Institute For Astronomie

Breitenfurt bei Wien, Austria

Institute For Astronomie

Breitenfurt bei Wien, Austria
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Murphy S.J.,University of Central Lancashire | Murphy S.J.,University of Porto | Pigulski A.,Wrocław University | Kurtz D.W.,University of Central Lancashire | And 15 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

The candidate SX Phe star KIC 11754974 shows a remarkably high number of combination frequencies in the Fourier amplitude spectrum: 123 of the 166 frequencies in our multifrequency fit are linear combinations of independent modes. Predictable patterns in frequency spacings are seen in the Fourier transform of the light curve. We present an analysis of 180 d of short-cadence Kepler photometry and of new spectroscopic data for this evolved, late A-type star. We infer from the 1150-d, long-cadence light curve, and in two different ways, that our target is the primary of a 343-d, non-eclipsing binary system. According to both methods, the mass function is similar, f (M) = 0.0207 ± 0.0003M⊙. The observed pulsations are modelled extensively, using separate, state-of-the-art, time-dependent convection (TDC) and rotating models. The models match the observed temperature and low metallicity, finding a mass of 1.50-1.56M⊙. The models suggest that the whole star is metal poor, and that the low metallicity is not just a surface abundance peculiarity. This is the best frequency analysis of an SX Phe star, and the only Kepler δ Sct star to be modelled with both TDC and rotating models. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Kepler S.O.,Federal University of Rio Grande do Sul | Pelisoli I.,Federal University of Rio Grande do Sul | Jordan S.,University of Heidelberg | Kleinman S.J.,Gemini Observatory | And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 to 733 MG, which amounts to 4 per cent of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Castanheira B.G.,Federal University of Rio Grande do Sul | Castanheira B.G.,Institute For Astronomie | Kepler S.O.,Federal University of Rio Grande do Sul | Kleinman S.J.,Gemini Observatory | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

Using the The SOuthern Astrophysical Research (SOAR) 4.1-m telescope, we report on the discovery of low amplitude pulsations for three stars previously reported as not-observed-to-vary (NOV) by Mukadam et al. and Mullally et al., which are inside the ZZ Ceti instability strip. With the two pulsators discovered by Castanheira et al., we have now found variability in a total of five stars previously reported as NOVs. We also report the variability of eight new pulsating stars, not previously observed, bringing the total number of known ZZ Ceti stars to 148. In addition, we lowered the detection limit for 10 NOVs located near the edges of the ZZ Ceti instability strip. Our results are consistent with a pure mass-dependent ZZ Ceti instability strip. © 2010 The Authors. Journal compilation © 2010 RAS.

Castanheira B.G.,Institute For Astronomie | Kepler S.O.,Federal University of Rio Grande do Sul | Kleinman S.J.,Gemini Observatory | Nitta A.,Gemini Observatory | Fraga L.,Southern Observatory for Astrophysical Research
Monthly Notices of the Royal Astronomical Society | Year: 2013

Using the SOuthern Astrophysical Research telescope (SOAR) Optical Imager at the SOAR 4.1 m telescope, we report on the discovery of five new massive pulsating white dwarf stars. Our results represent an increase of about 20 per cent in the number of massive pulsators. We have detected both short and long periods, low and high amplitude pulsation modes, covering the whole range of the ZZ Ceti instability strip. In this paper, we present a first seismological study of the new massive pulsators based on the few frequencies detected. Our analysis indicates that these stars have masses higher than average, in agreement with the spectroscopic determinations. In addition, we study for the first time the ensemble properties of the pulsating white dwarf stars with masses above 0.8 M⊙. We found a bimodal distribution of the main pulsation period with the effective temperature for the massive DAVs, which indicates mode selection mechanisms. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Zeidler S.,Astrophysikalisches Institute | Posch T.,Institute For Astronomie | Mutschke H.,Astrophysikalisches Institute | Richter H.,Institute For Astronomie | Wehrhan O.,Institute For Optik Und Quantenelektronik
Astronomy and Astrophysics | Year: 2011

Context. Several astrophysically relevant solid oxides and silicates have extremely small opacities in the visual and near-infrared in their pure forms. Datasets for the opacities and for the imaginary part k of their complex indices of refraction are hardly available in these wavelength ranges. Aims. We aimed at determining k for spinel, rutile, anatase, and olivine, especially in the near-infrared region. Our measurements were made with impurity-containing, natural, and synthetic stardust analogs. Methods. Two experimental methods were used: preparing small sections of natural minerals and synthesizing melt droplets under the electric arc furnace. In both cases, the aborption properties of the samples were measured by transmission spectroscopy. Results. For spinel (MgAl2O4), anatase, rutile (both TiO2), and olivine ((Mg,Fe)2SiO4), the optical constants have been extended to the visual and near-infrared. We highlight that the individual values of k(λ) and the absorption cross section Qabs(λ) depend strongly on the content in transition metals like iron. Based on our measurements, we infer that k values below 10-5 are very rare in natural minerals including stardust grains, if they occur at all. Conclusions. Data for k and Qabs(λ) are important for various physical properties of stardust grains such as temperature and radiation pressure. With increasing Qabs(λ) due to impurities, the equilibrium temperature of small grains in circumstellar shells increases as well. We discuss why and to what extent this is the case. © 2010 ESO.

Van Belle G.T.,Lowell Observatory | Paladini C.,Institute For Astronomie | Aringer B.,National institute for astrophysics | Hron J.,Institute For Astronomie | Ciardi D.,California Institute of Technology
Astrophysical Journal | Year: 2013

We report new interferometric angular diameter observations of 41 carbon stars observed with the Palomar Testbed Interferometer. Two of these stars are CH carbon stars and represent the first such measurements of this subtype. Of these, 39 have Yamashita spectral classes and are of sufficiently high quality that we can determine the dependence of effective temperature on spectral type. We find that there is a tendency for the effective temperature to increase with increasing temperature index by 120 K per step, starting at T EFF ≃ 2500 K for C3, y, although there is a large amount of scatter in this relationship. Overall, the median effective temperature of the carbon star sample is 2800 ± 270 K and the median linear radius is 360 ± 100 R. We also find agreement, on average within 15 K, with the T EFF determinations of Bergeat et al. and a refinement of the carbon star angular size prediction based on V & K magnitudes is presented that is good to an rms of 12%. A subsample of our stars have sufficient {u, v} coverage to permit non-spherical modeling of their photospheres, and a general tendency for detection of statistically significant departures from sphericity with increasing interferometric signal-to-noise is seen. The implications of most - and potentially all - carbon stars being non-spherical is considered in the context of surface inhomogeneities and a rotation-mass-loss connection. © 2013. The American Astronomical Society. All rights reserved.

Romero A.D.,National University of La Plata | Romero A.D.,CONICET | Corsico A.H.,National University of La Plata | Corsico A.H.,CONICET | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

ZZCeti stars form the most numerous group of degenerate variable stars. They are otherwise normal DA (H-rich atmospheres) white dwarfs that exhibit pulsations. Here, we present an asteroseismological analysis for 44 bright ZZCeti stars based on a new set of fully evolutionary DA white dwarf models characterized by detailed chemical profiles from the centre to the surface. One of our targets is the archetypal ZZCeti star G117-B15A, for which we obtain an asteroseismological model with an effective temperature and a surface gravity in excellent agreement with the spectroscopy. The asteroseismological analysis of a set of 44 ZZCeti stars has the potential to characterize the global properties of the class, in particular the thicknesses of the hydrogen envelope and the stellar masses. Our results support the belief that white dwarfs in the solar neighbourhood harbour a broad range of hydrogen-layer thickness. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Mitchell N.L.,Institute For Astronomie
Astrophysics and Space Science Proceedings | Year: 2011

Numerical simulations are now a fundamental tool with which modern astronomers test current theory. However an increasing number of authors have noted significant discrepancies between galaxy properties when run in different hydrodynamic codes. As we can now finally begin to run large cosmological simulations with complex gas physics it is necessary to understand the way in which these differences between codes affect the properties of the ISM. From the efficiency of supernova feedback to the large scale heating of gas during galaxy cluster mergers, I will show that there are notable differences between particle and grid based codes, explain their origin and demonstrate work that is being undertaken at Vienna to improve the way in which we model the properties of the ISM. © Springer-Verlag Berlin Heidelberg 2011.

Kaiser A.,Institute For Astronomie | Kuschnig R.,Institute For Astronomie | Weiss W.W.,Institute For Astronomie
Astronomische Nachrichten | Year: 2012

The BRITE-Constellation, short for "BRIght Target Explorer" Constellation, is a group of six nanosatellites from Austria, Poland and Canada carrying thirty millimeter aperture optical telescopes with a CCD camera. The goal of the mission is to photometrically measure low-level oscillations and temperature variations of the brightest stars in the sky down to a visual magnitude of 4.0, with unprecedented precision and time coverage not achievable from ground in two colors. The improved three-axis pointing performance which is a critical element that makes this high precision photometry mission feasible is a recent development by the University of Toronto's Space Flight Laboratory. The University of Vienna and the FFG/ALR (Austria's space agency) are financing the development of two satellites. The Polish Academy of Sciences is funding two additional satellites as well as the Canadian Space Agency resulting in six components for the constellation. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kaiser A.,Institute For Astronomie | Weiss W.W.,Institute For Astronomie
Astronomische Nachrichten | Year: 2012

The task of accurately determining the fundamental parameters of a star from medium or low resolution spectra is time consuming and requires substantial experience. Since the first large-scale space missions such as CoRoT or Kepler came along, the need for a fast and statistically sound method emerged to cope with the large amount of ground based support data. We present here a fully automated approach for the determination of stellar parameters from low-resolution stellar spectra using a Bayesian approach and discuss the reliability and limits of the method. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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