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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.

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.

Lavraud B.,CNRS Astrophysics and Planetology Research Institute | Larson D.E.,University of California at Berkeley
Journal of Geophysical Research A: Space Physics | Year: 2016

We first introduce previous works on spacecraft electrostatic charging and its effects on particle measurements and the calculation of moments from three-dimensional distribution functions. We illustrate the fact that the lack of use, or misuse, of Liouville's theorem may lead to misinterpretations and inappropriate corrections to those effects. We emphasize in particular that its appropriate use naturally accounts for what is often called the "sheath focusing effect" in moment calculation. In the case of a "perfect" particle detector we show that there exists a trivial and essentially exact formulation for the calculation of moments of particle distribution functions that accounts for the spacecraft potential and that is particularly useful for onboard moment calculations. The main limitations, but that are not specific to this formulation, are inaccuracies in angle information (from blurring of acceptance angles at low energies or spacecraft-skimming trajectory effects) and those arising from the detector properties (resolution, cutoffs, photoelectrons, etc.). We discuss this correction in the context of previous works and remind that it primarily affects low-energy measurements and for populations whose temperature is comparable to the potential. Based on spacecraft observations, we show that this correction is most needed for regimes such as that of solar wind electrons but also for cold ions as often observed, for instance, in the magnetosphere near the Earth's magnetopause. ©2016. American Geophysical Union.

Monteiller V.,CNRS Astrophysics and Planetology Research Institute | Monteiller V.,Aix - Marseille University | Chevrot S.,CNRS Astrophysics and Planetology Research Institute | Komatitsch D.,Aix - Marseille University | Fuji N.,CNRS Astrophysics and Planetology Research Institute
Geophysical Journal International | Year: 2013

We present a hybrid method to simulate the propagation of short-period teleseismic bodywaves through 3-D regional models. The incident wavefield is computed in an spherically symmetric reference earth model based on the direct solution method. The global and regional wavefields are matched at the boundaries of the regional mesh. In the regional domain, we implement a spectral-element method with absorbing boundaries to cancel the outgoing scatteredwavefield. The hybrid method is successfully benchmarked against the direct solution method in the reference earth model iasp91. The potential of the method is illustrated by computing shortperiod P-wave synthetic seismograms in a 3-D model with a 20 km Moho offset and/or topography on the free surface, focusing on the actual example of the Pyrenees. © The Authors 2012. Published by Oxford University Press on behalf of The Royal Astronomical Society.

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.

Chevrel M.O.,Ludwig Maximilians University of Munich | Baratoux D.,CNRS Astrophysics and Planetology Research Institute | Hess K.-U.,Ludwig Maximilians University of Munich | Dingwell D.B.,Ludwig Maximilians University of Munich
Geochimica et Cosmochimica Acta | Year: 2014

The chemical compositions of martian basalts are enriched in iron with respect to terrestrial basalts. Their rheology is poorly known and liquids of this chemical composition have not been experimentally investigated. Here, we determine the viscosity of five synthetic silicate liquids having compositions representative of the diversity of martian volcanic rocks including primary martian mantle melts and alkali basalts. The concentric cylinder method has been employed between 1500. °C and the respective liquidus temperatures of these liquids. The viscosity near the glass transition has been derived from calorimetric measurements of the glass transition. Although some glass heterogeneity limits the accuracy of the data near the glass transition, it was nevertheless possible to determine the parameters of the non-Arrhenian temperature-dependence of viscosity over a wide temperature range (1500. °C to the glass transition temperature). At superliquidus conditions, the martian basalt viscosities are as low as those of the Fe-Ti-rich lunar basalts, similar to the lowest viscosities recorded for terrestrial ferrobasalts, and 0.5 to 1 order of magnitude lower than terrestrial tholeiitic basalts. Comparison with empirical models reveals that Giordano et al. (2008) offers the best approximation, whereas the model proposed by Hui and Zhang (2007) is inappropriate for the compositions considered.The slightly lower viscosities exhibited by the melts produced by low degree of mantle partial melting versus melts produced at high degree of mantle partial melting (likely corresponding to the early history of Mars), is not deemed sufficient to lead to viscosity variations large enough to produce an overall shift of martian lava flow morphologies over time. Rather, the details of the crystallization sequence (and in particular the ability of some of these magmas to form spinifex texture) is proposed to be a dominant effect on the viscosity during martian lava flow emplacement and may explain the lower range of viscosities (102-104Pas) inferred from lava flow morphology. Further, the differences between the rheological behaviors of tholeiitic vs. trachy-basalts are significant enough to affect their emplacement as intrusive bodies or as effusive lava flows. The upper range of viscosities (106-108Pas) suggested from lava flow morphology is found consistent with the occurrence of alkali basalt documented from in situ analyses and does not necessarily imply the occurrence of basaltic-andesite or andesitic rocks. © 2013 The Authors.

Namur O.,University of Liège | Charlier B.,University of Liège | 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.

Gregory S.G.,California Institute of Technology | Donati J.-F.,CNRS Astrophysics and Planetology Research Institute
Astronomische Nachrichten | Year: 2011

Traditionally models of accretion of gas on to T Tauri stars have assumed a dipole stellar magnetosphere, partly for simplicity, but also due to the lack of information about their true magnetic field topologies. Before and since the first magnetic maps of an accreting T Tauri star were published in 2007 a new generation of magnetospheric accretion models have been developed that incorporate multipole magnetic fields. Three-dimensional models of the large-scale stellar magnetosphere with an observed degree of complexity have been produced via numerical field extrapolation from observationally derived T Tauri magnetic maps. Likewise, analytic and magnetohydrodynamic models with multipolar stellar magnetic fields have been produced. In this conference review article we compare and contrast the numerical field extrapolation and analytic approaches, and argue that the large-scale magnetospheres of some (but not all) accreting T Tauri stars can be well described by tilted dipole plus tilted octupole field components. We further argue that the longitudinal field curve, whether derived from accretion related emission lines, or from photospheric absorption lines, provides poor constrains on the large-scale magnetic field topology and that detailed modeling of the rotationally modulated Stokes V signal is required to recover the true field complexity. We conclude by examining the advantages, disadvantages and limitations of both the field extrapolation and analytic approaches, and also those of magnetohydrodynamic models. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Morin J.,Dublin Institute for Advanced Studies | Dormy E.,Ecole Normale Superieure de Paris | Schrinner M.,Ecole Normale Superieure de Paris | Donati J.-F.,CNRS Astrophysics and Planetology Research Institute
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

Observations of magnetism in very low mass stars recently made important progress, revealing characteristics that are now to be understood in the framework of dynamo theory. In parallel, there is growing evidence that dynamo processes in these stars share many similarities with planetary dynamos. We investigate the extent to which the weak-field versus strong-field bistability predicted for the geodynamo can apply to recent observations of two groups of very low mass fully-convective stars sharing similar stellar parameters but generating radically different types of magnetic fields. Our analysis is based on previously published spectropolarimetric and spectroscopic data. We argue that these can be interpreted in the framework of weak- and strong-field dynamos. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

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