996 research outputs found
Seismic diagnostics for transport of angular momentum in stars 1. Rotational splittings from the PMS to the RGB
Rotational splittings are currently measured for several main sequence stars
and a large number of red giants with the space mission Kepler. This will
provide stringent constraints on rotation profiles. Our aim is to obtain
seismic constraints on the internal transport and surface loss of angular
momentum of oscillating solar-like stars. To this end, we study the evolution
of rotational splittings from the pre-main sequence to the red-giant branch for
stochastically excited oscillation modes. We modified the evolutionary code
CESAM2K to take rotationally induced transport in radiative zones into account.
Linear rotational splittings were computed for a sequence of
models. Rotation profiles were derived from our evolutionary models and
eigenfunctions from linear adiabatic oscillation calculations. We find that
transport by meridional circulation and shear turbulence yields far too high a
core rotation rate for red-giant models compared with recent seismic
observations. We discuss several uncertainties in the physical description of
stars that could have an impact on the rotation profiles. For instance, we find
that the Goldreich-Schubert-Fricke instability does not extract enough angular
momentum from the core to account for the discrepancy. In contrast, an increase
of the horizontal turbulent viscosity by 2 orders of magnitude is able to
significantly decrease the central rotation rate on the red-giant branch. Our
results indicate that it is possible that the prescription for the horizontal
turbulent viscosity largely underestimates its actual value or else a mechanism
not included in current stellar models of low mass stars is needed to slow down
the rotation in the radiative core of red-giant stars.Comment: 15 pages, 13 figures, accepted for publication in A&
Slowly, rotating non-stationary, fluid solutions of Einstein's equations and their match to Kerr empty space-time
A general class of solutions of Einstein's equation for a slowly rotating
fluid source, with supporting internal pressure, is matched using Lichnerowicz
junction conditions, to the Kerr metric up to and including first order terms
in angular speed parameter. It is shown that the match applies to any
previously known non-rotating fluid source made to rotate slowly for which a
zero pressure boundary surface exists. The method is applied to the dust source
of Robertson-Walker and in outline to an interior solution due to McVittie
describing gravitational collapse. The applicability of the method to
additional examples is transparent. The differential angular velocity of the
rotating systems is determined and the induced rotation of local inertial frame
is exhibited
PHARAO Laser Source Flight Model: Design and Performances
In this paper, we describe the design and the main performances of the PHARAO
laser source flight model. PHARAO is a laser cooled cesium clock specially
designed for operation in space and the laser source is one of the main
sub-systems. The flight model presented in this work is the first
remote-controlled laser system designed for spaceborne cold atom manipulation.
The main challenges arise from mechanical compatibility with space constraints,
which impose a high level of compactness, a low electric power consumption, a
wide range of operating temperature and a vacuum environment. We describe the
main functions of the laser source and give an overview of the main
technologies developed for this instrument. We present some results of the
qualification process. The characteristics of the laser source flight model,
and their impact on the clock performances, have been verified in operational
conditions.Comment: Accepted for publication in Review of Scientific Instrument
The correlation of the Lyman-alpha forest in close pairs and groups of high-redshift quasars: clustering of matter on scales 1-5 Mpc
We study the clustering of matter in the intergalactic medium from the
Lyman-alpha forests seen in the spectra of pairs or groups of z ~ 2 quasars
observed with FORS2 and UVES at the VLT-UT2 Kueyen ESO telescope. The sample
consists of five pairs with separations 0.6, 1, 2.1, 2.6 and 4.4 arcmin and a
group of four quasars with separations from 2 up to 10 arcmin. This
unprecedented data set allows us to measure the transverse flux correlation
function for a range of angular scales. Correlations are clearly detectable at
separations smaller than 3 arcmin. The shape and correlation length of the
transverse correlation function on these scales is in good agreement with those
expected from absorption by the photoionized warm intergalactic medium
associated with the filamentary and sheet-like structures predicted in CDM-like
models for structure formation. At larger separation no significant correlation
is detected. Assuming that the absorbing structures are randomly orientated
with respect to the line of sight, the comparison of transverse and
longitudinal correlation lengths constrains the cosmological parameters (as a
modified version of the Alcock & Paczy\'nski test). The present sample is too
small to get significant constraints. Using N-body simulations, we investigate
the possibility to constrain Ol from future larger samples of QSO pairs with
similar separations. The observation of a sample of 30 pairs at 2, 4.5 and 7.5
arcmin should constrain the value of Ol at +- 15 % (2 sigma level). We further
use the observed spectra of the group of four quasars, to search for underdense
regions in the intergalactic medium. We find a quasi-spherical structure of
reduced absorption with radius 12.5 h^{-1} Mpc which we identify as an
underdense region.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Nanotrench for nano and microparticle electrical interconnects
We present a simple and versatile patterning procedure for the reliable and reproducible fabrication of high aspect ratio (10 4 ) electrical interconnects that have separation distances down to 20 nm and lengths of several hundreds of microns. The process uses standard optical lithography techniques and allows parallel processing of many junctions, making it easily scalable and industrially relevant. We demonstrate the suitability of these nanotrenches as electrical interconnects for addressing micro and nanoparticles by realizing several circuits with integrated species. Furthermore, low impedance metal-metal low contacts are shown to be obtained when trapping a single metal-coated microsphere in the gap, emphasizing the intrinsic good electrical conductivity of the interconnects, even though a wet process is used. Highly resistive magnetite-based nanoparticles networks also demonstrate the advantage of the high aspect ratio of the nanotrenches for providing access to electrical properties of highly resistive materials, with leakage current levels below 1 pA. © 2010 IOP Publishing Ltd
Finite axisymmetric charged dust disks in conformastatic spacetimes
An infinite family of axisymmetric charged dust disks of finite extension is
presented. The disks are obtained by solving the vacuum Einstein-Maxwell
equations for conformastatic spacetimes, which are characterized by only one
metric function. In order to obtain the solutions, it is assumed that the
metric function and the electric potential are functionally related and that
the metric function is functionally dependent of another auxiliary function,
which is taken as a solution of Laplace equation. The solutions for the
auxiliary function are then taken as given by the infinite family of
generalized Kalnajs disks recently obtained by Gonz\'alez and Reina (MNRAS 371,
1873, 2006), which is expressed in terms of the oblate spheroidal coordinates
and represents a well behaved family of finite axisymmetric flat galaxy models.
The so obtained relativistic thin disks have then a charge density that is
equal, except maybe by a sign, to their mass density, in such a way that the
electric and gravitational forces are in exact balance. The energy density of
the disks is everywhere positive and well behaved, vanishing at the edge.
Accordingly, as the disks are made of dust, their energy-momentum tensor it
agrees with all the energy conditions.Comment: Submitted to PR
Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei
Multifragmentation properties measured with INDRA are studied for single
sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV
and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both
types of sources is presented concerning Fisher scaling, Zipf law, fragment
size and fluctuation observables. A Fisher scaling is observed for all the
data. The pseudo-critical energies extracted from the Fisher scaling are
consistent between Xe+Sn central collisions and Au quasi-projectiles. In the
latter case it also corresponds to the energy region at which fluctuations are
maximal. The critical energies deduced from the Zipf analysis are higher than
those from the Fisher analysis.Comment: 30 pages, accepted for publication in Nuclear Physics A, references
correcte
- âŠ