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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.

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.

Budaj J.,Astronomical Institute | Burrows A.,Princeton | Hubeny I.,University of Arizona
Proceedings of the International Astronomical Union | Year: 2011

The internal heat loss or cooling of a planet determines its structure and evolution. We address in a consistent fashion the coupling between the day and the night sides by means of model atmosphere calculations with heat redistribution. We assume that a strong convection leads to the same entropy on the day and night side and that the gravity is the same on both hemispheres. We argue that the core cooling rate from the two hemispheres of a strongly irradiated planet may not be the same and that the difference depends on several important parameters. If the day-night heat redistribution is very effective, or if it takes place at a large optical depth, then the day-side and the night-side cooling may be comparable. However, if the day-night heat transport is not effective, or if it takes place at a shallow optical depth, then there can be a big difference between the day-side and the night-side cooling and the night side may cool more effectively. If the stellar irradiation gets stronger e.g. due to the stellar evolution or migration, this will reduce both the day and the night side cooling. Enhanced metallicity in the atmosphere acts as a blanket and reduces both the day- and the night-side cooling. However, the stratosphere on the day side of the planet can enhance the day-side cooling since its opacity acts as a shield which screens the stellar irradiation. These results might affect the well known gravity darkening and bolometric albedo effects in interacting binaries, especially for strongly irradiated cold objects. © International Astronomical Union 2012.

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.

Ostensen R.H.,Catholic University of Leuven | Geier S.,European Southern Observatory | Geier S.,Astronomical Institute | Schaffenroth V.,Astronomical Institute | And 19 more authors.
Astronomy and Astrophysics | Year: 2013

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M ⊙), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P = 0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 min and 2.5 h. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular mélange of flavours. © ESO, 2013.

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