Potential energy landscape-based extended van der Waals equation

The inherent structures ({\it IS}) are the local minima of the potential energy surface or landscape, $U({\bf r})$, of an {\it N} atom system. Stillinger has given an exact {\it IS} formulation of thermodynamics. Here the implications for the equation of state are investigated. It is shown that the van der Waals ({\it vdW}) equation, with density-dependent $a$ and $b$ coefficients, holds on the high-temperature plateau of the averaged {\it IS} energy. However, an additional ``landscape'' contribution to the pressure is found at lower $T$. The resulting extended {\it vdW} equation, unlike the original, is capable of yielding a water-like density anomaly, flat isotherms in the coexistence region {\it vs} {\it vdW} loops, and several other desirable features. The plateau energy, the width of the distribution of {\it IS}, and the ``top of the landscape'' temperature are simulated over a broad reduced density range, $2.0 \ge \rho \ge 0.20$, in the Lennard-Jones fluid. Fits to the data yield an explicit equation of state, which is argued to be useful at high density; it nevertheless reproduces the known values of $a$ and $b$ at the critical point

Sirt2 promotes white matter oligodendrogenesis during development and in models of neonatal hypoxia

Delayed oligodendrocyte (OL) maturation caused by hypoxia (Hx)-induced neonatal brain injury results in hypomyelination and leads to neurological disabilities. Previously, we characterized Sirt1 as a crucial regulator of OL progenitor cell (OPC) proliferation in response to Hx. We now identify Sirt2 as a critical promoter of OL differentiation during both normal white matter development and in a mouse model of Hx. Importantly, we find that Hx reduces Sirt2 expression in mature OLs and that Sirt2 overexpression in OPCs restores mature OL populations. Reduced numbers of Sirt2+ OLs were also observed in the white matter of preterm human infants. We show that Sirt2 interacts with p27Kip1/FoxO1, p21Cip1/Cdk4, and Cdk5 pathways, and that these interactions are altered by Hx. Furthermore, Hx induces nuclear translocation of Sirt2 in OPCs where it binds several genomic targets. Overall, these results indicate that a balance of Sirt1 and Sirt2 activity is required for developmental oligodendrogenesis, and that these proteins represent potential targets for promoting repair following white matter injury

Entropy, Dynamics and Instantaneous Normal Modes in a Random Energy Model

It is shown that the fraction f of imaginary frequency instantaneous normal modes (INM) may be defined and calculated in a random energy model(REM) of liquids. The configurational entropy S and the averaged hopping rate among the states R are also obtained and related to f, with the results R~f and S=a+b*ln(f). The proportionality between R and f is the basis of existing INM theories of diffusion, so the REM further confirms their validity. A link to S opens new avenues for introducing INM into dynamical theories. Liquid 'states' are usually defined by assigning a configuration to the minimum to which it will drain, but the REM naturally treats saddle-barriers on the same footing as minima, which may be a better mapping of the continuum of configurations to discrete states. Requirements of a detailed REM description of liquids are discussed

The Potential Energy Landscape and Mechanisms of Diffusion in Liquids

The mechanism of diffusion in supercooled liquids is investigated from the potential energy landscape point of view, with emphasis on the crossover from high- to low-T dynamics. Molecular dynamics simulations with a time dependent mapping to the associated local mininum or inherent structure (IS) are performed on unit-density Lennard-Jones (LJ). New dynamical quantities introduced include r2_{is}(t), the mean-square displacement (MSD) within a basin of attraction of an IS, R2(t), the MSD of the IS itself, and g_{loc}(t) the mean waiting time in a cooperative region. At intermediate T, r2_{is}(t) posesses an interval of linear t-dependence allowing calculation of an intrabasin diffusion constant D_{is}. Near T_{c} diffusion is intrabasin dominated with D = D_{is}. Below T_{c} the local waiting time tau_{loc} exceeds the time, tau_{pl}, needed for the system to explore the basin, indicating the action of barriers. The distinction between motion among the IS below T_{c} and saddle, or border dynamics above T_{c} is discussed.Comment: submitted to pr

Rôle et place du riz pluvial dans les exploitations du lac Alaotra

International audienceLa région de l'Alaotra repose sur un plateau situé à 750 mètres d'altitude avec au cœur de la plaine le lac Alaotra. La surface cultivée en riziculture est estimée à 148 500 ha en 2000 (FAO/UPDR, 2000), dont 75-80 000 ha dans la cuvette du Lac et plus de 65-70 000 ha au sud et dans les zones en périphérie, qui constitue 10% de la surface rizicole nationale pour seulement 4% des riziculteurs malgaches. Cette région réalise 33% de la valeur ajoutée de l'ensemble de la filière nationale et 15% de la richesse générée par le secteur riz. Le climat irrégulier constitue une contrainte majeure pour tous les agriculteurs du lac Alaotra. Les relations agriculture-élevage sont au cœur de la problématique de développement et d'évolution des exploitations agricoles.Ainsi, avec la saturation des rizières irrigables ou à Mauvaise Maîtrise de l'Eau (RMME), la colonisation agricole des tanety anciennement dévolus aux pâturages extensifs depuis les années 1980 s'est accélérer, induisant la perturbation des activités d'élevage bovin traditionnel extensif à objectif de capitalisation qui évoluent aujourd'hui vers un élevage productif (embouche et production laitière) ou de trait. Le diagnostic réalisé en 2007 a mis en évidence 7 types d'exploitations agricoles différents. Les différents systèmes de riziculture pluviale se retrouvent sur les plateaux sommitaux et les pentes des collines (en rotation jachère culture sur les sols très pauvres), les bas de pente et les baiboho, en rotation avec d'autres cultures pluviales (maïs, manioc, pois de terre...) avec ou sans jachère, ou en système SCV depuis leur introduction au début des années 2000. L'introduction de variétés composites poly-aptitudes de type Sebota a pu lever une contrainte majeure des zones ni totalement irriguées ni totalement pluviales mais intermédiaires dans les RMME (riziculture pluviale partiellement irriguée, selon l'accès à l'eau et les caractéristiques climatiques de la saison), Les systèmes SCV mis au point sur baiboho avec des successions culturales de type riz de saison / haricot de contre saison - maïs de saison ou riz de saison puis vesce de contre saison associée ou non à des cultures maraîchères montrent également un niveau de production et de sécurité par rapport aux aléas climatiques très proche voire supérieurs à ceux obtenus en riziculture irriguée. La part du riz pluvial dans la formation du revenu et la sécurité alimentaire a été analysée pour chaque type d'exploitation et varie de 20 à 30 % du revenu riz total (avant autoconsommation), et se révèle croissante en fonction du rapport tanety / rizière dans l'exploitation. Ainsi, sur certaines zones, le riz pluvial semble avoir de beaux jours devant lui. Mots clés : Riz pluvial, exploitation agricoles, lac Alaotra, SCV, riz poly-aptitudes

Saddles in the energy landscape probed by supercooled liquids

We numerically investigate the supercooled dynamics of two simple model liquids exploiting the partition of the multi-dimension configuration space in basins of attraction of the stationary points (inherent saddles) of the potential energy surface. We find that the inherent saddles order and potential energy are well defined functions of the temperature T. Moreover, decreasing T, the saddle order vanishes at the same temperature (T_MCT) where the inverse diffusivity appears to diverge as a power law. This allows a topological interpretation of T_MCT: it marks the transition from a dynamics between basins of saddles (T>T_MCT) to a dynamics between basins of minima (T<T_MCT).Comment: 4 pages, 3 figures, to be published on PR

VFISV: Very Fast Inversion of the Stokes Vector for the Helioseismic and Magnetic Imager

In this paper we describe in detail the implementation and main properties of a new inversion code for the polarized radiative transfer equation (VFISV: Very Fast inversion of the Stokes vector). VFISV will routinely analyze pipeline data from the Helioseismic and Magnetic Imager (HMI) on-board of the Solar Dynamics Observatory (SDO). It will provide full-disk maps (4096$\times$4096 pixels) of the magnetic field vector on the Solar Photosphere every 10 minutes. For this reason VFISV is optimized to achieve an inversion speed that will allow it to invert 16 million pixels every 10 minutes with a modest number (approx. 50) of CPUs. Here we focus on describing a number of important details, simplifications and tweaks that have allowed us to significantly speed up the inversion process. We also give details on tests performed with data from the spectropolarimeter on-board of the Hinode spacecraft.Comment: 23 pages, 9 figures (2 color). Submitted for publication to Solar Physic