8,821 research outputs found
Invariance principles for switched systems with restrictions
In this paper we consider switched nonlinear systems under average dwell time
switching signals, with an otherwise arbitrary compact index set and with
additional constraints in the switchings. We present invariance principles for
these systems and derive by using observability-like notions some convergence
and asymptotic stability criteria. These results enable us to analyze the
stability of solutions of switched systems with both state-dependent
constrained switching and switching whose logic has memory, i.e., the active
subsystem only can switch to a prescribed subset of subsystems.Comment: 29 pages, 2 Appendixe
Offline Signature Verification by Combining Graph Edit Distance and Triplet Networks
Biometric authentication by means of handwritten signatures is a challenging
pattern recognition task, which aims to infer a writer model from only a
handful of genuine signatures. In order to make it more difficult for a forger
to attack the verification system, a promising strategy is to combine different
writer models. In this work, we propose to complement a recent structural
approach to offline signature verification based on graph edit distance with a
statistical approach based on metric learning with deep neural networks. On the
MCYT and GPDS benchmark datasets, we demonstrate that combining the structural
and statistical models leads to significant improvements in performance,
profiting from their complementary properties
A common origin of magnetism from planets to white dwarfs
This is the final version of the article. Available from the publisher via the DOI in this record.Isolated magnetic white dwarfs have field strengths ranging from kilogauss to gigagauss. However, the origin of the magnetic field has not been hitherto elucidated. Whether these fields are fossil, hence the remnants of original weak magnetic fields amplified during the course of the evolution of their progenitor stars, or are the result of binary interactions, or, finally, they are produced by other internal physical mechanisms during the cooling of the white dwarf itself, remains a mystery. At sufficiently low temperatures, white dwarfs crystallize. Upon solidification, phase separation of its main constituents, 12C and 16O, and of the impurities left by previous evolution occurs. This process leads to the formation of a Rayleigh–Taylor unstable liquid mantle on top of a solid core. This convective region, as it occurs in solar system planets like the Earth and Jupiter, can produce a dynamo able to yield magnetic fields of strengths of up to 0.1 MG, thus providing a mechanism that could explain magnetism in single white dwarfs.This work has been supported by MINECO grants ESP2013-
47637-P, ESP2015-66134-R (J.I.), and AYA2014-59084-P
(E.G.-B.), by the European Union FEDER funds, by grants
2014SGR1458 (J.I.), 2014SGR0038 (E.G.-B.) of the AGAUR,
and by the CERCS program of the Generalitat de Catalunya
A white dwarf merger as progenitor of the anomalous X-ray pulsar 4U 0142+61?
It has been recently proposed that massive fast-rotating highly-magnetized
white dwarfs could describe the observational properties of some of Soft
Gamma-Ray Repeaters (SGRs) and Anomalous X-Ray Pulsars (AXPs). Moreover, it has
also been shown that high-field magnetic (HFMWDs) can be the outcome of white
dwarf binary mergers. The products of these mergers consist of a hot central
white dwarf surrounded by a rapidly rotating disk. Here we show that the merger
of a double degenerate system can explain the characteristics of the peculiar
AXP 4U 0142+61. This scenario accounts for the observed infrared excess. We
also show that the observed properties of 4U 0142+6 are consistent with an
approximately 1.2 M_{\sun} white dwarf, remnant of the coalescence of an
original system made of two white dwarfs of masses 0.6\, M_{\sun} and 1.0\,
M_{\sun}. Finally, we infer a post-merging age kyr,
and a magnetic field G. Evidence for such a magnetic
field may come from the possible detection of the electron cyclotron absorption
feature observed between the and bands at Hz in the
spectrum of 4U 0142+61.Comment: to appear in ApJ Letter
The Spin Evolution of Fast-Rotating, Magnetized Super-Chandrasekhar White Dwarfs in the Aftermath of White Dwarf Mergers
The evolution of the remnant of the merger of two white dwarfs is still an
open problem. Furthermore, few studies have addressed the case in which the
remnant is a magnetic white dwarf with a mass larger than the Chandrasekhar
limiting mass. Angular momentum losses might bring the remnant of the merger to
the physical conditions suitable for developing a thermonuclear explosion.
Alternatively, the remnant may be prone to gravitational or rotational
instabilities, depending on the initial conditions reached after the
coalescence. Dipole magnetic braking is one of the mechanisms that can drive
such losses of angular momentum. However, the timescale on which these losses
occur depend on several parameters, like the strength of the magnetic field. In
addition, the coalescence leaves a surrounding Keplerian disk that can be
accreted by the newly formed white dwarf. Here we compute the post-merger
evolution of a super-Chandrasekhar magnetized white dwarf taking into account
all the relevant physical processes. These include magnetic torques acting on
the star, accretion from the Keplerian disk, the threading of the magnetic
field lines through the disk, as well as the thermal evolution of the white
dwarf core. We find that the central remnant can reach the conditions suitable
to develop a thermonuclear explosion before other instabilities (such as the
inverse beta-decay instability or the secular axisymmetric instability) are
reached, which would instead lead to gravitational collapse of the magnetized
remnant.Comment: Accepted for publication in The Astrophysical Journa
Isidella elongata (Cnidaria: Alcyonacea) facies in the western Mediterranean Sea: Visual surveys and descriptions of its ecological role
Isidella elongata is a candelabrum-shaped alcyonacean forming important facies on the bathyal muddy bottoms of the Mediterranean Sea, currently considered a sensitive habitat and heavily impacted by deep-sea fisheries. Until a few decades ago, this facies was a widespread habitat of the deep Mediterranean seabed and I. elongata was a common species in the trawling fishery's bycatch. Despite its current persistence in dense aggregations being very scarce, a dense facies of I. elongata was revealed during several ROV (Remotely Operated Vehicle) surveys carried out from 2010 to 2014 on the muddy bottoms between two seamounts east of Ibiza (Balearic Sea). The facies developed in an area between 480 and 615 m in depth where trawling is forbidden, with an extraordinary density of about 2300–2683 colonies/ha, representing one of the biggest facies of I. elongata currently known for the Mediterranean Sea considering the surface covered and the colonies' density. The associated community was surveyed, with 50 taxa identified. Moreover, a canyon southwest of Formentera characterised by the presence of I. elongata together with a high trawling impact was investigated. The density of the colonies was 53–62 colonies/ha and only 19 taxa of associated fauna were observed. The results of the two areas are compared and discussed in the framework of the protection of such an important habitat
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