Are you V. Prat?

Claim your profile

Publications (12)20.34 Total impact

  • Source
    Vincent Prat, François Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: Turbulent transport of chemical elements in radiative zones of stars is taken into account in current stellar evolution codes thanks to phenomenologically derived diffusion coefficients. Recent local numerical simulations (Prat & Ligni\`eres 2013, A&A, 551, L3) suggest that the coefficient for radial turbulent diffusion due to radial differential rotation satisfies $D_{\rm t}\simeq0.058\kappa/Ri$, in qualitative agreement with Zahn's model. However, this model does not apply when differential rotation is strong with respect to stable thermal stratification or when chemical stratification has a significant dynamical effect, a situation encountered at the outer boundary of nuclear burning convective cores. We extend our numerical study to consider the effects of chemical stratification and of strong shear, and compare with prescriptions used in stellar evolution codes. We perform local, direct numerical simulations of stably stratified, homogeneous, sheared turbulence in the Boussinesq approximation. The regime of large thermal diffusivities, typical of stellar radiative zones, is reached thanks to the so-called small-P\'eclet-number approximation, which is an asymptotic development of the Boussinesq equations in this regime. The dependence of the diffusion coefficient with respect to chemical stratification is explored in this approximation. Maeder's extension of Zahn's model in the strong-shear regime is not supported by our results, which are better described by a model found in the geophysical literature. As regards the effect of chemical stratification, our quantitative estimate of the diffusion coefficient as a function of the mean gradient of mean molecular weight leads to the formula $D_{\rm t}\simeq 0.45\kappa(0.12-Ri_\mu)/Ri$, which is compatible in the weak-shear regime with the model of Maeder & Meynet (1996, A&A, 313, 140).
    04/2014;
  • Vincent Prat, François Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: Turbulent transport of chemical elements in radiative zones of stars is considered in current stellar evolution codes thanks to phenomenologically derived diffusion coefficients. Recent local numerical simulations (Prat & Ligni\`eres 2013, A&A;, 551, L3) suggest that the coefficient for radial turbulent diffusion due to radial differential rotation satisfies $D_{\rm t}\simeq0.058\kappa/Ri$, in qualitative agreement with Zahn's model. However, this model does not apply when differential rotation is strong with respect to stable thermal stratification or when chemical stratification has a significant dynamical effect, a situation encountered at the outer boundary of nuclear-burning convective cores. We extend our numerical study to consider the effects of chemical stratification and of strong shear, and compare the results with prescriptions used in stellar evolution codes. We performed local, direct numerical simulations of stably stratified, homogeneous, sheared turbulence in the Boussinesq approximation. The regime of high thermal diffusivities, typical of stellar radiative zones, is reached thanks to the so-called small-P\'eclet-number approximation, which is an asymptotic development of the Boussinesq equations in this regime. The dependence of the diffusion coefficient on chemical stratification was explored in this approximation. Maeder's extension of Zahn's model in the strong-shear regime is not supported by our results, which are better described by a model found in the geophysical literature. As regards the effect of chemical stratification, our quantitative estimate of the diffusion coefficient as a function of the mean gradient of mean molecular weight leads to the formula $D_{\rm t}\simeq 0.45\kappa(0.12-Ri_\mu)/Ri$, which is compatible in the weak-shear regime with the model of Maeder & Meynet (1996, A&A;, 313, 140).
    03/2014;
  • V. Prat, F. Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: One of the key issues of stellar evolution theory is the influence of the transport processes related to rotationally driven macroscopic motions on the internal structure and the evolution of stars. Turbulent mixing of chemical elements due to differential rotation in stellar radiative zones is currently taken into account in many stellar evolution codes through transport coefficients firstly derived by Zahn (1992, A&A, 265, 115). The purpose of our work is to constrain one of these coefficients, the radial diffusion coefficient driven by radial differential rotation through local direct numerical simulations of steady homogeneous stably stratified sheared turbulence, and to compare the results with phenomenological models. In particular, we have determined the dependence of the turbulent diffusion coefficient on thermal diffusivity and chemical stratification.
    EAS Publications Series 12/2013;
  • V. Prat, F. Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: A major issue of stellar evolution theory is the influence of the transport processes related to rotationally driven macroscopic motions on the internal structure and the evolution of stars. Turbulent mixing of chemical elements due to differential rotation in stellar radiative zones is currently taken into account in many stellar evolution codes through transport coefficients firstly derived by Zahn (1992, A&A, 265, 115). Our aim is to constrain one of these coefficients, the radial diffusion coefficient driven by radial differential rotation through local direct numerical simulations of steady homogeneous stably stratified sheared turbulence, and to compare the results with phenomenological models. In particular, we have determined the dependence of the turbulent diffusion coefficient on thermal diffusivity and chemical stratification.
    11/2013;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Context. The measurement of the Sun's surface motions with a high spatial and temporal resolution is still a challenge. Aims: We wish to validate horizontal velocity measurements all over the visible disk of the Sun from Solar Dynamics Observatory/ Helioseismic and Magnetic Imager (SDO/HMI) data. Methods: Horizontal velocity fields are measured by following the proper motions of solar granules using a newly developed version of the coherent structure tracking (CST) code. The comparison of the surface flows measured at high spatial resolution (Hinode, 0.1 arcsec) and low resolution (SDO/HMI, 0.5 arcsec) allows us to determine corrections to be applied to the horizontal velocity measured from HMI white light data. Results: We derive horizontal velocity maps with spatial and temporal resolutions of respectively 2.5 Mm and 30 min. From the two components of the horizontal velocity vx and vy measured in the sky plane and the simultaneous line of sight component from SDO/HMI dopplergrams vD, we derive the spherical velocity components (vr, vθ, vϕ). The azimuthal component vϕ gives the solar differential rotation with a high precision (± 0.037 km s-1) from a temporal sequence of only three hours. Conclusions: By following the proper motions of the solar granules, we can revisit the dynamics of the solar surface at high spatial and temporal resolutions from hours to months and years with the SDO data.
    Astronomy and Astrophysics 04/2013; · 5.08 Impact Factor
  • Source
    Vincent Prat, François Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: Context. In stellar interiors, rotation is able to drive turbulent motions, and the related transport processes have a significant influence on the evolution of stars. Turbulent mixing in the radiative zones is currently taken into account in stellar evolution models through a set of diffusion coefficients that are generally poorly constrained. Aims. We want to constrain the form of one of them, the radial diffusion coefficient of chemical elements due to the turbulence driven by radial differential rotation, derived by Zahn (1974, 1992) on phenomenological grounds and largely used since. Methods. We performed local, direct numerical simulations of stably stratified homogeneous sheared turbulence using the Boussinesq approximation. The domain of low P\'eclet numbers found in stellar interiors is currently inaccessible to numerical simulations without approximation. It is explored here thanks to a suitable asymptotic form of the Boussinesq equations. The turbulent transport of a passive scalar is determined in statistical steady states. Results. We provide a first quantitative determination of the turbulent diffusion coefficient and find that the form proposed by Zahn is in good agreement with the results of the numerical simulations.
    Astronomy and Astrophysics 01/2013; · 5.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Context: Mode identification is a crucial step to comparing observed frequencies with theoretical ones, but has proven to be difficult in rapidly rotating stars. Aims: To further constrain mode identification, we aim to accurately calculate mode visibilities and amplitude ratios in rapid rotators. Methods: We derive the relevant equations for calculating mode visibilities in different photometric bands while fully taking into account the geometric distortion from both the centrifugal deformation and the pulsation modes, the variations in effective gravity, and an approximate treatment of the temperature variations. These equations are then applied to 2D oscillation modes, calculated using the TOP code, in fully distorted 2D models based on the SCF method. The specific intensities come from a grid of Kurucz atmospheres, thereby taking into account limb and gravity darkening. Results: We obtain mode visibilities and amplitude ratios for 2 M_{\odot} models rotating at 0 to 80 % of the critical rotation rate. These calculations confirm previous results, such as the increased visibility of chaotic modes, the simpler frequency spectra of pole-on stars, or the dependence of amplitude ratios on inclination and azimuthal order. In addition, the geometric shape of the star reduces the contrast between pole-on and equator-on visibilities of island modes. We also show that modes with similar (ell, |m|) values frequently have similar amplitude ratios, even in the most rapidly rotating models.
    Astronomy and Astrophysics 12/2012; · 5.08 Impact Factor
  • V. Prat, F. Lignières
    [Show abstract] [Hide abstract]
    ABSTRACT: Macroscopic and turbulent motions created by rotation can significantly affect internal structure and evolution of stars in a way that is currently not well understood. In particular, turbulent mixing coefficients that are currently taken into account in many stellar evolution codes have been derived on phenomenological arguments and have not been tested through numerical simulations or laboratory experiments. Our purpose is to test the vertical turbulent diffusion coefficient generated by radial differential rotation of Zahn (1992) with 3D local direct numerical simulations of stably-stratified homogeneous stationary sheared turbulence. To reach the high thermal diffusivity regime found in stellar radiative zones, we use an asymptotic development of the Boussinesq equations in the domain of the small Péclet numbers. We present the results of simulations performed at different turbulent Péclet numbers (including one within the small-Péclet-number approximation). These results show that the form of the vertical turbulent diffusion coefficient initially proposed by Zahn is valid for Péclet numbers smaller than one. We also give a first quantitative estimate of this coefficient through numerical simulations.
    12/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Rotation makes oscillation spectra of rapidly rotating stars much more complicated. Hence, new strategies need to be developed in order to interpret such spectra. In what follows, we describe how multi-colour photometric mode visibilities can be generalised to rapidly rotating stars, while fully taking into account centrifugal deformation and gravity darkening. We then go on to describe some first results as well as a strategy for constraining mode identification.
    12/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present complete 2D computations of g-modes in distorted polytropic models of stars performed with the Two-dimensional Oscillation Program (TOP). We computed low-degree modes (ℓ =1 modes with radial order n = -1,…,-14, and ℓ = 2, 3 modes with n = -1,…,-5 and -16,…,-20) of a non-rotating model and followed them by slowly increasing the rotation rate up to 70 % of the Keplerian break-up velocity. We use these computations to determine the domain of validity of perturbative methods up to the 3rd order. We study the evolution of the regularities of the spectrum and show quantitative agreement with the traditional approximation for not too large values of the ratio of the rotation rate to the pulsation frequency. We also show the appearance of new types of modes, called “rosette” modes due to their spatial structure. Thanks to the ray theory for gravito-inertial waves that we developed, we can associate these modes with stable periodic rays.
    09/2012;
  • [Show abstract] [Hide abstract]
    ABSTRACT: We first present the important role played by the families of granule (or Tree of Fragmenting granules) in the formation of the photospheric network. Then, we describe the occurence and characteristics of acoustic events (AE), defined as spatially concentrated energy flux, in the quiet Sun. Finally, we present how horizontal velocities obtained from SDO/HMI data are calibrated by using Hinode/SOT observations.
    05/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: For the first time, the motion of granules (solar plasma on the surface on scales larger than 2.5 Mm) has been followed over the entire visible surface of the Sun, using SDO/HMI white-light data. Horizontal velocity fields are derived from image correlation tracking using a new version of the coherent structure tracking algorithm.The spatial and temporal resolutions of the horizontal velocity map are 2.5 Mm and 30 min respectively . From this reconstruction, using the multi-resolution analysis, one can obtain to the velocity field at different scales with its derivatives such as the horizontal divergence or the vertical component of the vorticity. The intrinsic error on the velocity is ~0.25 km/s for a time sequence of 30 minutes and a mesh size of 2.5 Mm.This is acceptable compared to the granule velocities, which range between 0.3 km/s and 1.8 km/s. A high correlation between velocities computed from Hinode and SDO/HMI has been found (85%). From the data we derive the power spectrum of the supergranulation horizontal velocity field, the solar differential rotation, and the meridional velocity.
    Astronomy and Astrophysics 03/2012; · 5.08 Impact Factor