Nikishin systems are perfect

K. Mahler introduced the concept of perfect systems in the general theory he developed for the simultaneous Hermite-Pade approximation of analytic functions. We prove that Nikishin systems are perfect providing, by far, the largest class of systems of functions for which this important property holds. As consequences, in the context of Nikishin systems, we obtain: an extension of Markov's theorem to simultaneous Hermite-Pade approximation, a general result on the convergence of simultaneous quadrature rules of Gauss-Jacobi type, the logarithmic asymptotics of general sequences of multiple orthogonal polynomials, and an extension of the Denisov-Rakhmanov theorem for the ratio asymptotics of mixed type multiple orthogonal polynomials.Comment: 39 page

Modeling a Slicer Mirror Using Zemax User-Defined Surface

A slicer mirror is a complex surface composed by many tilted and decentered mirrors sub-surfaces. The major difficulty to model such a complex surface is the large number of parameters used to define it. The Zemax's multi-configuration mode is usually used to specify each parameters (tilts, curvatures, decenters) for each mirror sub-surface which are then considered independently. Otherwise making use of the User-Defined Surface (UDS-DLL) Zemax capability, we are able to consider the set of sub-surfaces as a whole surface. In this paper, we present such a UDS-DLL tool comparing its performance with those of the classical multi-configuration mode. In particular, we explore the use of UDS-DLL to investigate the cross-talk due to the diffraction on the slicer array mirrors which has been a burden task when using multi-configuration mode.Comment: Submitted to the proceedings of the Durham Integral Field Spectroscopy Workshop July 4th-8th 200

In-Situ Particle Acceleration in Extragalactic Radio Hot Spots: Observations Meet Expectations

We discuss, in terms of particle acceleration, the results from optical VLT observations of hot spots associated with radio galaxies. On the basis of observational and theoretical grounds, it is shown that: 1. relatively low radio-radio power hot spots are the optimum candidates for being detected at optical waves. This is supported by an unprecedented optical detection rate of 70% out of a sample of low radio power hot spots. 2. the shape of the synchrotron spectrum of hot spots is mainly determined by the strength of the magnetic field in the region. In particular, the break frequency, related to the age of the oldest electrons in the hot spots, is found to increase with decreasing synchrotron power and magnetic field strength. Both observational results are in agreement with an in-situ particle acceleration scenario.Comment: 5 pages, TeX (or Latex, etc), 4 figures, to appear in MNRAS Letter, Updated reference

Validation of stellar population and kinematical analysis of galaxies

3D spectroscopy produces hundreds of spectra from which maps of the characteristics of stellar populations (age-metallicity) and internal kinematics of galaxies can be derived. We carried on simulations to assess the reliability of inversion methods and to define the requirements for future observations. We quantify the biases and show that to minimize the errors on the kinematics, age and metallicity (in a given observing time) the size of the spatial elements and the spectral dispersion should be chosen to obtain an instrumental velocity dispersion comparable to the physical dispersion.Comment: 5 pages, 3 figures, extended version of a poster proceeding to appear in "Science Perspectives for 3D Spectroscopy", eds. M. Kissler-Patig, M. M. Roth and J. R. Walsh, ESO Astrophysics Symposia. (The two last pages with figures are not in the conference proceedings.

Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid

To derive space velocities of stars along the line of sight from wavelength shifts in stellar spectra requires accounting for a number of second-order effects. For most stars, gravitational redshifts, convective blueshifts, and transverse stellar motion are the dominant contributors. We provide theoretical corrections for the net velocity shifts due to convection expected for the measurements from the Gaia Radial Velocity Spectrometer (RVS). We used a set of three-dimensional time-dependent simulations of stellar surface convection computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS range and to infer the net velocity offset that convective motions will induce in radial velocities derived by cross-correlation. The net velocity shifts derived by cross-correlation depend both on the wavelength range and spectral resolution of the observations. Convective shifts for Gaia RVS observations are less than 0.1 km/s for late-K-type stars, and they increase with stellar mass, reaching about 0.3 km/s or more for early F-type dwarfs. This tendency is the result of an increase with effective temperature in both temperature and velocity fluctuations in the line-forming region. Our simulations also indicate that the net RVS convective shifts can be positive (i.e. redshifts) in some cases. Overall, the blueshifts weaken slightly with increasing surface gravity, and are enhanced at low metallicity. Gravitational redshifts amount up to 0.7 km/s and dominate convective blueshifts for dwarfs, but become much weaker for giants.Comment: 13 pages, to appear in A&A; model fluxes available from ftp://leda.as.utexas.edu/pub/callende/Gaia3D and soon from CD