3,028 research outputs found
Spectrum in the broken phase of a theory
We derive the spectrum in the broken phase of a theory, in
the limit , showing that this goes as even integers of a
renormalized mass in agreement with recent lattice computations.Comment: 4 pages, 1 figure. Accepted for publication in International Journal
of Modern Physics
Photospheric and chromospheric activity in four young solar-type stars
We present a photometric and spectroscopic study of four G-K dwarfs, namely
HD 166, epsilon Eri, chi1 Ori and kappa1 Cet. In three cases, we find a clear
spatial association between photospheric and chromospheric active regions. For
chi1 Ori we do not find appreciable variations of photospheric temperature, and
chromospheric Halpha emission. We applied a spot/plage model to the observed
rotational modulation of temperature and flux to derive spot/plage parameters
and to reconstruct a rough three-dimensional map of the outer atmosphere of
kappa1 Cet, HD 166 and epsilon Eri.Comment: 12 pages, 3 tables, 9 figures. Submitted to Ap
Exact solutions of classical scalar field equations
We give a class of exact solutions of quartic scalar field theories. These
solutions prove to be interesting as are characterized by the production of
mass contributions arising from the nonlinear terms while maintaining a
wave-like behavior. So, a quartic massless equation has a nonlinear wave
solution with a dispersion relation of a massive wave and a quartic scalar
theory gets its mass term renormalized in the dispersion relation through a
term depending on the coupling and an integration constant. When spontaneous
breaking of symmetry is considered, such wave-like solutions show how a mass
term with the wrong sign and the nonlinearity give rise to a proper dispersion
relation. These latter solutions do not change the sign maintaining the
property of the selected value of the equilibrium state. Then, we use these
solutions to obtain a quantum field theory for the case of a quartic massless
field. We get the propagator from a first order correction showing that is
consistent in the limit of a very large coupling. The spectrum of a massless
quartic scalar field theory is then provided. From this we can conclude that,
for an infinite countable number of exact classical solutions, there exist an
infinite number of equivalent quantum field theories that are trivial in the
limit of the coupling going to infinity.Comment: 7 pages, no figures. Added proof of existence of a zero mode and two
more references. Accepted for publication in Journal of Nonlinear
Mathematical Physic
Green functions and nonlinear systems: Short time expansion
We show that Green function methods can be straightforwardly applied to
nonlinear equations appearing as the leading order of a short time expansion.
Higher order corrections can be then computed giving a satisfactory agreement
with numerical results. The relevance of these results relies on the
possibility of fully exploiting a gradient expansion in both classical and
quantum field theory granting the existence of a strong coupling expansion.
Having a Green function in this regime in quantum field theory amounts to
obtain the corresponding spectrum of the theory.Comment: 7 pages, 3 figures. Version accepted for publication in International
Journal of Modern Physics
Evidence from stellar rotation of enhanced disc dispersal: (I) The case of the triple visual system BD-21 1074 in the Pictoris association
The early stage of stellar evolution is characterized by a star-disc locking
mechanism. The disc-locking prevents the star to spin its rotation up, and its
timescale depends on the disc lifetime. Some mechanisms can significantly
shorten this lifetime, allowing a few stars to start spinning up much earlier
than other stars. In the present study, we aim to investigate how the
properties of the circumstellar environment can shorten the disc lifetime. We
have identified a few multiple stellar systems, composed of stars with similar
masses, which belong to associations with a known age. Since all parameters
that are responsible for the rotational evolution, with the exception of
environment properties and initial stellar rotation, are similar for all
components, we expect that significant differences among the rotation periods
can only arise from differences in the disc lifetimes. A photometric timeseries
allowed us to measure the rotation periods of each component, while
high-resolution spectra provided us with the fundamental parameters,
and chromospheric line fluxes. The rotation periods of the components differ
significantly, and the component B, which has a closer companion C, rotates
faster than the more distant and isolated component A. We can ascribe the
rotation period difference to either different initial rotation periods or
different disc-locking phases arising from the presence of the close companion
C. In the specific case of BD21 1074, the second scenario seems to be more
favored. In our hypothesis of different disc-locking phase, any planet orbiting
this star is likely formed very rapidly owing to a gravitational instability
mechanism, rather than core accretion. Only a large difference of initial
rotation periods alone could account for the observed period difference,
leaving comparable disc lifetimes.Comment: Accepted by Astronomy & Astrophysics on July 31, 2014; Pages 12,
Figs.
A new data analysis framework for the search of continuous gravitational wave signals
Continuous gravitational wave signals, like those expected by asymmetric
spinning neutron stars, are among the most promising targets for LIGO and Virgo
detectors. The development of fast and robust data analysis methods is crucial
to increase the chances of a detection. We have developed a new and flexible
general data analysis framework for the search of this kind of signals, which
allows to reduce the computational cost of the analysis by about two orders of
magnitude with respect to current procedures. This can correspond, at fixed
computing cost, to a sensitivity gain of up to 10%-20%, depending on the search
parameter space. Some possible applications are discussed, with a particular
focus on a directed search for sources in the Galactic center. Validation
through the injection of artificial signals in the data of Advanced LIGO first
observational science run is also shown.Comment: 21 pages, 8 figure
An improved algorithm for narrow-band searches of continuous gravitational waves
Continuous gravitational waves signals, emitted by asymmetric spinning
neutron stars, are among the main targets of current detectors like Advanced
LIGO and Virgo. In the case of sources, like pulsars, which rotational
parameters are measured through electromagnetic observations, typical searches
assume that the gravitational wave frequency is at a given known fixed ratio
with respect to the star rotational frequency. For instance, for a neutron star
rotating around one of its principal axis of inertia the gravitational signal
frequency would be exactly two times the rotational frequency of the star. It
is possible, however, that this assumption is wrong. This is why search
algorithms able to take into account a possible small mismatch between the
gravitational waves frequency and the frequency inferred from electromagnetic
observations have been developed. In this paper we present an improved pipeline
to perform such narrow-band searches for continuous gravitational waves from
neutron stars, about three orders of magnitude faster than previous
implementations. The algorithm that we have developed is based on the {\it
5-vectors} framework and is able to perform a fully coherent search over a
frequency band of width (Hertz) and for hundreds of spin-down
values running a few hours on a standard workstation. This new algorithm opens
the possibility of long coherence time searches for objects which rotational
parameters are highly uncertain.Comment: 19 pages, 8 figures, 6 tables, submitted to CQ
Learning node labels with multi-category Hopfield networks
In several real-world node label prediction problems on graphs, in fields ranging from computational
biology to World Wide Web analysis, nodes can be partitioned into categories different from the classes to be predicted, on the basis of their characteristics or their common properties. Such partitions may provide further information about node classification that classical machine learning algorithms do not take into account. We introduce a novel family of parametric Hopfield networks (m-category Hopfield networks) and a novel algorithm (Hopfield multi-category \u2014 HoMCat ), designed to appropriately exploit the presence of property-based partitions of nodes into multiple categories. Moreover, the proposed model adopts a cost-sensitive learning strategy to prevent the remarkable decay in performance usually observed when instance labels are unbalanced, that is, when one class of labels is highly underrepresented than the other one. We validate the proposed model on both synthetic and real-world data, in the context of multi-species function
prediction, where the classes to be predicted are the Gene Ontology terms and the categories the different species in the multi-species protein network. We carried out an intensive experimental validation, which on the one hand compares HoMCat with several state-of-the-art graph-based algorithms, and on the other hand reveals that exploiting meaningful prior partitions of input data can substantially improve classification performances
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