27,048 research outputs found
Dispersive representation of the pion vector form factor in decays
We propose a dispersive representation of the charged pion vector form factor
that is consistent with chiral symmetry and fulfills the constraints imposed by
analyticity and unitarity. Unknown parameters are fitted to the very precise
data on decays obtained by Belle, leading to a
good description of the corresponding spectral function up to a
squared invariant mass GeV. We discuss the effect of isospin
corrections, and obtain the values of low energy observables. For larger values
of , this representation is complemented with a phenomenological description
to allow its implementation in the new TAUOLA hadronic currents.Comment: 22 pages, 4 figures. Determination of rho(770) pole parameters
substantially improved using a new method, based on rational approximants.
Other results unchanged. Version to be published in EPJ
Faraday rotation and polarization gradients in the jet of 3C~120: Interaction with the external medium and a helical magnetic field?
We present a sequence of 12 monthly polarimetric 15, 22, and 43 GHz VLBA
observations of the radio galaxy 3C 120 revealing a systematic presence of
gradients in Faraday rotation and degree of polarization across and along the
jet. The degree of polarization increases with distance from the core and
toward the jet edges, and has an asymmetric profile in which the northern side
of the jet is more highly polarized. The Faraday rotation measure is also
stratified across the jet width, with larger values for the southern side. We
find a localized region of high Faraday rotation measure superposed on this
structure between approximately 3 and 4 mas from the core, with a peak of about
6000 rad/m^2. Interaction of the jet with the external medium or a cloud would
explain the confined region of enhanced Faraday rotation, as well as the
stratification in degree of polarization and the flaring of superluminal knots
when crossing this region. The data are also consistent with a helical field in
a two-fluid jet model, consisting of an inner, emitting jet and a sheath
containing nonrelativistic electrons. However, this helical magnetic field
model cannot by itself explain the localized region of enhanced Faraday
rotation. The polarization electric vectors, predominantly perpendicular to the
jet axis once corrected for Faraday rotation, require a dominant component
parallel to the jet axis (in the frame of the emitting plasma) for the magnetic
field in the emitting region.Comment: Accepted for publication in ApJ Letters. 4 pages (including 5
figures
Effects of electron inertia in collisionless magnetic reconnection
We present a study of collisionless magnetic reconnection within the
framework of full two-fluid MHD for a completely ionized hydrogen plasma,
retaining the effects of the Hall current, electron pressure and electron
inertia. We performed 2.5D simulations using a pseudo-spectral code with no
dissipative effects. We check that the ideal invariants of the problem are
conserved down to round-off errors. Our results show that the change in the
topology of the magnetic field lines is exclusively due to the presence of
electron inertia. The computed reconnection rates remain a fair fraction of the
Alfv\'en velocity, which therefore qualifies as fast reconnection
Footprints in the sand: What can globular clusters tell us about NGC 4753 past?
NGC 4753 is a bright (M_V approx -22.3) lenticular galaxy. It is a very
interesting target to test different theories of formation of lenticular
galaxies, due to its low-density environment and complex structure. We perform
the first comprehensive study of NGC 4753 globular cluster system (GCS), using
Gemini/GMOS and CTIO/MosaicII images. Our results indicate a rather poor GCS of
approx 1000 members. Its azimuthal distribution follows the shape of the galaxy
bulge. The GC colour distribution is peculiar, presenting an intermediate
subpopulation in addition to blue and red ones. This intermediate subgroup can
be explained by a single stellar population with an age of 1.5-3 Gyr and 0.5-1
Z_o. The GC specific frequency S_N = 1.3+/-0.15 is surprisingly low for a
galaxy of its class. The GC luminosity function (GCLF) is also peculiar, with
an excess of bright GCs compared to the expected gaussian distribution. The
underlying galaxy body has significant substructure, with remnants of spiral
arms, dust filaments, and isophote twisting. This, and the fact that NGC 4753
hosted two type Ia SNe, support the possibility that the intermediate GC
subpopulation may have originated during a recent merger, 1-3 Gyr ago.Comment: 10 pages, 10 figures, accepted on MNRA
First-order quasilinear canonical representation of the characteristic formulation of the Einstein equations
We prescribe a choice of 18 variables in all that casts the equations of the
fully nonlinear characteristic formulation of general relativity in
first--order quasi-linear canonical form. At the analytical level, a
formulation of this type allows us to make concrete statements about existence
of solutions. In addition, it offers concrete advantages for numerical
applications as it now becomes possible to incorporate advanced numerical
techniques for first order systems, which had thus far not been applicable to
the characteristic problem of the Einstein equations, as well as in providing a
framework for a unified treatment of the vacuum and matter problems. This is of
relevance to the accurate simulation of gravitational waves emitted in
astrophysical scenarios such as stellar core collapse.Comment: revtex4, 7 pages, text and references added, typos corrected, to
appear in Phys. Rev.
Gravitational waveforms with controlled accuracy
A partially first-order form of the characteristic formulation is introduced
to control the accuracy in the computation of gravitational waveforms produced
by highly distorted single black hole spacetimes. Our approach is to reduce the
system of equations to first-order differential form on the angular
derivatives, while retaining the proven radial and time integration schemes of
the standard characteristic formulation. This results in significantly improved
accuracy over the standard mixed-order approach in the extremely nonlinear
post-merger regime of binary black hole collisions.Comment: Revised version, published in Phys. Rev. D, RevTeX, 16 pages, 4
figure
The formation of planetary disks and winds: an ultraviolet view
Planetary systems are angular momentum reservoirs generated during star
formation. This accretion process produces very powerful engines able to drive
the optical jets and the molecular outflows. A fraction of the engine energy is
released into heating thus the temperature of the engine ranges from the 3000K
of the inner disk material to the 10MK in the areas where magnetic reconnection
occurs. There are important unsolved problems concerning the nature of the
engine, its evolution and the impact of the engine in the chemical evolution of
the inner disk. Of special relevance is the understanding of the shear layer
between the stellar photosphere and the disk; this layer controls a significant
fraction of the magnetic field building up and the subsequent dissipative
processes ougth to be studied in the UV.
This contribution focus on describing the connections between 1 Myr old suns
and the Sun and the requirements for new UV instrumentation to address their
evolution during this period. Two types of observations are shown to be needed:
monitoring programmes and high resolution imaging down to, at least,
milliarsecond scales.Comment: Accepted for publication in Astrophysics and Space Science 9 figure
Copernicus Downstream Service Supports Nature-Based Flood Defense: Use of Sentinel Earth Observation Satellites for Coastal Needs
This is a copy of the article published in Sea-Technology Magazine, which the authors have bought the rights to redistribute.With an uncertain future that includes climate change,
sea level rise and increasing coastal populations, being able to make informed policy decisions in coastal zones will be critical for ensuring the well-being of citizens, the
environment and the sustainability of economic activities.
Earth observation (EO) can be used to efficiently and systematically provide the key information needed to make these decisions. However, getting access to the right EO in-
formation can be a complicated and costly business, limiting availability. However, the launch in April 2014 of the first Sentinel satellite from Europe’s flagship EO program,
Copernicus, represents a major advance in the availability of EO data, which has great potential to benefit numerous sectors involved in marine and coastal activities. We discuss some examples of applications being developed and give an example of a new service which intends to support nature-based flood defense schemes.The research leading to these results has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under grant agreement n° 607131. All views presented are those of the authors. The EU is not liable for any use that may be made of the information contained herein.PDF, 5 pages, 20.4 M
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