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Grappin R.,LUTH | Grappin R.,Ecole Polytechnique - Palaiseau | Velli M.,California Institute of Technology
Astronomy and Astrophysics | Year: 2012

Context. Coronal loops act as resonant cavities for low-frequency fluctuations that are transmitted from the deeper layers of the solar atmosphere. These fluctuations are amplified in the corona and lead to the development of turbulence that in turn is able to dissipate the accumulated energy, thus heating the corona. However, trapping is not perfect, because some energy leaks down to the chromosphere on a long timescale, limiting the turbulent heating. Aims. We consider the combined effects of turbulence and energy leakage from the corona to the photosphere in determining the turbulent energy level and associated heating rate in models of coronal loops, which include the chromosphere and transition region. Methods. We use a piece-wise constant model for the Alfvén speed in loops and a reduced MHD-shell model to describe the interplay between turbulent dynamics in the direction perpendicular to the mean field and propagation along the field. Turbulence is sustained by incoming fluctuations that are equivalent, in the line-tied case, to forcing by the photospheric shear flows. While varying the turbulence strength, we systematically compare the average coronal energy level and dissipation in three models with increasing complexity: the classical closed model, the open corona, and the open corona including chromosphere (or three-layer model), with the last two models allowing energy leakage. Results. We find that (i) leakage always plays a role. Even for strong turbulence, the dissipation time never becomes much lower than the leakage time, at least in the three-layer model; therefore, both the energy and the dissipation levels are systematically lower than in the line-tied model; (ii) in all models, the energy level is close to the resonant prediction, i.e., assuming an effective turbulent correlation time longer than the Alfvén coronal crossing time; (iii) the heating rate is close to the value given by the ratio of photospheric energy divided by the Alfvén crossing time; (iv) the coronal spectral range is divided in two: an inertial range with 5/3 spectral slope, and a large-scale peak where nonlinear couplings are inhibited by trapped resonant modes; (v) in the realistic three-layer model, the two-component spectrum leads to a global decrease in damping equal to Kolmogorov damping reduced by a factor u rms/V a c where V a c is the coronal Alfvén speed. © 2012 ESO.


Nore C.,CNRS LIMSI | Nore C.,University Paris - Sud | Nore C.,Institut Universitaire de France | Leorat J.,Luth | And 3 more authors.
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2011

It is numerically demonstrated by means of a magnetohydrodynamics code that precession can trigger the dynamo effect in a cylindrical container. When the Reynolds number, based on the radius of the cylinder and its angular velocity, increases, the flow, which is initially centrosymmetric, loses its stability and bifurcates to a quasiperiodic motion. This unsteady and asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. The magnetic field thus generated is unsteady and quadrupolar. These numerical evidences of dynamo action in a precessing cylindrical container may be useful for an experiment now planned at the Dresden sodium facility for dynamo and thermohydraulic studies in Germany. © 2011 American Physical Society.


Zabaydullin O.,Fedorov Institute of Applied Geophysics | Dubau J.,LUTH | Dubau J.,University Paris - Sud
Journal of Physics B: Atomic, Molecular and Optical Physics | Year: 2012

For He-like ions, photoabsorption and photoionization cross-sections of 1s 2 can differ in the resonance region due to possible resonance radiative decays. This difference increases along isoelectronic sequence. As an example, the 2s2p autoionizing state is considered for different atomic elements (Z = 16, 20, 26). For these elements, we use atomic data computed, without radiative damping, by the R-matrix BreitPauli code for 1s 2 and 1s2s photoionization processes, which are available on atomic data websites. Applying Davies and Seaton's (1969 J. Phys. B: At. Mol. Phys. 2 757) radiative damping theory, we insert the damping effect. Comparisons are made with similar data computed by another code, which uses Robicheaux et al's (1995 Phys. Rev. A 52 1319) damping theory, also available on an atomic code website. We also present a very simple approximation of Davies and Seaton's theory, and test it by comparisons of data computed with and without the approximation. © 2012 IOP Publishing Ltd.


Esteban C.,Institute of Astrophysics of Canarias | Esteban C.,University of La Laguna | Carigi L.,National Autonomous University of Mexico | Copetti M.V.F.,Federal University of Santa Maria | And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present deep echelle spectrophotometry of the Galactic Hii region NGC 2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3550-10 400 Å range. This object, which has been largely neglected, shows however a rather high surface brightness, a high ionization degree and is located at a galactocentric distance of 12.4 ± 0.7 kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of the gas phase radial abundance gradients in the outer disc of theMilkyWay. We derive the physical conditions of the nebula using several emission line-intensity ratios as well as the abundances of several ionic species from the intensity of collisionally excited lines. We also determine the ionic abundances of C2+, O+ and O2+ - and therefore the total O abundance - from faint pure recombination lines. The results for NGC 2579 permit to extend our previous determinations of the C, O and C/O gas phase radial gradients of the inner Galactic disc to larger galactocentric distances. We find that the chemical composition of NGC 2579 is consistent with flattened gradients at its galactocentric distance. In addition, we have built a tailored chemical evolution model that reproduces the observed radial abundance gradients of O, C and N and other observational constraints. We find that a levelling out of the star formation efficiency about and beyond the isophotal radius can explain the flattening of chemical gradients observed in the outer Galactic disc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Cappanera L.,University Paris - Sud | Guermond J.-L.,Texas A&M University | Leorat J.,Luth | Nore C.,University Paris - Sud
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2016

It is numerically demonstrated by means of a magnetohydrodynamic code that precession can trigger dynamo action in a cylindrical container. Fixing the angle between the spin and the precession axis to be 12π, two limit configurations of the spinning axis are explored: either the symmetry axis of the cylinder is parallel to the spin axis (this configuration is henceforth referred to as the axial spin case), or it is perpendicular to the spin axis (this configuration is referred to as the equatorial spin case). In both cases, the centro-symmetry of the flow breaks when the kinetic Reynolds number increases. Equatorial spinning is found to be more efficient in breaking the centro-symmetry of the flow. In both cases, the average flow in the reference frame of the mantle converges to a counter-rotation with respect to the spin axis as the Reynolds number grows. We find a scaling law for the average kinetic energy in term of the Reynolds number in the axial spin case. In the equatorial spin case, the unsteady asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. The magnetic field is mainly dipolar in the equatorial spin case, while it is is mainly quadrupolar in the axial spin case. © 2016 American Physical Society.


Monceau-Baroux R.,Catholic University of Leuven | Keppens R.,Catholic University of Leuven | Meliani Z.,LUTh
Astronomy and Astrophysics | Year: 2012

Context. Relativistic jets emerging from active galactic nuclei (AGN) cores transfer energy from the core of the AGN to their surrounding interstellar/intergalactic medium through shock-related and hydrodynamic instability mechanisms. Because jets are observed to have finite opening angles, one needs to quantify the role of conical versus cylindrical jet propagation in this energy transfer. Aims. We adopt parameters representative for Faranoff-Riley class II AGN jets with finite opening angles. We study how such an opening angle affects the overall dynamics of the jet and its interaction with its surrounding medium and therefore how it influences the energy transfer between the AGN and the external medium. We also point out how the characteristics of this external medium, such as its density profile, play a role in the dynamics. Methods. This study exploits our parallel adaptive mesh refinement code MPI-AMRVAC with its special relativistic hydrodynamic model, incorporating an equation of state with varying effective polytropic index. We initially studied mildly underdense jets up to opening angles of 10 degrees, at Lorentz factors of about 10, inspired by input parameters derived from observations. Instantaneous quantifications of the various interstellar medium (ISM) volumes affected by jet injection and their energy content allows one to quantify the role of mixing versus shock-heated cocoon regions over the simulated time intervals. Results. We show that a wider opening angle jet results in a faster deceleration of the jet and leads to a wider radial expansion zone dominated by Kelvin-Helmholtz and Rayleigh-Taylor instabilities. The energy transfer mainly occurs in the shocked ISM region by both the frontal bow shock and cocoon-traversing shock waves, in a roughly 3 to 1 ratio to the energy transfer of the mixing zone, for a 5 degree opening angle jet. The formation of knots along the jet may be related to X-ray emission blobs known from observations. A rarefaction wave induces a dynamically formed layered structure of the jet beam. Conclusions. Finite opening angle jets can efficiently transfer significant fractions (25% up to 70%) of their injected energy over a growing region of shocked ISM matter. The role of the ISM stratification is prominent for determining the overall volume that is affected by relativistic jet injection. While our current 2D simulations give us clear insights into the propagation characteristics of finite opening angle, hydrodynamic relativistic jets, we need to expand this work to 3D. © 2012 ESO.


Monceau-Baroux R.,Catholic University of Leuven | Porth O.,Catholic University of Leuven | Porth O.,University of Leeds | Meliani Z.,LUTh | Keppens R.,Catholic University of Leuven
Astronomy and Astrophysics | Year: 2014

Context. Modern high-resolution radio observations allow us a closer look into the objects that power relativistic jets. This is especially the case for SS433, an X-ray binary that emits a precessing jet that is observed down to the subparsec scale. Aims. We aim to study full 3D dynamics of relativistic jets associated with active galactic nuclei or X-ray binaries (XRB). In particular, we incorporate the precessing motion of a jet into a model for the jet associated with the XRB SS433. Our study of the jet dynamics in this system focuses on the subparsec scales. We investigate the impact of jet precession and the variation of the Lorentz factor of the injected matter on the general 3D jet dynamics and its energy transfer to the surrounding medium. After visualizing and quantifying jet dynamics, we aim to realize synthetic radio mapping of the data, to compare our results with observations. Methods. For our study we used a block-tree adaptive mesh refinement scheme and an inner time-dependent boundary prescription to inject precessing bipolar supersonic jets. Parameters extracted from observations were used. Different 3D jet realizations that match the kinetic flux of the SS433 jet were intercompared, which vary in density contrast and jet beam velocity. We tracked the energy content deposited in different regions of the domain affected by the jet. Our code allows us to follow the adiabatic cooling of a population of relativistic particles injected by the jet. This evolving energy spectrum of accelerated electrons, using a pressure-based proxy for the magnetic field, allowed us to obtain the radio emission from our simulation. Results. We find a higher energy transfer for a precessing jet than for standing jets with otherwise identical parameters as a result of the effectively increased interaction area. We obtain synthetic radio maps for all jets, from which one can see that dynamical flow features are clearly linked with enhanced emission sites. Conclusions. The synthetic radio map best matches a jet model with the canonical propagation speed of 0.26c and a precession angle of 20°. Overdense precessing jets experience significant deceleration in their propagation through the interstellar medium, while the overall jet is of helical shape. Our results show that the kinematic model for SS433 has to be corrected for deceleration assuming ballistic propagation on a subparsec scale. © ESO, 2013.


Tests are presented of the 1D Accelerated Lambda Iteration method, which is widely used for solving the radiative transfer equation for a stellar atmosphere. We use our ARTY code as a reference solution and tables for these tests are provided. We model a static idealized stellar atmosphere, which is illuminated on its inner face and where internal sources are distributed with weak or strong gradients. This is an extension of published tests for a slab without incident radiation and gradients. Typical physical conditions for the continuum radiation and spectral lines are used, as well as typical values for the numerical parameters in order to reach a 1% accuracy. It is shown that the method is able to reach such an accuracy for most cases but the spatial discretization has to be refined for strong gradients and spectral lines, beyond the scope of realistic stellar atmospheres models. Discussion is provided on faster methods. © EAS, EDP Sciences 2010.


Izotov Y.I.,LUTH | Izotov Y.I.,Ukrainian Academy of Sciences | Izotov Y.I.,Max Planck Institute for Radio Astronomy | Stasinska G.,LUTH | And 2 more authors.
Astronomy and Astrophysics | Year: 2013

We verified the validity of the empirical method to derive the 4He abundance used in our previous papers by applying it to CLOUDY (v13.01) models. Using newly published He i emissivities for which we present convenient fits as well as the output CLOUDY case B hydrogen and He i line intensities, we found that the empirical method is able to reproduce the input CLOUDY 4He abundance with an accuracy of better than 1%. The CLOUDY output data also allowed us to derive the non-recombination contribution to the intensities of the strongest Balmer hydrogen Ha, H, H?, and Hd emission lines and the ionisation correction factors for He. With these improvements we used our updated empirical method to derive the 4He abundances and to test corrections for several systematic effects in a sample of 1610 spectra of low-metallicity extragalactic H ii regions, the largest sample used so far. From this sample we extracted a subsample of 111 H ii regions with H equivalent width EW(H) = 150 A, with excitation parameter x = O2+/O = 0.8, and with helium mass fraction Y derived with an accuracy better than 3%. With this subsample we derived the primordial 4He mass fraction Yp = 0.254 ± 0.003 from linear regression Y O/H. The derived value of Yp is higher at the 68% confidence level (CL) than that predicted by the standard big bang nucleosynthesis (SBBN) model, possibly implying the existence of different types of neutrino species in addition to the three known types of active neutrinos. Using the most recently derived primordial abundances D/H = (2.60 ± 0.12) 10-5 and Yp = 0.254 ± 0.003 and the ?2 technique, we found that the best agreement between abundances of these light elements is achieved in a cosmological model with baryon mass density Obh2 = 0.0234 ± 0.0019 (68% CL) and an effective number of the neutrino species Neff = 3.51 ± 0.35 (68% CL). © Author(s) 2013. CC Attribution 3.0 License.


Carter B.,LuTh
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

The principles of a previously developed formalism for the covariant treatment of multiscalar fields for which (as in a nonlinear sigma model) the relevant target space is not of affine type-but curved-are recapitulated. Their application is extended from ordinary harmonic models to a more general category of harmonious field models, with emphasis on cases in which the field is confined to a string or higher brane world sheet, and for which the relevant internal symmetry group is non-Abelian, so that the conditions for conservation of the corresponding charge currents become rather delicate, particularly when the symmetry is gauged. Attention is also given to the conditions for conservation of currents of a different kind-representing surface fluxes of generalized momentum or energy-associated with symmetries not of the internal target space but of the underlying space-time background structure, including the metric and any relevant gauge field. For the corresponding current to be conserved the latter need not be manifestly invariant: preservation modulo a gauge adjustment will suffice. The simplest case is that of "strong" symmetry, meaning invariance under the action of an effective Lie derivative (an appropriately gauge adjusted modification of an ordinary Lie derivative). When the effective symmetry is of the more general "weak" kind, the kinetic part of the current is not conserved by itself but only after being supplemented by a suitable contribution from the background. © 2010 The American Physical Society.

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