471 research outputs found
2D Rutherford-Like Scattering in Ballistic Nanodevices
Ballistic injection in a nanodevice is a complex process where electrons can
either be transmitted or reflected, thereby introducing deviations from the
otherwise quantized conductance. In this context, quantum rings (QRs) appear as
model geometries: in a semiclassical view, most electrons bounce against the
central QR antidot, which strongly reduces injection efficiency. Thanks to an
analogy with Rutherford scattering, we show that a local partial depletion of
the QR close to the edge of the antidot can counter-intuitively ease ballistic
electron injection. On the contrary, local charge accumulation can focus the
semi-classical trajectories on the hard-wall potential and strongly enhance
reflection back to the lead. Scanning gate experiments on a ballistic QR, and
simulations of the conductance of the same device are consistent, and agree to
show that the effect is directly proportional to the ratio between the strength
of the perturbation and the Fermi energy. Our observation surprisingly fits the
simple Rutherford formalism in two-dimensions in the classical limit
Detecting heterozygosity in shotgun genome assemblies: Lessons from obligately outcrossing nematodes
The majority of nematodes are gonochoristic (dioecious) with distinct male and female sexes, but the best-studied species, Caenorhabditis elegans, is a self-fertile hermaphrodite. The sequencing of the genomes of C. elegans and a second hermaphrodite, C. briggsae, was facilitated in part by the low amount of natural heterozygosity, which typifies selfing species. Ongoing genome projects for gonochoristic Caenorhabditis species seek to approximate this condition by intense inbreeding prior to sequencing. Here we show that despite this inbreeding, the heterozygous fraction of the whole genome shotgun assemblies of three gonochoristic Caenorhabditis species, C. brenneri, C. remanei, and C. japonica, is considerable. We first demonstrate experimentally that independently assembled sequence variants in C. remanei and C. brenneri are allelic. We then present gene-based approaches for recognizing heterozygous regions of WGS assemblies. We also develop a simple method for quantifying heterozygosity that can be applied to assemblies lacking gene annotations. Consistently we find that ∼10% and 30% of the C. remanei and C. brenneri genomes, respectively, are represented by two alleles in the assemblies. Heterozygosity is restricted to autosomes and its retention is accompanied by substantial inbreeding depression, suggesting that it is caused by multiple recessive deleterious alleles and not merely by chance. Both the overall amount and chromosomal distribution of heterozygous DNA is highly variable between assemblies of close relatives produced by identical methodologies, and allele frequencies have continued to change after strains were sequenced. Our results highlight the impact of mating systems on genome sequencing projects
Gap Formation in the Dust Layer of 3D Protoplanetary Disks
We numerically model the evolution of dust in a protoplanetary disk using a
two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is
non-self-gravitating and locally isothermal. The code follows the three
dimensional distribution of dust in a protoplanetary disk as it interacts with
the gas via aerodynamic drag. In this work, we present the evolution of a disk
comprising 1% dust by mass in the presence of an embedded planet for two
different disk configurations: a small, minimum mass solar nebular (MMSN) disk
and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the
grain size and planetary mass to see how they effect the resulting disk
structure. We find that gap formation is much more rapid and striking in the
dust layer than in the gaseous disk and that a system with a given stellar,
disk and planetary mass will have a different appearance depending on the grain
size and that such differences will be detectable in the millimetre domain with
ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk
while not in the gas disk. We also note that dust accumulates at the external
edge of the planetary gap and speculate that the presence of a planet in the
disk may facilitate the growth of planetesimals in this high density region.Comment: 5 page, 4 figures. Accepted for publication in Astrophysics & Space
Scienc
Gathering in Dynamic Rings
The gathering problem requires a set of mobile agents, arbitrarily positioned
at different nodes of a network to group within finite time at the same
location, not fixed in advanced.
The extensive existing literature on this problem shares the same fundamental
assumption: the topological structure does not change during the rendezvous or
the gathering; this is true also for those investigations that consider faulty
nodes. In other words, they only consider static graphs. In this paper we start
the investigation of gathering in dynamic graphs, that is networks where the
topology changes continuously and at unpredictable locations.
We study the feasibility of gathering mobile agents, identical and without
explicit communication capabilities, in a dynamic ring of anonymous nodes; the
class of dynamics we consider is the classic 1-interval-connectivity.
We focus on the impact that factors such as chirality (i.e., a common sense
of orientation) and cross detection (i.e., the ability to detect, when
traversing an edge, whether some agent is traversing it in the other
direction), have on the solvability of the problem. We provide a complete
characterization of the classes of initial configurations from which the
gathering problem is solvable in presence and in absence of cross detection and
of chirality. The feasibility results of the characterization are all
constructive: we provide distributed algorithms that allow the agents to
gather. In particular, the protocols for gathering with cross detection are
time optimal. We also show that cross detection is a powerful computational
element.
We prove that, without chirality, knowledge of the ring size is strictly more
powerful than knowledge of the number of agents; on the other hand, with
chirality, knowledge of n can be substituted by knowledge of k, yielding the
same classes of feasible initial configurations
Noradrenergic Modulation of Intrinsic and Synaptic Properties of Lumbar Motoneurons in the Neonatal Rat Spinal Cord
Although it is known that noradrenaline (NA) powerfully controls spinal motor networks, few data are available regarding the noradrenergic (NAergic) modulation of intrinsic and synaptic properties of neurons in motor networks. Our work explores the cellular basis of NAergic modulation in the rat motor spinal cord. We first show that lumbar motoneurons express the three classes of adrenergic receptors at birth. Using patch-clamp recordings in the newborn rat spinal cord preparation, we characterized the effects of NA and of specific agonists of the three classes of adrenoreceptors on motoneuron membrane properties. NA increases the motoneuron excitability partly via the inhibition of a KIR like current. Methoxamine (α1), clonidine (α2) and isoproterenol (β) differentially modulate the motoneuron membrane potential but also increase motoneuron excitability, these effects being respectively inhibited by the antagonists prazosin (α1), yohimbine (α2) and propranolol (β). We show that the glutamatergic synaptic drive arising from the T13-L2 network is enhanced in motoneurons by NA, methoxamine and isoproterenol. On the other hand, NA, isoproterenol and clonidine inhibit both the frequency and amplitude of miniature glutamatergic EPSCs while methoxamine increases their frequency. The T13-L2 synaptic drive is thereby differentially modulated from the other glutamatergic synapses converging onto motoneurons and enhanced by presynaptic α1 and β receptor activation. Our data thus show that the NAergic system exerts a powerful and complex neuromodulation of lumbar motor networks in the neonatal rat spinal cord
Effect of cathode material and its size on the abundance of nitrogen removal functional genes in microcosms of integrated bioelectrochemical-wetland systems
Funding Information: Funding: This study was supported by the Estonian Research Council (PUTJD715, IUT2-16, PRG352 and PRG676); the EU through the European Regional Development Fund (Centre of Excellence EcolChange, Estonia) and by the European Structural and Investment Funds. The financial support from The French National Research Agency (ANR-17-CE04-0004) is gratefully acknowledged. The financial support from the International mobility support for PhD students–DrEAM (University of Lorraine) is gratefully acknowledged.Peer reviewedPublisher PD
Planet gaps in the dust layer of 3D protoplanetary disks. II. Observability with ALMA
[Abridged] Aims: We provide predictions for ALMA observations of planet gaps
that account for the specific spatial distribution of dust that results from
consistent gas+dust dynamics. Methods: In a previous work, we ran full 3D,
two-fluid Smoothed Particle Hydrodynamics (SPH) simulations of a planet
embedded in a gas+dust T Tauri disk for different planet masses and grain
sizes. In this work, the resulting dust distributions are passed to the Monte
Carlo radiative transfer code MCFOST to construct synthetic images in the ALMA
wavebands. We then use the ALMA simulator to produce images that include
thermal and phase noise for a range of angular resolutions, wavelengths, and
integration times, as well as for different inclinations, declinations and
distances. We also produce images which assume that gas and dust are well mixed
with a gas-to-dust ratio of 100 to compare with previous ALMA predictions, all
made under this hypothesis. Results: Our findings clearly demonstrate the
importance of correctly incorporating the dust dynamics. We show that the gap
carved by a 1 M_J planet orbiting at 40 AU is visible with a much higher
contrast than the well-mixed assumption would predict. In the case of a 5 M_J
planet, we clearly see a deficit in dust emission in the inner disk, and point
out the risk of interpreting the resulting image as that of a transition disk
with an inner hole if observed in unfavorable conditions. Planet signatures are
fainter in more distant disks but declination or inclination to the
line-of-sight have little effect on ALMA's ability to resolve the gaps.
Conclusions: ALMA has the potential to see signposts of planets in disks of
nearby star-forming regions. We present optimized observing parameters to
detect them in the case of 1 and 5 M_J planets on 40 AU orbits.Comment: 15 pages, 21 figures, accepted by Astronomy & Astrophysics, a higher
resolution version of the paper is available at
http://www-obs.univ-lyon1.fr/labo/perso/jean-francois.gonzalez/Papers/Gaps_ALMA.pd
The X-ray Flux Distribution of Sagittarius A* as Seen by Chandra
We present a statistical analysis of the X-ray flux distribution of Sgr A*
from the Chandra X-ray Observatory's 3 Ms Sgr A* X-ray Visionary Project (XVP)
in 2012. Our analysis indicates that the observed X-ray flux distribution can
be decomposed into a steady quiescent component, represented by a Poisson
process with rate cts s and a variable
component, represented by a power law process (
). This slope matches our recently-reported
distribution of flare luminosities. The variability may also be described by a
log-normal process with a median unabsorbed 2-8 keV flux of
erg s cm and a shape parameter
but the power law provides a superior description of the
data. In this decomposition of the flux distribution, all of the intrinsic
X-ray variability of Sgr A* (spanning at least three orders of magnitude in
flux) can be attributed to flaring activity, likely in the inner accretion
flow. We confirm that at the faint end, the variable component contributes ~10%
of the apparent quiescent flux, as previously indicated by our statistical
analysis of X-ray flares in these Chandra observations. Our flux distribution
provides a new and important observational constraint on theoretical models of
Sgr A*, and we use simple radiation models to explore the extent to which a
statistical comparison of the X-ray and infrared can provide insights into the
physics of the X-ray emission mechanism.Comment: 13 pages, 7 figures, accepted for publication in ApJ. Comments
welcom
- …