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Randall S.K.,ESO | Fontaine G.,University of Montreal | Geier S.,ESO | Van Grootel V.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | Brassard P.,University of Montreal
Astronomy and Astrophysics | Year: 2014

We present an analysis of time-resolved spectrophotometry gathered with FORS/VLT for the rapidly pulsating hot B subdwarf EC 01541-1409 with the aim of identifying the degree index ℓ of the larger amplitude modes. This mode identification can be extremely useful in detailed searches for viable asteroseismic models in parameter space, and can be crucial for testing the validity of a solution a posteriori. To achieve it, we exploit the ℓ-dependence of the monochromatic amplitude, phase, and velocity-to- amplitude ratio of a mode as a function of wavelength. We use the ℓ-sensitive phase lag between the flux perturbation and the radial velocity as an additional diagnostic tool. On this basis, we are able to unambiguously identify the dominant 140.5 s pulsation of our target as a radial mode, and the second-highest amplitude periodicity at 145.8 s as an ℓ = 2 mode. We further exploit the exceptionally high-sensitivity data that we gathered for the dominant mode to infer modal properties that are usually quite difficult to estimate in sdB pulsators, namely the physical values of the dimensionless radius, temperature, and surface gravity perturbations. © ESO, 2014. Source

Saesen S.,Observatoire de Geneva | Saesen S.,Catholic University of Leuven | Briquet M.,Catholic University of Leuven | Briquet M.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | And 4 more authors.
Astronomical Journal | Year: 2013

Recent progress in the seismic interpretation of field β Cep stars has resulted in improvements of the physical description in the stellar structure and evolution model computations of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance, and chemical composition. We present an observational asteroseismology study based on the discovery of numerous multi-periodic and mono-periodic B stars in the open cluster NGC 884. We describe a thorough investigation of the pulsational properties of all B-type stars in the cluster. Overall, our detailed frequency analysis resulted in 115 detected frequencies in 65 stars. We found 36 mono-periodic, 16 bi-periodic, 10 tri-periodic, and 2 quadru-periodic stars and one star with nine independent frequencies. We also derived the amplitudes and phases of all detected frequencies in the U, B, V, and I filter, if available. We achieved unambiguous identifications of the mode degree for 12 of the detected frequencies in nine of the pulsators. Imposing the identified degrees and measured frequencies of the radial, dipole, and quadrupole modes of five pulsators led to a seismic cluster age estimate of log (age/yr) = 7.12-7.28 from a comparison with stellar models. Our study is a proof-of-concept for and illustrates the current status of ensemble asteroseismology of a young open cluster. © 2013. The American Astronomical Society. All rights reserved. Source

Girardi L.,National institute for astrophysics | Eggenberger P.,Observatoire de Geneva | Miglio A.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

Many intermediate-age star clusters in the Magellanic Clouds present multiple main-sequence turn-offs (MMSTOs), which challenge the classical idea that star formation in such objects took place over short time-scales. It has been recently suggested that the presence of fast rotators among main-sequence stars could be the cause of such features, hence relaxing the need for extended periods of star formation. In this Letter, we compute evolutionary tracks and isochrones of models with and without rotation. We find that, for the same age and input physics, both kinds of models present turn-offs with an almost identical position in the colour-magnitude diagrams (CMDs). As a consequence, a dispersion of rotational velocities in coeval ensembles of stars could not explain the presence of MMSTOs. We construct several synthetic CMDs for the different kinds of tracks and combinations of them. The models that best reproduce the morphology of observed MMSTOs are clearly those assuming a significant spread in the stellar ages - as long as ~400 Myr - added to a moderate amount of convective core overshooting. Only these models produce the detailed 'golf club' shape of observed MMSTOs. A spread in rotational velocities alone cannot do anything similar. We also discuss models involving a mixture of stars with and without overshooting, as an additional scenario to producing MMSTOs with coeval populations. We find that they produce turn-offs with a varying extension in the CMD direction perpendicular to the lower main sequence, which are clearly not present in observed MMSTOs. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Bouabid M.-P.,University of Nice Sophia Antipolis | Bouabid M.-P.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | Dupret M.-A.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | Salmon S.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | And 3 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

γ Doradus stars pulsate with high-order gravity modes having typical frequencies which can be comparable to or higher than their rotation frequencies. Therefore, rotation has a nonnegligible effect on their oscillation properties. To explore the rotation-pulsation coupling in γ Dor stars, we perform a non-adiabatic study including the traditional approximation of rotation on a grid of spherical stellar models covering the mass range 1.4 < M* < 2.1M⊙. This approximation allows us to treat the effect of the Coriolis force on the frequencies and the stability of high-order g modes. The effect of the Coriolis force depends on the kind of mode considered (prograde sectoral or not) and increases with their periods. As a consequence, we first find that the period spacing between modes is no longer periodically oscillating around a constant value. Secondly, we showthat the frequency gap (5-15 cycles day-1) arising from stablemodes between γ Dor-type high-order g modes and δ Scuti-type modes can be easily filled by g-mode frequencies shifted to higher values by the rotation. Thirdly, we analyse the combined effect of diffusive mixing and the Coriolis force on the period spacings. And finally, we predict a slight broadening of the γ Dor instability strip. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Ouazzani R.-M.,University Pierre and Marie Curie | Ouazzani R.-M.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | Dupret M.-A.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege | Reese D.R.,Institute Dastrophysique Et Of Geophysique Of Luniversite Of Liege
Astronomy and Astrophysics | Year: 2012

Context. Very high precision seismic space missions such as CoRoT and Kepler provide the means of testing the modeling of transport processes in stellar interiors. For some stars, such as solar-like and red giant stars, a rotational splitting is measured. However, to fully exploit these splittings and constrain the rotation profile, one needs to be able to calculate them accurately. For some other stars, such as δ Scuti and Be stars, for instance, the observed pulsation spectra are modified by rotation to such an extent that a perturbative treatment of the effects of rotation is no longer valid. Aims. We present here a new two-dimensional non-perturbative code called ACOR (adiabatic code of oscillation including rotation) that allows us to compute adiabatic non-radial pulsations of rotating stars without making any assumptions on the sphericity of the star, the fluid properties (i.e., baroclinicity) or the rotation profile. Methods. The 2D non-perturbative calculations fully take into account the centrifugal distortion of the star and include the full influence of the Coriolis acceleration. The numerical method is based on a spectral approach for the angular part of the modes and a fourth-order finite differences approach for the radial part. Results. We test and evaluate the accuracy of the calculations by comparing them with those coming from the TOP (two-dimensional oscillation program) for the same polytropic models. We illustrate the effects of rapid rotation on stellar pulsations through the phenomenon of avoided crossings. Conclusions. As shown by the comparison with the TOP for simple models, the code is stable, and gives accurate results up to near-critical rotation rates. © 2012 ESO. Source

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