82 research outputs found
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
Approche méthodologique de la modélisation du transport des HAP dans les sols et les eaux
Afin de préciser le cadre d'utilisation de codes de transport de polluants dans les sols et les eaux, un programme d'intercomparaison de méthodes et de codes a été mis en oeuvre avec la collaboration de huit équipes appartenant à des instituts de recherche ou des bureaux d'études. Parmi les types de polluants étudiés, les hydrocarbures aromatiques polycycliques (HAP) ont fait l'objet de simulations par deux groupes de travail. Le premier groupe, composé de cinq équipes, s'est intéressé à leur comportement en phase dissoute à partir d'un cas réel de pollution. Disposant des mêmes données d'étude, chaque équipe avait pour mission de concevoir un modèle conceptuel (description géologique, définition de la source, paramètres d'écoulement et de transport) et de simuler la pollution de la nappe avec le code de son choix. Les résultats obtenus montrent des différences significatives qui trouvent leur explication dans les paramètres des modèles conceptuels adoptés plus que dans les simulateurs choisis. Cet exercice a confirmé l'importance des paramètres suivants qui sont souvent incertains mais justifieraient d'être mieux appréciés lors de diagnostics : paramètres hydrodynamiques (détermination du champ de vitesse), coefficient de partition (pour chaque horizon géologique), temps de demi-vie du ou des polluants, extension de la source de pollution. Le second groupe a étudié les écoulements en phase libre (phase liquide non aqueuse) sur un cas théorique inspiré d'un cas réel de déversement massif de naphtalène. La modélisation a été conduite par trois équipes avec trois codes polyphasiques différents (SIMUSCOPP, TOUGH/T2VOC, UTCHEM). Cet exercice comprend le suivi du déversement et de la migration du naphtalène sous formes liquide et dissoute sur une distance de 300 m et une durée de 10 ans. Le naphtalène révèle un comportement d'hydrocarbure " lourd " avec une phase huile (partiellement miscible) qui tend à descendre à travers la nappe. Ce comportement a été reproduit par chacune des équipes, de manière plus ou moins complète en fonction des possibilités de modélisation offertes par les codes. Les résultats obtenus permettent de vérifier la cohérence des différentes approches polyphasiques entre elles. Bien que plus complexe (paramètres plus nombreux, difficulté numérique accrue), l'approche polyphasique se justifie pour mieux comprendre et déterminer la répartition spatiale de HAP en profondeur dans une nappe
Beam mask and sliding window-facilitated deep learning-based accurate and efficient dose prediction for pencil beam scanning proton therapy
Purpose: To develop a DL-based PBSPT dose prediction workflow with high
accuracy and balanced complexity to support on-line adaptive proton therapy
clinical decision and subsequent replanning.
Methods: PBSPT plans of 103 prostate cancer patients and 83 lung cancer
patients previously treated at our institution were included in the study, each
with CTs, structure sets, and plan doses calculated by the in-house developed
Monte-Carlo dose engine. For the ablation study, we designed three experiments
corresponding to the following three methods: 1) Experiment 1, the conventional
region of interest (ROI) method. 2) Experiment 2, the beam mask (generated by
raytracing of proton beams) method to improve proton dose prediction. 3)
Experiment 3, the sliding window method for the model to focus on local details
to further improve proton dose prediction. A fully connected 3D-Unet was
adopted as the backbone. Dose volume histogram (DVH) indices, 3D Gamma passing
rates, and dice coefficients for the structures enclosed by the iso-dose lines
between the predicted and the ground truth doses were used as the evaluation
metrics. The calculation time for each proton dose prediction was recorded to
evaluate the method's efficiency.
Results: Compared to the conventional ROI method, the beam mask method
improved the agreement of DVH indices for both targets and OARs and the sliding
window method further improved the agreement of the DVH indices. For the 3D
Gamma passing rates in the target, OARs, and BODY (outside target and OARs),
the beam mask method can improve the passing rates in these regions and the
sliding window method further improved them. A similar trend was also observed
for the dice coefficients. In fact, this trend was especially remarkable for
relatively low prescription isodose lines. The dose predictions for all the
testing cases were completed within 0.25s
Long term aging of Selenide glasses: Evidence of sub-Tg endotherms and pre-Tg exotherms
Long term aging is studied on several families of chalcogenide glasses
including the Ge-Se, As-Se, Ge-P-Se and Ge-As-Se systems. Special attention is
given to the As-Se binary, a system that displays a rich variety of aging
behavior intimately tied to sample synthesis conditions and the ambient
environment in which samples are aged. Calorimetric (Modulated DSC) and Raman
scattering experiments are undertaken. Our results show all samples display a
sub-Tg endotherm below Tg in glassy networks possessing a mean coordination
number r in the 2.25 < r < 2.45 range. Two sets of AsxSe1-x samples aged for 8
years were compared, set A consisted of slow cooled samples aged in the dark,
and set B consisted of melt quenched samples aged at laboratory environment.
Samples of set B in the As concentration range, 35% < x < 60%, display a pre-Tg
exotherm, but the feature is not observed in samples of set A. The aging
behavior of set A presumably represents intrinsic aging in these glasses, while
that of set B is extrinsic due to presence of light. The reversibility window
persists in both sets of samples but is less well defined in set B. These
findings contrast with a recent study by Golovchak et al., which finds the
onset of the reversibility window moved up to the stoichiometric composition (x
= 40%). Here we show that the upshifted window is better understood as
resulting due to demixing of As4Se4 and As4Se3 molecules from the backbone,
i.e., Nanoscale phase separation (NSPS). We attribute sub-Tg endotherms to
compaction of the flexible part of networks upon long term aging, while the
pre-Tg exotherm to NSPS. Finally, the narrowing and sharpening of the
reversibility window upon aging is interpreted as the slow 'self-organizing'
stress relaxation of the phases just outside the Intermediate phase.Comment: In press - J. of Physics: Condensed Matte
Inertial focusing of small particles in wavy channels: Asymptotic analysis at weak particle inertia
International audienceThe motion of tiny non-Brownian inertial particles in a two-dimensional channel flow with periodic corrugations is investigated analytically, to determine the trapping rate as well as the exact position of the attractor, and understand the conditions under which particle trapping and long-term suspension occur. This phenomenon has been observed numerically in previous works and happens under the combined effects of confinement and inertia. Starting from the particle motion equations, a Poincare map is constructed analytically in the limit of weak inertia and weak channel corrugations. It enables to derive the equation of the attractor, if any, and the corresponding trapping rate. The attractor is close to a streamline, the so-called ``attracting streamline'', and is shown to persist in the presence of transverse gravity, provided the channel Froude number is large enough. Particles which are trapped by this streamline can therefore travel over long distances, avoiding deposition. Numerical simulations confirm the theoretical results at small particle response times tau and reveal some non-linear effects at larger tau: the asymptotic attractor becomes unstable at some critical value and splits into multiple branches each with its own basin of attraction
A dual-porosity theory for solute transport in biofilm-coated porous media
International audienceIn this work, we derive a Darcy-scale model for solute transport in porous media colonized by biofilms. Biofilms refer to structured communities of microbial cells that grow attached to an aqueous interface, e.g., the pore walls of a water-saturated porous medium. First, we present the pore-scale description of mass transport within and between the phases (water phase and biofilm phase) including the biologically-mediated reactions. Then, a two-equation Darcy-scale mass balance model (classically referred to as dual-porosity model), suitable both for local equilibrium and non-equilibrium conditions, is obtained from the method of volume averaging. Application of this macroscopic model to complex pore geometry of a two-dimensional porous medium supporting biofilms is presented. Comparisons with direct numerical simulation results confirm the large range of validity of the model. Finally, an illustrative example based on a scenario of reactive transport experimental tests is used to point out conditions for when the use of the proposed model is required
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