GalPak3D: A Bayesian parametric tool for extracting morpho-kinematics of galaxies from 3D data

We present a method to constrain galaxy parameters directly from three-dimensional data cubes. The algorithm compares directly the data with a parametric model mapped in $x,y,\lambda$ coordinates. It uses the spectral lines-spread function (LSF) and the spatial point-spread function (PSF) to generate a three-dimensional kernel whose characteristics are instrument specific or user generated. The algorithm returns the intrinsic modeled properties along with both an `intrinsic' model data cube and the modeled galaxy convolved with the 3D-kernel. The algorithm uses a Markov Chain Monte Carlo (MCMC) approach with a nontraditional proposal distribution in order to efficiently probe the parameter space. We demonstrate the robustness of the algorithm using 1728 mock galaxies and galaxies generated from hydrodynamical simulations in various seeing conditions from 0.6" to 1.2". We find that the algorithm can recover the morphological parameters (inclination, position angle) to within 10% and the kinematic parameters (maximum rotation velocity) to within 20%, irrespectively of the PSF in seeing (up to 1.2") provided that the maximum signal-to-noise ratio (SNR) is greater than $\sim3$ pixel$^{-1}$ and that the ratio of the galaxy half-light radius to seeing radius is greater than about 1.5. One can use such an algorithm to constrain simultaneously the kinematics and morphological parameters of (nonmerging) galaxies observed in nonoptimal seeing conditions. The algorithm can also be used on adaptive-optics (AO) data or on high-quality, high-SNR data to look for nonaxisymmetric structures in the residuals.Comment: 16 pages, 10 figures, accepted to publication in AJ, revised version after proofs corrections. Algorithm available at http://galpak.irap.omp.e

Long-term magnetic field stability of Vega

We present new spectropolarimetric observations of the normal A-type star Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard Lyot (Pic du Midi Observatory). This new time-series is constituted of 615 spectra collected over 6 different nights. We use the Least-Square-Deconvolution technique to compute, from each spectrum, a mean line profile with a signal-to-noise ratio close to 20,000. After averaging all 615 polarized observations, we detect a circularly polarized Zeeman signature consistent in shape and amplitude with the signatures previously reported from our observations of 2008 and 2009. The surface magnetic geometry of the star, reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the maps obtained in 2008 and 2009, showing that most recognizable features of the photospheric field of Vega are only weakly distorted by large-scale surface flows (differential rotation or meridional circulation).Comment: Proceedings of the conference "Stellar polarimetry: from birth to death", 2011 Jun 27-30, Madiso

Discovery of starspots on Vega - First spectroscopic detection of surface structures on a normal A-type star

The theoretically studied impact of rapid rotation on stellar evolution needs to be confronted with the results of high resolution spectroscopy-velocimetry observations. A weak surface magnetic field had recently been detected in the A0 prototype star Vega, potentially leading to a (yet undetected) structured surface. The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order reveal potential activity tracers, exoplanet companions and stellar oscillations. Vega was monitored in high-resolution spectroscopy with the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained during 5 nights (August 2012) at R = 75000 and covering the visible domain. For each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask. Several methods were applied to study the dynamic behavior of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others). We present the discovery of a starspotted stellar surface in an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, confirming the results of previous spectropolarimetric studies. Either a very thin convective layer can be responsible for magnetic field generation at small amplitudes, or a new mechanism has to be invoked in order to explain the existence of activity tracing starspots. This first strong evidence that standard A-type stars can show surface structures opens a new field of research and asks the question about a potential link with the recently discovered weak magnetic field discoveries in this category of stars.Comment: accepted for publication by Astronomy & Astrophysics (23rd of March 2015

Sparsity driven ultrasound imaging

An image formation framework for ultrasound imaging from synthetic transducer arrays based on sparsity-driven regularization functionals using single-frequency Fourier domain data is proposed. The framework involves the use of a physics-based forward model of the ultrasound observation process, the formulation of image formation as the solution of an associated optimization problem, and the solution of that problem through efficient numerical algorithms. The sparsity-driven, model-based approach estimates a complex-valued reflectivity field and preserves physical features in the scene while suppressing spurious artifacts. It also provides robust reconstructions in the case of sparse and reduced observation apertures. The effectiveness of the proposed imaging strategy is demonstrated using experimental data

Measurement of the Damping Rate of High-n Toroidal Alfven Eigenmodes in JET

A new set of compact in-vessel antennas has been built and installed in JET to provide for the first time the direct measurement of the damping rate (/) of stable Alfvén Eigenmodes (AEs) with toroidal mode number (n) in the range n=3-15. This paper reports the first quantitative analysis of the measurements of the damping rate for these modes as function of the edge plasma elongation (95). We find that the scaling of / vs. 95 for medium-n Toroidal AEs, with n=3-7, follows the same trend previously measured and explained theoretically for the n=1 and n=2 modes. This confirms the possibility of using the edge shape parameters as a real-time actuator for control of the stability of alpha-particles driven AEs in burning plasma experiments, such as ITER

AN ALGORITHM FOR THE REAL-TIME, BLIND DETECTION, DECOMPOSITION AND TRACKING OF THE INDIVIDUAL COMPONENTS IN A DEGENERATE, MULTI-HARMONICS SPECTRUM

In this work we report the successful application of an innovative method, based on the Sparse Representation of signals, to perform a real-time, unsupervised detection of the individual components in a frequency degenerate, multi-harmonic spectrum, using a small number of data un-evenly sampled in the spatial domain. This method has been developed from its original applications in astronomy, and is now routinely used in the JET thermonuclear fusion experiment to obtain the decomposition of a spectrum of high-frequency (~10-500kHz range) magnetic instabilities with a sub-ms time resolution, allowing the real-time tracking of its individual components as the plasma background evolves. This work opens a path towards developing real-time control tools for electro-magnetic instabilities in future fusion devices aimed at achieving a net energy gain. More generally, the speed and accuracy of this algorithm is recommended for instances of physics measurements and control engineering where an unsupervised, real-time decomposition of a degenerate signal is required from a small number of data

The Dependence of the Damping Rate of Medium-n Toroidal Alfvén Eigenmodes on the Edge Plasma Elongation in JET

This paper reports the first quantitative analysis of the measurements of the damping rate (gamma/omega) for stable Alfvén Eigenmodes (AEs) with toroidal mode number (n) in the range |n|=3-15 as function of the edge plasma elongation (kappa95). We find that the damping rate gamma/omega vs. kappa95 for medium-n Toroidal AEs, with n=3 and n=7, increases for increasing elongation, i.e. its scaling vs. kappa95 follows the same trend previously measured and explained theoretically for the n=1 and n=2 TAE modes. Theoretical analysis of the measurements for the n=3 TAEs has been performed using the LEMan code. The results are in good agreement (within a factor 2) for all the magnetic configurations where there is only a very minor up/down asymmetry in the poloidal cross-section of the plasma. These experimental results further confirm the possibility of using the edge shape parameters as a real-time actuator for control of the stability of alpha-particles driven AEs in burning plasma experiments, such as ITER

Measurement and Theoretical Modeling of the Damping Rate of Medium-N Toroidal Alfven Eigenmodes in JET

This paper reports on the results of recent experiments performed on the JET tokamak on Alfven Eigenmodes (AEs) with toroidal mode number (n) in the range |n|=3-15. The stability properties of these medium-n AEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. First, we describe the development of a new algorithm for real-time mode detection and discrimination using the Sparse Signal Representation theory. Second, we present measurements of the dependence of the damping rate for Toroidal AEs with |n|<=8 upon various background plasma parameters. Finally, the results of theoretical modeling of the damping rate for n=3 Toroidal AEs, performed with the LEMan, CASTOR and TAEFL codes, are shown as function of the edge plasma elongation

The radius and mass of the close solar twin 18 Sco derived from asteroseismology and interferometry

The growing interest in solar twins is motivated by the possibility of comparing them directly to the Sun. To carry on this kind of analysis, we need to know their physical characteristics with precision. Our first objective is to use asteroseismology and interferometry on the brightest of them: 18 Sco. We observed the star during 12 nights with HARPS for seismology and used the PAVO beam-combiner at CHARA for interferometry. An average large frequency separation $134.4\pm0.3$ $\mu$Hz and angular and linear radiuses of $0.6759 \pm 0.0062$ mas and $1.010\pm0.009$ R$_{\odot}$ were estimated. We used these values to derive the mass of the star, $1.02\pm0.03$ M$_{\odot}$.Comment: 5 pages, 5 figure