Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023

Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory. ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum. These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. Singly ionized PAH dimers appear as good candidates but PAH$^{++}$ molecules seem to be excluded.Comment: Accepted for publication in A&

Three-dimensional flow instability in a lid-driven isosceles triangular cavity

Linear three-dimensional modal instability of steady laminar two-dimensional states developing in a lid-driven cavity of isosceles triangular cross-section is investigated theoretically and experimentally for the case in which the equal sides form a rectangular corner. An asymmetric steady two-dimensional motion is driven by the steady motion of one of the equal sides. If the side moves away from the rectangular corner, a stationary three-dimensional instability is found. If the motion is directed towards the corner, the instability is oscillatory. The respective critical Reynolds numbers are identified both theoretically and experimentally. The neutral curves pertinent to the two configurations and the properties of the respective leading eigenmodes are documented and analogies to instabilities in rectangular lid-driven cavities are discussed

Resistance noise scaling in a 2D system in GaAs

The 1/f resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well has been measured on both sides of the 2D metal-insulator transition (MIT) at zero magnetic field (B=0), and deep in the insulating regime. The two measurement methods used are described: I or V fixed, and measurement of resp. V or I fluctuations. The normalized noise magnitude SR/R^2 increases strongly when the hole density is decreased, and its temperature (T) dependence goes from a slight increase with T at the largest densities, to a strong decrease at low density. We find that the noise magnitude scales with the resistance, SR /R^2 ~ R^2.4. Such a scaling is expected for a second order phase transition or a percolation transition. The possible presence of such a transition is investigated by studying the dependence of the conductivity as a function of the density. This dependence is consistent with a critical behavior close to a critical density p* lower than the usual MIT critical density pc.Comment: 13 pages, 8 figures, Proceedings of SPIE: Fluctuations and noise in materials, D. Popovic, M.B. Weissman, Z.A. Racz Eds., Vol. 5469, pp. 101-113, Mspalomas, Spain, 200

Adaptive mesh refinement with spectral accuracy for magnetohydrodynamics in two space dimensions

We examine the effect of accuracy of high-order spectral element methods, with or without adaptive mesh refinement (AMR), in the context of a classical configuration of magnetic reconnection in two space dimensions, the so-called Orszag-Tang vortex made up of a magnetic X-point centered on a stagnation point of the velocity. A recently developed spectral-element adaptive refinement incompressible magnetohydrodynamic (MHD) code is applied to simulate this problem. The MHD solver is explicit, and uses the Elsasser formulation on high-order elements. It automatically takes advantage of the adaptive grid mechanics that have been described elsewhere in the fluid context [Rosenberg, Fournier, Fischer, Pouquet, J. Comp. Phys. 215, 59-80 (2006)]; the code allows both statically refined and dynamically refined grids. Tests of the algorithm using analytic solutions are described, and comparisons of the Orszag-Tang solutions with pseudo-spectral computations are performed. We demonstrate for moderate Reynolds numbers that the algorithms using both static and refined grids reproduce the pseudo--spectral solutions quite well. We show that low-order truncation--even with a comparable number of global degrees of freedom--fails to correctly model some strong (sup--norm) quantities in this problem, even though it satisfies adequately the weak (integrated) balance diagnostics.Comment: 19 pages, 10 figures, 1 table. Submitted to New Journal of Physic

Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction of surface and volume excitations of liquid helium, which rapidly decreases with temperature. Thus, the temperature dependence of the low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V s), where n_e is the surface electron density. The relation T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and \omega/2\pi < 30 Hz.Comment: 4 pages, 1 figur

An efficient iterative solution method for the Chebyshev collocation of advection-dominated transport problems

A new Chebyshev collocation algorithm is proposed for the iterative solution of advection-diffusion problems. The main features of the method lie in the original way in which a finite-difference preconditioner is built and in the fact that the solution is collocated on a set of nodes matching the standard Gauss-Lobatto-Chebyshev set only in the case of pure diffusion problems. The key point of the algorithm is the capability of the preconditioner to represent the high-frequency modes when dealing with advection-dominated problems. The basic idea is developed for a one-dimensional case and is extended to two-dimensional problems. A series of numerical experiments is carried out to demonstrate the efficiency of the algorithm. The proposed algorithm can also be used in the context of the incompressible Navier-Stokes equations

Prevalence and family-related factors associated with suicidal ideation, suicide attempts, and self-injury in children aged 9 to 10 years

Importance: Although suicide is a leading cause of death for children in the United States, and the rate of suicide in childhood has steadily increased, little is known about suicidal ideation and behaviors in children. Objective: To assess the overall prevalence of suicidal ideation, suicide attempts, and nonsuicidal self-injury, as well as family-related factors associated with suicidality and self-injury among preadolescent children. Design, Setting, and Participants: Cross-sectional study using retrospective analysis of the baseline sample from the Adolescent Brain Cognitive Development (ABCD) study. This multicenter investigation used an epidemiologically informed school-based recruitment strategy, with consideration of the demographic composition of the 21 ABCD sites and the United States as a whole. The sample included children aged 9 to 10 years and their caregivers. Main Outcomes and Measures: Lifetime suicidal ideation, suicide attempts, and nonsuicidal self-injury as reported by children and their caregivers in a computerized version of the Kiddie Schedule for Affective Disorders and Schizophrenia. Results: A total of 11 814 children aged 9 to 10 years (47.8% girls; 52.0% white) and their caregivers were included. After poststratification sociodemographic weighting, the approximate prevalence rates were 6.4% (95% CI, 5.7%-7.3%) for lifetime history of passive suicidal ideation; 4.4% (95% CI, 3.9%-5.0%) for nonspecific active suicidal ideation; 2.4% (95% CI, 2.1%-2.7%) for active ideation with method, intent, or plan; 1.3% (95% CI, 1.0%-1.6%) for suicide attempts; and 9.1% (95% CI, 8.1-10.3) for nonsuicidal self-injury. After covarying by sex, family history, internalizing and externalizing problems, and relevant psychosocial variables, high family conflict was significantly associated with suicidal ideation (odds ratio [OR], 1.12; 95% CI, 1.07-1.16) and nonsuicidal self-injury (OR, 1.09; 95% CI, 1.05-1.14), and low parental monitoring was significantly associated with ideation (OR, 0.97; 95% CI, 0.95-0.98), attempts (OR, 0.91; 95% CI, 0.86-0.97), and nonsuicidal self-injury (OR, 0.95; 95% CI, 0.93-0.98); these findings were consistent after internal replication. Most of children\u27s reports of suicidality and self-injury were either unknown or not reported by their caregivers. Conclusions and Relevance: This study demonstrates the association of family factors, including high family conflict and low parental monitoring, with suicidality and self-injury in children. Future research and ongoing prevention and intervention efforts may benefit from the examination of family factors

Integrate and Fire Neural Networks, Piecewise Contractive Maps and Limit Cycles

We study the global dynamics of integrate and fire neural networks composed of an arbitrary number of identical neurons interacting by inhibition and excitation. We prove that if the interactions are strong enough, then the support of the stable asymptotic dynamics consists of limit cycles. We also find sufficient conditions for the synchronization of networks containing excitatory neurons. The proofs are based on the analysis of the equivalent dynamics of a piecewise continuous Poincar\'e map associated to the system. We show that for strong interactions the Poincar\'e map is piecewise contractive. Using this contraction property, we prove that there exist a countable number of limit cycles attracting all the orbits dropping into the stable subset of the phase space. This result applies not only to the Poincar\'e map under study, but also to a wide class of general n-dimensional piecewise contractive maps.Comment: 46 pages. In this version we added many comments suggested by the referees all along the paper, we changed the introduction and the section containing the conclusions. The final version will appear in Journal of Mathematical Biology of SPRINGER and will be available at http://www.springerlink.com/content/0303-681

Magneto-shear modes and a.c. dissipation in a two-dimensional Wigner crystal

The a.c. response of an unpinned and finite 2D Wigner crystal to electric fields at an angular frequency $\omega$ has been calculated in the dissipative limit, $\omega \tau \ll 1$, where $\tau ^{-1}$ is the scattering rate. For electrons screened by parallel electrodes, in zero magnetic field the long-wavelength excitations are a diffusive longitudinal transmission line mode and a diffusive shear mode. A magnetic field couples these modes together to form two new magneto-shear modes. The dimensionless coupling parameter $\beta =2(c_{t}/c_{l})|\sigma_{xy}/\sigma_{xx}|$ where $c_{t}$ and $c_{l}$ are the speeds of transverse and longitudinal sound in the collisionless limit and $\sigma_{xy}$ and $\sigma_{xx}$ are the tensor components of the magnetoconductivity. For $\beta \geqslant 1$, both the coupled modes contribute to the response of 2D electrons in a Corbino disk measurement of magnetoconductivity. For $\beta \gg 1$, the electron crystal rotates rigidly in a magnetic field. In general, both the amplitude and phase of the measured a.c. currents are changed by the shear modulus. In principle, both the magnetoconductivity and the shear modulus can be measured simultaneously.Comment: REVTeX, 7 pp., 4 eps figure

Pelizaeus-Merzbacher-Like disease presentation of MCT8 mutated male subjects.

Pelizaeus-Merzbacher Disease is an X-linked hypomyelinatiing leukodystrophy. We report mutations in the thyroid hormone transporter gene MCT8 in 11% of 53 families affected by hypomyelinating leukodystrophies of unknown aetiology. The 12 MCT8 mutated patients express initially a Pelizaeus-Merzbacher-Like disease phenotype with a latter unusual improvement of magnetic resonance imaging white matter signal despite absence of clinical progression. This observation underlines the interest of determining both free T3 and free T4 serum concentrations to screen for MCT8 mutations in young patients (<3 y) with a severe Pelizaeus-Merzbacher-Like disease presentation or older severe mentally retarded male patients with "hypomyelinated" regions