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Saint Petersburg, Russia

Latour M.,University of Montreal | Latour M.,Astronomical Institute | Randall S.K.,ESO | Fontaine G.,University of Montreal | And 4 more authors.
Astrophysical Journal | Year: 2014

Taking advantage of a recent FORS2/VLT spectroscopic sample of extreme horizontal branch (EHB) stars in ω Cen, we isolate 38 spectra well suited for detailed atmospheric studies and determine their fundamental parameters (T eff, log g, and log N(He)/N(H)) using NLTE, metal line-blanketed models. We find that our targets can be divided into three groups: 6 stars are hot (T eff ≳ 45,000 K) H-rich subdwarf O stars, 7 stars are typical H-rich sdB stars (T eff ≲ 35,000 K), and the remaining 25 targets at intermediate effective temperatures are He-rich (log N(He)/N(H) ≳ -1.0) subdwarfs. Surprisingly, and quite interestingly, these He-rich hot subdwarfs in ω Cen cluster in a narrow temperature range (35,000 K to 40,000 K). We additionally measure the atmospheric carbon abundance and find a most interesting positive correlation between the carbon and helium atmospheric abundances. This correlation certainly bears the signature of diffusion processes - most likely gravitational settling impeded by stellar winds or internal turbulence - but also constrains possible formation scenarios proposed for EHB stars in ω Cen. For the He-rich objects in particular, the clear link between helium and carbon enhancement points toward a late hot flasher evolutionary history. © 2014. The American Astronomical Society. All rights reserved. Source

Zboril M.,Astronomical Institute
Astrophysics and Space Science | Year: 2010

We present new results on the extended atmosphere of the chromospherically active system UZ Lib. Doppler tomography confirms that the atmosphere of primary is extended to about 3.0 times its radius. Significant downfalls and probably plage regions are present in the atmosphere and we find a rather good correlation between the hydrogen integrated emission and the light curve. We also fix stellar parameters and distance to the system. An unseen companion revolves close to co-rotation radius, as well as close to the Roche L1 point, and in the corona of the primary. At this distance, tidal effects are significant. Despite the fact that the UV spectra are contaminated by the active giant atmosphere, we speculate that the unseen companion might be (a yellow-type) white dwarf (evidence for accretion, however, was not found), or alternatively an object like a brown dwarf. Finally, the IR observations indicate hydrogen emission without a recourse to extra-atmospheric material. © Springer Science+Business Media B.V. 2010. Source

Budaj J.,Astronomical Institute
Proceedings of the International Astronomical Union | Year: 2011

Program SHELLSPEC is designed to calculate light-curves, spectra and images of interacting binaries and extrasolar planets immersed in a moving circumstellar environment which is optically thin. It solves simple radiative transfer along the line of sight in moving media. The assumptions include LTE and optional known state quantities and velocity fields in 3D. Optional (non)transparent objects such as a spot, disc, stream, jet, shell or stars may be defined (embedded) in 3D and their composite synthetic spectrum calculated. The Roche model can be used as a boundary condition for the radiative transfer. Recently, a new model of the reflection effect, dust and Mie scattering were incorporated into the code. Ï Aurigae is one of the most mysterious objects on the sky. Prior modeling of its light-curve assumed a dark, inclined, disk of dust with a central hole to explain the light-curve with a sharp mid-eclipse brightening. Our model consists of two geometrically thick flared disks: an internal optically thick disk and an external optically thin disk which absorbs and scatters radiation. Shallow mid-eclipse brightening may result from eclipses by nearly edge-on flared (dusty or gaseous) disks. Mid-eclipse brightening may also be due to strong forward scattering and optical properties of the dust which can have an important effect on the light-curves. There are many similarities between interacting binary stars and transiting extrasolar planets. The reflection effect which is briefly reviewed is one of them. The exact Roche shape and temperature distributions over the surface of all currently known transiting extrasolar planets have been determined. In some cases (HAT-P-32b, WASP-12b, WASP-19b), departures from the spherical shape can reach 7-15%. © International Astronomical Union 2012. Source

Agency: Narcis | Branch: Project | Program: Completed | Phase: Physics, Chemistry and Medicine | Award Amount: | Year: 2006

Telescopes have to operate in the open air without any objects nearby to reach the highest spatial resolution. At the same time, they must be protected from environmental influences when not in use. A revolutionary new solution is a foldable tent construction based on tensing the cloth between movable but still stiff bows. The cloth parts are tensed in saddle shape for maximum stability against wind. Experiences with a first, very experimental tent of 7m diameter for the Dutch Open Telescope (DOT) at a height of 2350m on the Canary Island La Palma are very promising. This new solution needs a thorough scientific basis concerning a reliable way to calculate strength, stiffness and deformations. Essential are extensive measurements in all possible weather conditions on realized foldable tent constructions, at the moment two experimental prototypes DOT and GREGOR. |Utilization: A first utilization is the construction of a tent with 9m diameter for the GREGOR telescope at the Observatorio del Teide on Tenerife with significant improvements. The research must give insight into the best way to construct this type of tent: how to obtain maximum strength and stiffness with a construction as simple as possible. It must show where the limitations are when the diameter of these tents scales up. There is much interest in applications for astronomical instruments. There will be a much larger market for many other applications, like protection of radar and laser installations on land and on ships, and recreative objects like swimming pools. Large diameters are of high interest to exhibition areas and attraction parks.

Agency: Narcis | Branch: Project | Program: Completed | Phase: Physics, Chemistry and Medicine | Award Amount: | Year: 2007


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