could be a link between the pulsation, the mass loss and the formation of the envelopes.
Cepheids are commonly used as distance indicators, thanks to the existence of a basic relation between their intrinsic brightness and their pulsation period. By measuring the period of a Cepheid star, its intrinsic brightness can be deduced and from the observed apparent brightness, the distance may then be calculated.
As they are intrinsically very bright stars, and can be observed in distant galaxies, this remarkable property has turned these yellow supergiant stars into primary 'standard candles' for extragalactic distance estimations.

L Carinae is the brightest Cepheid in the sky, and also the one that presents the largest apparent angular diameter. This is a massive supergiant star, having about 10 times the mass of the Sun and a radius approximately 180 times that of the Sun.
Polaris is a peculiar star as it is located very close to the North celestial pole (hence its name). It is classified as a Cepheid, but it shows very weak pulsations compared to the other stars of its class.
Delta Cephei is the prototype of the Cepheids. It was discovered to be a variable star in the 18th century by the English amateur John Goodricke, and it is still one of the brightest members of the Cepheid class. Its short period is characteristic of a relatively small supergiant, with a radius of "only" 43 times that of the Sun.

These results are published in two articles to appear in Astronomy and Astrophysics: "Extended envelopes around Galactic Cepheids I. L Car from near and mid-infrared interferometry with the VLTI" by P. Kervella et al., and "Extended envelopes around Cepheids II.

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