10,914 research outputs found
Classification of Triadic Chord Inversions Using Kohonen Self-organizing Maps
In this paper we discuss the application of the Kohonen Selforganizing
Maps to the classification of triadic chords in inversions and root
positions. Our motivation started in the validation of Schönberg´s hypotheses of
the harmonic features of each chord inversion. We employed the Kohonen
network, which has been generally known as an optimum pattern classification
tool in several areas, including music, to verify that hypothesis. The outcomes
of our experiment refuse the Schönberg´s assumption in two aspects: structural
and perceptual/functional
An extended solution space for Chern-Simons gravity: the slowly rotating Kerr black hole
In the Einstein-Cartan formulation, an iterative procedure to find solutions
in non-dynamical Chern-Simons (CS) gravity in vacuum is proposed. The
iterations, in powers of a small parameter which codifies the CS
coupling, start from an arbitrary torsionless solution of Einstein equations.
With Schwarzschild as the zeroth-order choice, we derive a second-order
differential equation for the corrections to the metric,
for an arbitrary zeroth-order embedding parameter. In particular, the slowly
rotating Kerr metric is an solution in either the
canonical or the axial embeddings.Comment: 5 pages, PRD accepte
Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
Ultracompact objects are self-gravitating systems with a light ring. It was
recently suggested that fluctuations in the background of these objects are
extremely long-lived and might turn unstable at the nonlinear level, if the
object is not endowed with a horizon. If correct, this result has important
consequences: objects with a light ring are black holes. In other words, the
nonlinear instability of ultracompact stars would provide a strong argument in
favor of the "black hole hypothesis," once electromagnetic or
gravitational-wave observations confirm the existence of light rings. Here we
explore in some depth the mode structure of ultracompact stars, in particular
constant-density stars and gravastars. We show that the existence of very
long-lived modes -- localized near a second, stable null geodesic -- is a
generic feature of gravitational perturbations of such configurations. Already
at the linear level, such modes become unstable if the object rotates
sufficiently fast to develop an ergoregion. Finally, we conjecture that the
long-lived modes become unstable under fragmentation via a
Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the
structure of the star, it is also possible that nonlinearities lead to the
formation of small black holes close to the stable light ring. Our results
suggest that the mere observation of a light ring is a strong evidence for the
existence of black holes.Comment: 10 pages, RevTeX
Quantum interference oscillations of the superparamagnetic blocking in an Fe8 molecular nanomagnet
We show that the dynamic magnetic susceptibility and the superparamagnetic
blocking temperature of an Fe8 single molecule magnet oscillate as a function
of the magnetic field Hx applied along its hard magnetic axis. These
oscillations are associated with quantum interferences, tuned by Hx, between
different spin tunneling paths linking two excited magnetic states. The
oscillation period is determined by the quantum mixing between the ground S=10
and excited multiplets. These experiments enable us to quantify such mixing. We
find that the weight of excited multiplets in the magnetic ground state of Fe8
amounts to approximately 11.6%.Comment: Accepted in Phys. Rev. Let
Pre- and Post-burst Radio Observations of the Class 0 Protostar HOPS 383 in Orion
There is increasing evidence that episodic accretion is a common phenomenon
in Young Stellar Objects (YSOs). Recently, the source HOPS 383 in Orion was
reported to have a mid-infrared -- and bolometric -- luminosity
increase between 2004 and 2008, constituting the first clear example of a class
0 YSO (a protostar) with a large accretion burst. The usual assumption that in
YSOs accretion and ejection follow each other in time needs to be tested. Radio
jets at centimeter wavelengths are often the only way of tracing the jets from
embedded protostars. We searched the Very Large Array archive for the available
observations of the radio counterpart of HOPS 383. The data show that the radio
flux of HOPS 383 varies only mildly from January 1998 to December 2014, staying
at the level of to 300 Jy in the X band ( GHz), with a
typical uncertainty of 10 to 20 Jy in each measurement. We interpret the
absence of a radio burst as suggesting that accretion and ejection enhancements
do not follow each other in time, at least not within timescales shorter than a
few years. Time monitoring of more objects and specific predictions from
simulations are needed to clarify the details of the connection betwen
accretion and jets/winds in YSOs.Comment: ApJ Letters, in pres
Ultralight boson cloud depletion in binary systems
Ultralight scalars can extract rotational energy from astrophysical black
holes through superradiant instabilities, forming macroscopic boson clouds.
This process is most efficient when the Compton wavelength of the boson is
comparable to the size of the black hole horizon, i.e. when the "gravitational
fine structure constant" . If the black
hole/cloud system is in a binary, tidal perturbations from the companion can
produce resonant transitions between the energy levels of the cloud, depleting
it by an amount that depends on the nature of the transition and on the
parameters of the binary. Previous cloud depletion estimates considered
binaries in circular orbit and made the approximation . Here we
use black hole perturbation theory to compute instability rates and decay
widths for generic values of , and we show that this leads to much
larger cloud depletion estimates when . We also study
eccentric binary orbits. We show that in this case resonances can occur at all
harmonics of the orbital frequency, significantly extending the range of
frequencies where cloud depletion may be observable with gravitational wave
interferometers.Comment: 12 pages, 6 figures. v2: references added, matches published versio
Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8
Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and
the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe
nuclear spins leads to a huge enhancement of the specific heat below 1 K,
ascribed to a strong increase in the spin-lattice relaxation rate Gamma arising
from incoherent, nuclear-spin-mediated magnetic quantum tunneling in the
ground-doublet. Since Gamma is found comparable to the expected tunneling rate,
the latter process has to be inelastic. A model for the coupling of the
tunneling levels to the lattice is presented. Under transverse field, a
crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.Comment: Replaced with version accepted for publication in Physical Review
Letter
On the 2D Dirac oscillator in the presence of vector and scalar potentials in the cosmic string spacetime in the context of spin and pseudospin symmetries
The Dirac equation with both scalar and vector couplings describing the
dynamics of a two-dimensional Dirac oscillator in the cosmic string spacetime
is considered. We derive the Dirac-Pauli equation and solve it in the limit of
the spin and the pseudo-spin symmetries. We analyze the presence of cylindrical
symmetric scalar potentials which allows us to provide analytic solutions for
the resultant field equation. By using an appropriate ansatz, we find that the
radial equation is a biconfluent Heun-like differential equation. The solution
of this equation provides us with more than one expression for the energy
eigenvalues of the oscillator. We investigate these energies and find that
there is a quantum condition between them. We study this condition in detail
and find that it requires the fixation of one of the physical parameters
involved in the problem. Expressions for the energy of the oscillator are
obtained for some values of the quantum number . Some particular cases which
lead to known physical systems are also addressed.Comment: 15 pages, 8 figures, matches published versio
Undecidability as solution to the problem of measurement: fundamental criterion for the production of events
In recent papers we put forth a new interpretation of quantum mechanics,
colloquially known as ``the Montevideo interpretation''. This interpretation is
based on taking into account fundamental limits that gravity imposes on the
measurement process. As a consequence one has that situations develop where a
reduction process is undecidable from an evolution operator. When such a
situation is achieved, an event has taken place. In this paper we sharpen the
definition of when and how events occur, more precisely we give sufficient
conditions for the occurrence of events. We probe the new definition in an
example. In particular we show that the concept of undecidability used is not
``FAPP'' (for all practical purposes), but fundamental.Comment: 10 pages, contributed to the Castagnino Festschrif
- …