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Lagarde N.,University of Geneva | Decressin T.,University of Geneva | Charbonnel C.,University of Geneva | Charbonnel C.,CNRS Astrophysics and Planetology Research Institute | And 3 more authors.
Astronomy and Astrophysics | Year: 2012

Context. The availability of asteroseismic constraints for a large sample of stars from the missions CoRoT and Kepler paves the way for various statistical studies of the seismic properties of stellar populations. Aims. We evaluate the impact of rotation-induced mixing and thermohaline instability on the global asteroseismic parameters at different stages of the stellar evolution from the zero age main sequence to the thermally pulsating asymptotic giant branch to distinguish stellar populations. Methods. We present a grid of stellar evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and 0.014) in the mass range from 0.85 to 6.0 M·. The models are computed either with standard prescriptions or including both thermohaline convection and rotation-induced mixing. For the whole grid, we provide the usual stellar parameters (luminosity, effective temperature, lifetimes,..), together with the global seismic parameters, i.e. the large frequency separation and asymptotic relations, the frequency corresponding to the maximum oscillation power νmax, the maximal amplitude Amax, the asymptotic period spacing of g-modes, and different acoustic radii. Results. We discuss a signature of rotation-induced mixing on the global asteroseismic quantities, that can be detected observationally. Thermohaline mixing whose effects can be identified using spectroscopic studies cannot be characterized by the global seismic parameters studied here. However, we cannot exclude that individual mode frequencies or other well chosen asteroseismic quantities might help us to constrain this mixing. © 2012 ESO.

Gregory S.G.,California Institute of Technology | Donati J.-F.,CNRS Astrophysics and Planetology Research Institute | Morin J.,University of Gottingen | Morin J.,Dublin Institute for Advanced Studies | And 4 more authors.
Astrophysical Journal | Year: 2012

Zeeman-Doppler imaging studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (H-R) diagrams for the stars in the sample. Intriguingly, the large-scale field topology of a given pre-main-sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we argue that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the H-R diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the H-R diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core mass in excess of 40% of the stellar mass, host highly complex and dominantly non-axisymmetric magnetic fields, while those with smaller radiative cores host axisymmetric fields with field modes of higher order than the dipole dominant (typically, but not always, the octupole). Fully convective stars above ≳ 0.5 M appear to host dominantly axisymmetric fields with strong (kilo-Gauss) dipole components. Based on similarities between the magnetic properties of PMS stars and main-sequence M-dwarfs with similar internal structures, we speculate that a bistable dynamo process operates for lower mass stars (≲ 0.5 M at an age of a few Myr) and that they will be found to host a variety of magnetic field topologies. If the magnetic topology trends across the H-R diagram are confirmed, they may provide a new method of constraining PMS stellar evolution models. © 2012. The American Astronomical Society. All rights reserved.

Namur O.,University of Liege | Charlier B.,University of Liege | Toplis M.J.,CNRS Astrophysics and Planetology Research Institute | Higgins M.D.,University of Quebec at Chicoutimi | And 3 more authors.
Journal of Petrology | Year: 2011

The undeformed 564 Ma Sept Iles layered intrusion (Quebec, Canada) is a large igneous body of c. 20 000 km3. From the base to the top, it consists of a Layered Series dominated by troctolite and gabbro, an anorthositic Upper Border Series and a dominantly granitic Upper Series. The parent magma of the Layered Series is inferred to be an iron-rich tholeiitic basalt (48 wt % SiO2; 15 wt % FeOt). Whole-rock compositions from the chilled margin, dykes cross-cutting the Layered Series and silicic rocks from the Upper Series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous A-type granites (77 wt % SiO2). Initial 143Nd/144Nd (0·51201-0·51207) and 87Sr/86Sr (0·70353-0·70548) indicate a juvenile-mantle source and minimal contamination by old crust (1-2%) during crystallization. Geochemical modeling, using the MELTS thermodynamic calculator combined with equations predicting mineral-melt equilibria from experiments on tholeiitic basalts, indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the Layered Series. Fe-Ti oxides saturate early in the Layered Series, after 14% fractionation of plagioclase-olivine cumulates. Further fractionation of Fe-Ti oxide-bearing gabbros drives the residual liquids toward silica enrichment and iron depletion. Major and trace element modeling indicates that the A-type granites from the Upper Series were produced by protracted fractional crystallization of an iron-rich basaltic parent magma, at a fraction of residual liquid of only 8%. The observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations. Most of the intermediate compositions correspond to magmatic mafic enclave-bearing granitoids and display geochemical evidence of hybridization. Intermediate compositions produced by fractional crystallization are scarce and a Daly gap occurs from 57 to 67 wt % SiO2. This gap could result either from the fractional crystallization process or from silicate-liquid immiscibility during that compositional interval. © The Author 2011. Published by Oxford University Press. All rights reserved.

Chevrot S.,CNRS Astrophysics and Planetology Research Institute | Martin R.,University Paul Sabatier | Komatitsch D.,Aix - Marseille University
Geophysical Journal International | Year: 2012

Wavelets are extremely powerful to compress the information contained in finite-frequency sensitivity kernels and tomographic models. This interesting property opens the perspective of reducing the size of global tomographic inverse problems by one to two orders of magnitude. However, introducing wavelets into global tomographic problems raises the problem of computing fast wavelet transforms in spherical geometry. Using a Cartesian cubed sphere mapping, which grids the surface of the sphere with six blocks or 'chunks', we define a new algorithm to implement fast wavelet transforms with the lifting scheme. This algorithm is simple and flexible, and can handle any family of discrete orthogonal or bi-orthogonal wavelets. Since wavelet coefficients are local in space and scale, aliasing effects resulting from a parametrization with global functions such as spherical harmonics are avoided. The sparsity of tomographic models expanded in wavelet bases implies that it is possible to exploit the power of compressed sensing to retrieve Earth's internal structures optimally. This approach involves minimizing a combination of a ℓ 2 norm for data residuals and a ℓ 1 norm for model wavelet coefficients, which can be achieved through relatively minor modifications of the algorithms that are currently used to solve the tomographic inverse problem. © 2012 The Authors Geophysical Journal International © 2012 RAS.

Maeder A.,University of Geneva | Meynet G.,University of Geneva | Lagarde N.,University of Birmingham | Charbonnel C.,University of Geneva | Charbonnel C.,CNRS Astrophysics and Planetology Research Institute
Astronomy and Astrophysics | Year: 2013

Aims. We examine the interactions of various instabilities in rotating stars, which usually are considered as independent. Methods. An analytical study of the problem is performed accounting for radiative losses, μ-gradients, and horizontal turbulence. Results. The diffusion coefficient for an ensemble of instabilities is not given by the sum of the specific coefficients for each instability, but by the solution of a general equation. We find that thermohaline mixing is possible in low-mass red giants only if the horizontal turbulence is very weak. In rotating stars the Rayleigh-Taylor and the shear instabilities need simultaneous treating. This leads to rotation laws of the form 1/rα being predicted to be unstable for α > 1.6568, while the usual Rayleigh criterion only predicts instability for α > 2. Also, the shear instabilities are somehow reduced in main sequence stars by the effect of the Rayleigh-Taylor criterion. Various instability criteria should be expressed differently in rotating stars than in simplified geometries. © 2013 ESO.

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