1,281 research outputs found
Alternative symplectic structures for SO(3,1) and SO(4) four-dimensional BF theories
The most general action, quadratic in the B fields as well as in the
curvature F, having SO(3,1) or SO(4) as the internal gauge group for a
four-dimensional BF theory is presented and its symplectic geometry is
displayed. It is shown that the space of solutions to the equations of motion
for the BF theory can be endowed with symplectic structures alternative to the
usual one. The analysis also includes topological terms and cosmological
constant. The implications of this fact for gravity are briefly discussed.Comment: 13 pages, LaTeX file, no figure
Construction and Processing of the Variable RF Power Couplers for the LHC Superconducting Cavities
The Large Hadron Collider (LHC) will accelerate beams of protons with a main RF system of sixteen 400 MHz superconducting cavities. The different requirements at injection and collision together with the heavy beam loading have imposed the use of a variable power coupler with a coupling value varying by a factor 20. The power requirements are high; 250 kW cw forward for several hours and pulses of 1.85 MW local peak power. In order to withstand these levels, copper sealing rings are used on the main ceramic. To avoid multipactor occurring at high powers and beam intensities, a second ceramic is used to provide a second DC polarization of the antenna. Twenty-five couplers have already been fully conditioned using a special dedicated FM+AM conditioning process, with 16 installed in the cavities in the LHC tunnel for first powering tests
Searching for signatures of planet formation in stars with circumstellar debris discs
(Abridged) Tentative correlations between the presence of dusty debris discs
and low-mass planets have been presented. In parallel, detailed chemical
abundance studies have reported different trends between samples of planet and
non-planet hosts. We determine in a homogeneous way the metallicity, and
abundances of a sample of 251 stars including stars with known debris discs,
with debris discs and planets, and only with planets. Stars with debris discs
and planets have the same [Fe/H] behaviour as stars hosting planets, and they
also show a similar -Tc trend. Different behaviour in the -Tc
trend is found between the samples of stars without planets and the samples of
planet hosts. In particular, when considering only refractory elements,
negative slopes are shown in cool giant planet hosts, whilst positive ones are
shown in stars hosting low-mass planets. Stars hosting exclusively close-in
giant planets show higher metallicities and positive -Tc slope. A
search for correlations between the -Tc slopes and the stellar
properties reveals a moderate but significant correlation with the stellar
radius and as well as a weak correlation with the stellar age. The fact that
stars with debris discs and stars with low-mass planets do not show neither
metal enhancement nor a different -Tc trend might indicate a
correlation between the presence of debris discs and the presence of low-mass
planets. We extend results from previous works which reported differences in
the -Tc trends between planet hosts and non hosts. However, these
differences tend to be present only when the star hosts a cool distant planet
and not in stars hosting exclusively low-mass planets.Comment: Accepted for publication in Astronomy and Astrophysic
Lorentz-covariant Hamiltonian analysis of BF gravity with the Immirzi parameter
We perform the Lorentz-covariant Hamiltonian analysis of two Lagrangian
action principles that describe general relativity as a constrained BF theory
and that include the Immirzi parameter. The relation between these two
Lagrangian actions has been already studied through a map among the fields
involved. The main difference between these is the way the Immirzi parameter is
included, since in one of them the Immirzi parameter is included explicitly in
the BF terms, whereas in the other (the CMPR action) it is in the constraint on
the B fields. In this work we continue the analysis of their relationship but
at the Hamiltonian level. Particularly, we are interested in seeing how the
above difference appears in the constraint structure of both action principles.
We find that they both possess the same number of first-class and second-class
constraints and satisfy a very similar (off-shell) Poisson-bracket algebra on
account of the type of canonical variables employed. The two algebras can be
transformed into each other by making a suitable change of variablesComment: LaTeX file, no figure
Dynamical stabilization of matter-wave solitons revisited
We consider dynamical stabilization of Bose-Einstein condensates (BEC) by
time-dependent modulation of the scattering length. The problem has been
studied before by several methods: Gaussian variational approximation, the
method of moments, method of modulated Townes soliton, and the direct averaging
of the Gross-Pitaevskii (GP) equation. We summarize these methods and find that
the numerically obtained stabilized solution has different configuration than
that assumed by the theoretical methods (in particular a phase of the
wavefunction is not quadratic with ). We show that there is presently no
clear evidence for stabilization in a strict sense, because in the numerical
experiments only metastable (slowly decaying) solutions have been obtained. In
other words, neither numerical nor mathematical evidence for a new kind of
soliton solutions have been revealed so far. The existence of the metastable
solutions is nevertheless an interesting and complicated phenomenon on its own.
We try some non-Gaussian variational trial functions to obtain better
predictions for the critical nonlinearity for metastabilization but
other dynamical properties of the solutions remain difficult to predict
Distribució, dinàmica poblacional i caracterització de Pseudomonas syringae, formadors de nuclis de gel i fotopatògens de Pyrus communis a les comarques gironines
Accretion variability of Herbig Ae/Be stars observed by X-Shooter. HD 31648 and HD 163296
This work presents X-Shooter/VLT spectra of the prototypical, isolated Herbig
Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by
timescales ranging from days to months. Each spectrum spans over a wide
wavelength range covering from 310 to 2475 nm. We have monitored the continuum
excess in the Balmer region of the spectra and the luminosity of twelve
ultraviolet, optical and near infrared spectral lines that are commonly used as
accretion tracers for T Tauri stars. The observed strengths of the Balmer
excesses have been reproduced from a magnetospheric accretion shock model,
providing a mean mass accretion rate of 1.11 x 10^-7 and 4.50 x 10^-7 Msun
yr^-1 for HD 31648 and HD 163296, respectively. Accretion rate variations are
observed, being more pronounced for HD 31648 (up to 0.5 dex). However, from the
comparison with previous results it is found that the accretion rate of HD
163296 has increased by more than 1 dex, on a timescale of ~ 15 years. Averaged
accretion luminosities derived from the Balmer excess are consistent with the
ones inferred from the empirical calibrations with the emission line
luminosities, indicating that those can be extrapolated to HAe stars. In spite
of that, the accretion rate variations do not generally coincide with those
estimated from the line luminosities, suggesting that the empirical
calibrations are not useful to accurately quantify accretion rate variability.Comment: 14 pages, 7 Figures, Accepted in Ap
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