Prediction of the elastic modulus of the trabecular bone based on X-ray computed tomography

International audienceThis work aims to estimate the apparent Young's modulus of real human trabecular bones using a numerical micro-macro approach. Cylindrical specimens of trabecular bone were extracted from human femur heads, cleaned and scanned using a SkyScan-1072 micro-computed tomography system. 3D volumetric tetrahedral grids were generated from the exploitation of the reconstructed images using original meshing techniques. Numerical compressive tests were simulated, assuming isotropic tissue Young's modulus for all elements. The large size of the volumes implies grids with a high number of nodes, which required the use of a large number of parallel processors in order to perform the finite element calculations. Numerical Young's moduli varied between 1300 MPa and 1600 MPa, with a good agreement with experiments

Caractérisation de milieux poreux par étude de leur géométrie 3D : Application à l'os trabéculaire

Cette communication présente le développement d'un ensemble d'outils permettant de caractériser un milieu poreux tel que l'os trabéculaire. Ce travail est basé sur une nouvelle technique permettant de localiser et d'individualiser les arches du milieu poreux. Nous nous intéressons principalement à la mesure d'anisotropie en 3 dimensions et au calcul d'indices d'orientation et de courbure pour chaque travée. Les différentes techniques mises en place sont comparées sur 2 populations différentes composées d'échantillons osseux ostéoporotiques et coxarthriques. Nous montrons que seuls les indices de courbure des travées permettent de discriminer de manière significative les 2 populations étudiées

Nouvelle approche de modélisation de milieux poreux. Application à l'os trabéculaire

Dans de nombreux domaines tels que la science des matériaux ou l'imagerie médicale, il est intéressant d'évaluer les propriétés mécaniques d'une structure. Pour ce faire, l'analyse par éléments finis est souvent utilisée. Cependant, son application à des milieux poreux complexes est limitée, car le nombre d'éléments nécessaires pour représenter la structure interne est très grand. Dans cette communication, nous proposons une nouvelle approche par éléments finis qui prend en compte la topologie de la structure étudiée. Pour cela, nous implémentons et améliorons une technique récente basée sur le squelette 3D, permettant de caractériser des milieux poreux complexes. Chaque travée de la structure interne peut alors être représentée par une chaîne de poutres rectilignes auxquelles sont attribuées les propriétés de l'arche. Cette approche permet de réduire considérablement le temps de calcul nécessaire à la simulation mécanique par éléments finis tout en présentant une bonne adéquation avec une technique de référence. Nous avons évalué cette technique sur des vecteurs de test, puis appliqué notre méthode sur des échantillons d'os trabéculaire afin d'en quantifier précisément l'élasticité. Ce nouveau procédé de modélisation donne de meilleurs résultats de rigidité par rapport aux techniques à éléments poutre existantes sur des vecteurs de test. Cette tendance se confirme également lors de son application à l'étude de la microarchitecture de l'os trabéculaire

Constraining the recent star formation history of galaxies: an approximate Bayesian computation approach

Although galaxies are found to follow a tight relation between their star formation rate and stellar mass, they are expected to exhibit complex star formation histories (SFH) with short-term fluctuations. The goal of this pilot study is to present a method that identifies galaxies that undergo strong variation in star formation activity in the last ten to some hundred million years. In other words, the proposed method determines whether a variation in the last few hundred million years of the SFH is needed to properly model the spectral energy distribution (SED) rather than a smooth normal SFH. To do so, we analyzed a sample of COSMOS galaxies with 0.5   8.5 using high signal-to-noise ratio broadband photometry. We applied approximate Bayesian computation, a custom statistical method for performing model choice, which is associated with machine-learning algorithms to provide the probability that a flexible SFH is preferred based on the observed flux density ratios of galaxies. We present the method and test it on a sample of simulated SEDs. The input information fed to the algorithm is a set of broadband UV to NIR (rest-frame) flux ratios for each galaxy. The choice of using colors is made to remove any difficulty linked to normalization when classification algorithms are used. The method has an error rate of 21% in recovering the correct SFH and is sensitive to SFR variations larger than 1 dex. A more traditional SED-fitting method using CIGALE is tested to achieve the same goal, based on fit comparisons through the Bayesian information criterion, but the best error rate we obtained is higher, 28%. We applied our new method to the COSMOS galaxies sample. The stellar mass distribution of galaxies with a strong to decisive evidence against the smooth delayed-τ SFH peaks at lower M* than for galaxies where the smooth delayed-τ SFH is preferred. We discuss the fact that this result does not come from any bias due to our training. Finally, we argue that flexible SFHs are needed to be able to cover the largest possible SFR-M* parameter space

Investigating the delay between dust radiation and star-formation in local and distant quenching galaxies

International audienceWe investigate the timescale over which the infrared (IR) luminosity decreases after a complete and rapid quenching of star formation using observations of local and high-redshift galaxies. From spectral energy distribution modelling, we derive the time since quenching of a subsample of 14 galaxies from the Herschel Reference Survey that suffer from ram-pressure stripping due to the environment of the Virgo cluster and of a subsample of 7 rapidly quenched COSMOS galaxies selected through a state-of-the-art statistical method already tested on the determination of galaxy star formation history (SFH). Three out of the seven COSMOS galaxies have an optical spectrum with no emission line, confirming their quenched nature. We obtained the present physical properties of the combined sample (local plus high-redshift) from the long-term SFH properties, as well as the past LIR of these galaxies just before their quenching. We show that this past LIR is consistent with the LIR of reference samples of normally star-forming galaxies with same stellar mass and redshift as each of our quenched galaxies. We put constraints on the present to past IR luminosity ratio as a function of quenching time. The two samples probe different dynamical ranges in terms of quenching age with the HRS galaxies exhibiting longer timescales (0.2–3 Gyr) compared to the COSMOS ones (< 100 Myr). Assuming an exponential decrease in the LIR after quenching, the COSMOS quenched galaxies are consistent with short e-folding times of less than a couple of hundred million years, while the properties of the HRS quenched galaxies are compatible with larger timescales of several hundred million years. For the HRS sample, this result is consistent with the known quenching mechanism that affected them, namely ram pressure stripping due to the environment. For the COSMOS sample, different quenching processes are acting on short to intermediate timescales. Processes such as galaxy mergers, disk instabilities, and environmental effects can produce such strong star formation variability

Diffusion and viscosity in polysaccharide solutions: Beyond Stokes-Einstein

Non-starch polysaccharides such as arabinoxylans (AXs) have important roles in diet, especially in so-called resistant starch, which results in cholesterol lowering.1 This beneficial effect may arise from viscosity or diffusion phenomena in the gut. In the present study, the diffusion coefficients of a dextran probe in solutions of three AXs of different viscosities were measured. The diffusion coefficients of a dextran probe (MW = 7.0×104) in solutions of three different arabinoxylans over a wide concentration range were carried out using fluorescent recovery after photobleaching (FRAP