Long-term & large-scale viscous evolution of dense planetary rings

We investigate the long-term and large-scale viscous evolution of dense planetary rings using a simple 1D numerical code. We use a physically realistic viscosity model derived from N-body simulations (Daisaka et al., 2001), and dependent on the disk's local properties (surface mass density, particle size, distance to the planet). Particularly, we include the effects of gravitational instabilities (wakes) that importantly enhance the disk's viscosity. We show that common estimates of the disk's spreading time-scales with constant viscosity significantly underestimate the rings' lifetime. With a realistic viscosity model, an initially narrow ring undergoes two successive evolutionary stages: (1) a transient rapid spreading when the disk is self-gravitating, with the formation of a density peak inward and an outer region marginally gravitationally stable, and with an emptying time-scale proportional to 1/M_0^2 (where M_0 is the disk's initial mass) (2) an asymptotic regime where the spreading rate continuously slows down as larger parts of the disk become not-self-gravitating due to the decrease of the surface density, until the disk becomes completely not-self-gravitating. At this point its evolution dramatically slows down, with an emptying time-scale proportional to 1/M_0, which significantly increases the disk's lifetime compared to the case with constant viscosity. We show also that the disk's width scales like t^{1/4} with the realistic viscosity model, while it scales like t^{1/2} in the case of constant viscosity, resulting in much larger evolutionary time-scales in our model. We find however that the present shape of Saturn's rings looks like a 100 million-years old disk in our simulations. Concerning Jupiter's, Uranus' and Neptune's rings that are faint today, it is not likely that they were much more massive in the past and lost most of their mass due to viscous spreading alone.Comment: 18 pages, 18 figures, 2 tables. Accepted for publication in Icaru

Velocity profiles in shear-banding wormlike micelles

Using Dynamic Light Scattering in heterodyne mode, we measure velocity profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to exhibit both shear-banding and stress plateau behavior. Our data provide evidence for the simplest shear-banding scenario, according to which the effective viscosity drop in the system is due to the nucleation and growth of a highly sheared band in the gap, whose thickness linearly increases with the imposed shear rate. We discuss various details of the velocity profiles in all the regions of the flow curve and emphasize on the complex, non-Newtonian nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let

Shear-banding in a lyotropic lamellar phase, Part 2: Temporal fluctuations

We analyze the temporal fluctuations of the flow field associated to a shear-induced transition in a lyotropic lamellar phase: the layering transition of the onion texture. In the first part of this work [Salmon et al., submitted to Phys. Rev. E], we have evidenced banded flows at the onset of this shear-induced transition which are well accounted for by the classical picture of shear-banding. In the present paper, we focus on the temporal fluctuations of the flow field recorded in the coexistence domain. These striking dynamics are very slow (100--1000s) and cannot be due to external mechanical noise. Using velocimetry coupled to structural measurements, we show that these fluctuations are due to a motion of the interface separating the two differently sheared bands. Such a motion seems to be governed by the fluctuations of $\sigma^\star$, the local stress at the interface between the two bands. Our results thus provide more evidence for the relevance of the classical mechanical approach of shear-banding even if the mechanism leading to the fluctuations of $\sigma^\star$ remains unclear

Determining the metallicity of the solar envelope using seismic inversion techniques

peer reviewedThe solar metallicity issue is a long-lasting problem of astrophysics, impacting multi- ple fields and still subject to debate and uncertainties. While spectroscopy has mostly been used to determine the solar heavy elements abundance, helioseismologists at- tempted providing a seismic determination of the metallicity in the solar convective enveloppe. However, the puzzle remains since two independent groups prodived two radically different values for this crucial astrophysical parameter. We aim at provid- ing an independent seismic measurement of the solar metallicity in the convective enveloppe. Our main goal is to help provide new information to break the current stalemate amongst seismic determinations of the solar heavy element abundance. We start by presenting the kernels, the inversion technique and the target function of the inversion we have developed. We then test our approach in multiple hare-and-hounds exercises to assess its reliability and accuracy. We then apply our technique to solar data using calibrated solar models and determine an interval of seismic measurements for the solar metallicity. We show that our inversion can indeed be used to estimate the solar metallicity thanks to our hare-and-hounds exercises. However, we also show that further dependencies in the physical ingredients of solar models lead to a low accuracy. Nevertheless, using various physical ingredients for our solar models, we determine metallicity values between 0.008 and 0.014

The IACOB project. IV. New predictions for high-degree non-radial mode instability domains in massive stars and their connection with macroturbulent broadening

Context. Asteroseismology is a powerful tool to access the internal structure of stars. Apart from the important impact of theoretical developments, progress in this field has been commonly associated with the analysis of time-resolved observations. Recently, the so-called macroturbulent broadening has been proposed as a complementary and less expensive way - in terms of observational time - to investigate pulsations in massive stars. Aims: We assess to what extent this ubiquitous non-rotational broadening component which shapes the line profiles of O stars and B supergiants is a spectroscopic signature of pulsation modes driven by a heat mechanism. Methods: We compute stellar main-sequence and post-main-sequence models from 3 to 70 M[SUB]⊙[/SUB] with the ATON stellar evolution code, and determine the instability domains for heat-driven modes for degrees ℓ = 1-20 using the adiabatic and non-adiabatic codes LOSC and MAD. We use the observational material compiled in the framework of the IACOB project to investigate possible correlations between the single snapshot line-broadening properties of a sample of ≈260 O and B-type stars and their location inside or outside the various predicted instability domains. Results: We present an homogeneous prediction for the non-radial instability domains of massive stars for degree ℓ up to 20. We provide a global picture of what to expect from an observational point of view in terms of the frequency range of excited modes, and we investigate the behavior of the instabilities with respect to stellar evolution and the degree of the mode. Furthermore, our pulsational stability analysis, once compared to the empirical results, indicates that stellar oscillations originated by a heat mechanism cannot explain alone the occurrence of the large non-rotational line-broadening component commonly detected in the O star and B supergiant domain. Based on observations made with the Nordic Optical Telescope, operated by NOTSA, and the Mercator Telescope, operated by the Flemish Community, both at the Observatorio del Roque de los Muchachos (La Palma, Spain) of the Instituto de Astrofísica de Canarias

Migration of a moonlet in a ring of solid particles : Theory and application to Saturn's propellers

Hundred meter sized objects have been identified by the Cassini spacecraft in Saturn's A ring through the so-called "propeller" features they create in the ring. These moonlets should migrate, due to their gravitational interaction with the ring ; in fact, some orbital variation have been detected. The standard theory of type I migration of planets in protoplanetary disks can't be applied to the ring system, as it is pressureless. Thus, we compute the differential torque felt by a moonlet embedded in a two-dimensional disk of solid particles, with flat surface density profile, both analytically and numerically. We find that the corresponding migration rate is too small to explain the observed variations of the propeller's orbit in Saturn's A-ring. However, local density fluctuations (due to gravity wakes in the marginally gravitationally stable A-ring) may exert a stochastic torque on a moonlet. Our simulations show that this torque can be large enough to account for the observations, depending on the parameters of the rings. We find that on time scales of several years the migration of propellers is likely to be dominated by stochastic effects (while the former, non-stochastic migration dominates after ~ 10^{4-5} years). In that case, the migration rates provided by observations so far suggests that the surface density of the A ring should be of the order of 700 kg/m^2. The age of the propellers shouldn't exceed 1 to 100 million years, depending on the dominant migration regime.Comment: 17 pages, 5 figures, submitted to Astronomical Journal on february, the 23

Charge conserving FEM-PIC schemes on general grids

This version is a complete rewriting of the first version submitted in 2008.In this article we aim at proposing a general mathematical formulation for charge conserving finite elements Maxwell solvers coupled with particle schemes. In particular, we identify the finite-element continuity equations that must be satisfied by the discrete current sources for several classes of time domain Vlasov-Maxwell simulations to preserve the Gauss law at each time step, and propose a generic algorithm for computing such consistent sources. Since our results cover a wide range of schemes (namely curl-conforming finite element methods of arbitrary degree, general meshes in 2 or 3 dimensions, several classes of time discretization schemes, particles with arbitrary shape factors and piecewise polynomial trajectories of arbitrary degree), we believe that they provide a useful roadmap in the design of high order charge conserving FEM-PIC numerical schemes

Sondage stellaire sismique intégral

Aims: We develop a method that provides a comprehensive analysis of the oscillation spectra of solar-like pulsators. We define new seismic indicators that should be as uncorrelated and as precise as possible and should hold detailed information about stellar interiors. This is essential to improve the quality of the results obtained from asteroseismology as it will provide better stellar models which in turn can be used to refine inferences made in exoplanetology and galactic archeology. Method: The presented method – WhoSGlAd – relies on Gram-Schmidt’s orthogonalisation process. A Euclidean vector subspace of functions is defined and the oscillation frequencies are projected over an orthonormal basis in a specific order. This allows the obtention of independent coefficients that we combine to define independent seismic indicators. Results: The developed method has been shown to be stable and to converge efficiently for solar-like pulsators. Thus, detailed and precise inferences can be obtained on the mass, the age, the chemical composition and the undershooting in the interior of the studied stars. However, attention has to be paid when studying the helium glitch as there seems to be a degeneracy between the influence of the helium abundance and that of the heavy elements on the glitch amplitude. As an example, we analyse the 16CygA (HD 186408) oscillation spectrum to provide an illustration of the capabilities of the method.Thèse de doctorat : New seismic probing method for solar-type stars, red subgiants and g pulsator