514 research outputs found
Switching times in long-overlap Josephson junctions subject to thermal fluctuations and non-Gaussian noise sources
We investigate the superconducting lifetime of long current-biased Josephson
junctions, in the presence of Gaussian and non-Gaussian noise sources. In
particular, we analyze the dynamics of a Josephson junction as a function of
the noise signal intensity, for different values of the parameters of the
system and external driving currents. We find that the mean lifetime of the
superconductive state is characterized by nonmonotonic behavior as a function
of noise intensity, driving frequency and junction length. We observe that
these nonmonotonic behaviours are connected with the dynamics of the junction
phase string during the switching towards the resistive state. An important
role is played by the formation and propagation of solitons, with two different
dynamical regimes characterizing the dynamics of the phase string. Our analysis
allows to evidence the effects of different bias current densities, that is a
simple spatially homogeneous distribution and a more realistic inhomogeneous
distribution with high current values at the junction edges. Stochastic
resonant activation, noise enhanced stability and temporary trapping phenomena
are observed in the system investigated.Comment: 16 pages, 9 figures, Physical Review B, in pres
Effects of L\'evy noise on the dynamics of sine-Gordon solitons in long Josephson junctions
We numerically investigate the generation of solitons in current-biased long
Josephson junctions in relation to the superconducting lifetime and the voltage
drop across the device. The dynamics of the junction is modelled with a
sine-Gordon equation driven by an oscillating field and subject to an external
non-Gaussian noise. A wide range of -stable L\'evy distributions is
considered as noise source, with varying stability index and asymmetry
parameter . In junctions longer than a critical length, the mean
switching time (MST) from superconductive to the resistive state assumes a
values independent of the device length. Here, we demonstrate that such a value
is directly related to the mean density of solitons which move into or from the
washboard potential minimum corresponding to the initial superconductive state.
Moreover, we observe: (i) a connection between the total mean soliton density
and the mean potential difference across the junction; (ii) an inverse behavior
of the mean voltage in comparison with the MST, with varying the junction
length; (iii) evidences of non-monotonic behaviors, such as stochastic resonant
activation and noise enhanced stability, of MST versus the driving frequency
and noise intensity for different values of and ; (iv) finally,
these non-monotonic behaviors are found to be related to the mean density of
solitons formed along the junction.Comment: 24 pages, 8 figures, submitted to J. Stat. Mech.: Theory Exp. arXiv
admin note: text overlap with arXiv:1406.481
Sine-Gordon breathers generation in driven long Josephson junctions
We consider a long Josephson junction excited by a suitable external
ac-signal, in order to generate control and detect breathers. Studying the
nonlinear supratransmission phenomenon in a nonlinear sine-Gordon chain
sinusoidally driven, Geniet and Leon explored the bifurcation of the energy
transmitted into the chain and calculated a threshold for the
external driving signal amplitude, at which the energy flows into the system by
breathers modes. I numerically study the continuous sine-Gordon model,
describing the dynamics of the phase difference in a long Josephson junction,
in order to deeply investigate the "continuous limit" modifications to this
threshold. Wherever the energy flows into the system due to the nonlinear
supratransmission, a peculiar breather localization areas appear in a parameters space. The emergence of these areas depends on the damping
parameter value, the bias current, and the waveform of driving external signal.
The robustness of generated breathers is checked by introducing into the model
a thermal noise source to mimic the environmental fluctuations. Presented
results allows one to consider a cryogenic experiment for creation and
detection of Josephson breathers.Comment: 8 pages, 3 figure
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied
star-forming regions, thanks to their large content of both OB stars and stars
with disks and the observed ongoing star formation. We identified 834
disk-bearing stars associated with the cloud, after detecting their excesses in
NIR bands from J band to 8.0 micron. In this paper, we study in detail the
nature of a subsample of disk-bearing stars that show peculiar characteristics.
They appear older than the other members in the V vs. V-I diagram, and/or they
have one or more IRAC colors at pure photospheric values, despite showing NIR
excesses, when optical and infrared colors are compared. We confirm the
membership of these stars to M16 by a spectroscopic analysis. The physical
properties of these stars with disks are studied by comparing their spectral
energy distributions (SEDs) with the SEDs predicted by models of T-Tauri stars
with disks and envelopes. We show that the age of these stars estimated from
the V vs. V-I diagram is unreliable since their V-I colors are altered by the
light scattered by the disk into the line of sight. Only in a few cases their
SEDs are compatible with models with excesses in V band caused by optical
veiling. Candidate members with disks and photospheric IRAC colors are selected
by the used NIR disk diagnostic, which is sensitive to moderate excesses, such
as those produced by disks with low masses. In 1/3 of these cases, scattering
of stellar flux by the disks can also be invoked. The photospheric light
scattered by the disk grains into the line of sight can affect the derivation
of physical parameters of ClassII stars from photometric optical and NIR data.
Besides, the disks diagnostic we defined are useful for selecting stars with
disks, even those with moderate excesses or whose optical colors are altered by
veiling or photospheric scattered light.Comment: Accepted for publication in A&
Chronology of star formation and disk evolution in the Eagle Nebula
Massive SFR are characterized by intense ionizing fluxes, strong stellar
winds and supernovae explosions, all of which have important effects on the
surrounding media, on the star-formation (SF) process and on the evolution of
YSOs and their disks. We present a multiband study of the massive young cluster
NGC6611 and M16, to study how OB stars affect the early stellar evolution and
the SF. We search for evidence of triggered SF by OB stars in NGC6611 on a
large spatial scale (~10 pc) and how the efficiency of disks photoevaporation
depends on the central stars mass. We assemble a multiband catalog with
photometric data, from B band to 8.0micron, and X-ray data obtained with 2 new
and 1 archival ACIS-I observation. We select the stars with disks from IR
photometry and disk-less from X-ray emission, both in NGC6611 and the outer
region of M16. We study induced photoevaporation searching for the spatial
variation of disk frequency for distinct stellar mass ranges. The triggering of
SF by OB stars has been investigated by deriving the history of SF across the
nebula. We find evidence of sequential SF in the Eagle Nebula going from the SE
(2.6 Myrs) to the NW (0.3 Myrs), with the median age of ~1 Myear. We observe a
drop of the disk frequency close to OB stars (up to an average distance of 1
pc), without effects at larger distances. Furthermore, disks are more frequent
around low-mass stars (<1 M(solar)) than in high-mass stars, regardless of the
distance from OB stars. The SF chronology in M16 does not support the
hypothesis of a large-scale SF triggered by OB stars in NGC6611. Instead, we
speculate that it was triggered by the encounter (~3 Myrs ago) with a giant
molecular shell created ~6 Myrs ago.Comment: Accepted for publication at Astronomy and Astrophysic
Genome-wide analysis of the repertoire of TRIM genes in sea urchins
The eukaryotic TRIM (TRIpartite Motif) super-family represents one of the largest classes of
putative E3 ubiquitin ligases involved in several processes, including epigenetic control of
development and disease. In the post-genomic era, new approaches allow genome-wide studies
of gene family. In particular, we performed a comprehensive analysis of the TRIM repertoire in
selected sea urchin species. By combining iterations of ab initio predictions and pairwise
comparative methods, we first retrieved the full complement of TRIM genes in
Strongylocentrotus purpuratus, whose full genome sequence was available. Interestingly, such a
DNA sequence set includes not previously classified, echinoderm-specific, TRIM genes as well
as multiple copies of known ones. We also retrieved a landscape of cDNA sequences from staged
EST libraries, indicating that most of these genes are actively transcribed during development.
Phylogenetic analysis of the deduced proteins, using set of TRIMs from various species, revealed
a degree of genetic variation between species. Worth of mention, we predicted the occurrence of
transposition events involving some of these genes, according with the documented rapid
evolution of this family. Next, we adopted heuristic algorithms and post-processing steps to
investigate the evolutionarily distant Paracentrotus lividus, Allocentrotus fragilis and Lytechinus
variegatus genomes, whose sequencing projects are actually in progress. We assembled partial
pools of TRIM genes and specifically associated them to EST-derived cDNA sequences. Such a
collection of data should provide a framework for unravel gene regulatory networks involving
TRIM genes from an evolutionary perspective. Indeed, in the Pl species, we have previously
isolated and functionally characterized the cDNA sequence encoding the first echinoderm
TRIM factor, Strim1. Here, we identified five strim1 genes, all sharing a intronless organization,
and roughly located their cis-regulatory apparatus
Low mass star formation and subclustering in the HII regions RCW 32, 33 and 27 of the Vela Molecular Ridge. A photometric diagnostics to identify M-type stars
Most stars born in clusters and recent results suggest that star formation
(SF) preferentially occurs in subclusters. Studying the morphology and SF
history of young clusters is crucial to understanding early SF. We identify the
embedded clusters of young stellar objects (YSOs) down to M stars, in the HII
regions RCW33, RCW32 and RCW27 of the Vela Molecular Ridge. Our aim is to
characterise their properties, such as morphology and extent of the clusters in
the three HII regions, derive stellar ages and the connection of the SF history
with the environment. Through public photometric surveys such as Gaia, VPHAS,
2MASS and Spitzer/GLIMPSE, we identify YSOs with IR, Halpha and UV excesses, as
signature of circumstellar disks and accretion. In addition, we implement a
method to distinguish M dwarfs and giants, by comparing the reddening derived
in several optical/IR color-color diagrams, assuming suitable theoretical
models. Since this diagnostic is sensitive to stellar gravity, the procedure
allows us to identify pre-main sequence stars. We find a large population of
YSOs showing signatures of circumstellar disks with or without accretion. In
addition, with the new technique of M-type star selection, we find a rich
population of young M stars with a spatial distribution strongly correlated to
the more massive population. We find evidence of three young clusters, with
different morphology. In addition, we identify field stars falling in the same
region, by securely classifying them as giants and foreground MS stars. We
identify the embedded population of YSOs, down to about 0.1 Msun, associated
with the HII regions RCW33, RCW32 and RCW27 and the clusters Vela T2, Cr197 and
Vela T1, respectively, showing very different morphologies. Our results suggest
a decreasing SF rate in Vela T2 and triggered SF in Cr197 and Vela T1.Comment: Accepted for publication in A&A; 20 pages, 22 figures, 6 table
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