The Dependence of the Galactic Star Formation Laws on Metallicity

We describe results from semi-analytical modelling of star formation in protocluster clumps of different metallicities. In this model, gravitationally bound cores form uniformly in the clump following a prescribed core formation efficiency per unit time. After a contraction timescale which is equal to a few times their free-fall times, the cores collapse into stars and populate the IMF. Feedback from the newly formed OB stars is taken into account in the form of stellar winds. When the ratio of the effective energy of the winds to the gravitational energy of the system reaches unity, gas is removed from the clump and core and star formation are quenched. The power of the radiation driven winds has a strong dependence on metallicity and it increases with increasing metallicity. Thus, winds from stars in the high metallicity models lead to a rapid evacuation of the gas from the protocluster clump and to a reduced star formation efficiency, as compared to their low metallicity counterparts. We derive the metallicity dependent star formation efficiency per unit time in this model as a function of the gas surface density Sigma_g. This is combined with the molecular gas fraction in order to derive the dependence of the surface density of star formation Sigma_SFR on Sigma_g. This feedback regulated model of star formation reproduces very well the observed star formation laws in galaxies extending from low gas surface densities up to the starburst regime. Furthermore, the results show a dependence of Sigma_SFR on metallicity over the entire range of gas surface densities, and can also explain part of the scatter in the observations.Comment: In the proceedings of the French Astronomical Society meeting SF2A 2011. 8 pages, 5 figure

Surface convection: from the Sun to red giant stars

We check how the change in surface conditions between the Sun and red giant branch stars changes the characteristic surface convection length scale to be used in models. We investigate the question in the case of the mixing length theory and of the phenomenology of full spectrum of turbulence. For the observational part, we rely on independent measurements of effective temperatures and interferometric radii of nearby red giants. We find that the local red giant branch cannot be explained taking into account the solar calibrated convective length scale.Comment: In the proceedings of the French Astronomical Society meeting SF2A 201

Thermographic monitoring of asphalt concrete surface with phase change materials inclusions for icing delays purposes

International audienceIf de-icers still are the main solution to avoid black ice occurrence and snow accumulation on pavements, some alternatives based on transportation infrastructure modifications have been tested over the past years. They aim at the reduction of environmental impacts and at coping with budgets constraints and cuts, including in winter maintenance. Among existing ones, the implementation of phase changes materials (PCM) into the infrastructure was evaluated to delay the occurrence of water in its solid phase on the surface. Additional results were obtained with an asphalt concrete in simulated winter conditions, indicating a tenuous thermal effect at the surface

Un schéma d'intégration temporelle pour la réponse transitoire de systèmes mécaniques avec butées de contact

Dans le but de modéliser en temps réel le comportement dynamique en régime transitoire de train tiges de forage confinées dans un puits, il faut utiliser des techniques numériques économes en temps de calcul. Les méthodes d'intégration temporelle pas-à-pas [1], telles que celles de Runge-Kutta (RK), [2, 3, 4] ou de Newmark (NM) [5] sont nécessaires pour calculer la réponse transitoire non linéaire, mais sont coûteuses en temps. Il convient donc d'améliorer leur efficacité. Dans les méthodes de RK, c'est le schéma explicite d'ordre 4 qui est le plus largement répandu. Comme celui-ci exprime l'état actuel du système en fonction de son état précédent, il est donc simple à implémenter. Néanmoins, le pas de temps doit être suffisamment petit pour satisfaire la condition de stabilité. De plus, la taille du système d'équations à résoudre est doublée car le système est décomposé en deux systèmes d'équations différentielles du 1er ordre. Pour les systèmes linéaires, les schémas de NM [5] sont inconditionnellement stables même avec un pas de temps plus grand que celui des schémas explicites. Cependant la nécessité d'inverser les matrices à chaque pas de temps pénalise les temps de calcul. D'autre part, à cause des non-linéarités des systèmes mécaniques, il est usuel d'introduire à chaque pas de temps l'algorithme itératif de Newton-Raphson, (NR) qui impose le traitement délicat des matrices jacobiennes. Afin d'éviter l'appel à NR, une technique approximative est d'affecter au temps i+1 la valeur de la force non linéaire connue au temps i.  D'une façon générale, adapter le pas de temps améliore grandement l'efficacité des schémas numériques, notamment ceux de RK [4] et NM [6, 7]. Le modèle proposé ici, repose sur le schéma de NM à accélération constante qui allie la technique pour s'affranchir de NR et l'algorithme à pas adaptatif calculé utilisé dans [6, 7]. Son efficacité est démontrée en le comparant à d'autres schémas existants dans les cas d'un oscillateur à double butées et de la déflexion d'une poutre avec butée latérale répartie. Remerciements Cette recherche est réalisée dans le cadre de DRILLAB, laboratoire commun à la société DrillScan et le LaMCoS, membre de l'institut Carnot Ingénierie@Lyon. DRILLAB a été fondé grâce au programme de l'ANR Laboratoires communs organismes de recherche publique ? PME/ETI, ANR-15-LCV4-0010. Références [1] Y. M. Xie, An Assessment of time integration schemes for non-linear dynamic equations, J.ournal of Sound and Vibration, 192 , 321-331 (1996) [2] C. Runge, Über die numerische Auflösung von Differentialgleichungen. Math. Ann. 46, 167?178 (1895) [3] Kutta, M. Wilhelm, Beitrag zur näherungsweisen Integration totaler Differentialgleichungen, Zeitschrift für Mathematik und Physik, 46, 435-453 (1901) [4] W. H. Press, S. A. Teukolsky, W. T. Vetterling, B. P. Flannery, Numerical Recipes in Fortran, Cambridge University Press, (1992) [5] N. M. Newmark, A method of computation for structural dynamics, Journal of the Engineering Mechanics Division, ASCE 85, 67-94 (1959) [6] O. C. Zienkiewicz, Y. M. Xie, A simple error estimator and adaptive time stepping procedure for dynamic analysis, Earthquake engineering and structural dynamics, 20, 871-887 (1991) [7] D. Kuhl, E. Ramm, Generalized energy-momentum method for non-linear adaptive shell dynamics, Comput. Methods Appl. Mech.

Star formation efficiency as a function of metallicity: from star clusters to galaxies

We explore how the star formation efficiency in a protocluster clump is regulated by metallicity dependent stellar winds from the newly formed massive OB stars (Mstar >5 Msol). The model describes the co-evolution of the mass function of gravitationally bound cores and of the IMF in a protocluster clump. Dense cores are generated uniformly in time at different locations in the clump, and contract over lifetimes that are a few times their free fall times. The cores collapse to form stars that power strong stellar winds whose cumulative kinetic energy evacuates the gas from the clump and quenches further core and star formation. This sets the final star formation efficiency, SFEf. Models are run with various metallicities in the range Z/Zsol=[0.1,2]. We find that the SFEf decreases strongly with increasing metallicity.The SFEf-metallicity relation is well described by a decaying exponential whose exact parameters depend weakly on the value of the core formation efficiency. We find that there is almost no dependence of the SFEf-metallicity relation on the clump mass. This is due to the fact that an increase (decrease) in the clump mass leads to an increase (decrease) in the feedback from OB stars which is opposed by an increase (decrease) in the gravitational potential of the clump. The clump mass-cluster mass relations we find for all of the different metallicity cases imply a negligible difference between the exponent of the mass function of the protocluster clumps and that of the young clusters mass function. By normalizing the SFEs to their value for the solar metallicity case, we compare our results to SFE-metallicity relations derived on galactic scales and find a good agreement. As a by-product of this study, we also provide ready-to-use prescriptions for the power of stellar winds of main sequence OB stars in the mass range [5,80] Msol in the metallicity range we have consideredComment: accepted to MNRAS. More discussion added. Figures and conclusions unchange

Coupling protoplanetary disk formation with early protostellar evolution: influence on planet traps

International audienceProtoplanetary disk structures are known to be shaped by various thermal and compositional effects such as (though not limited to) shadowed regions, sublimation lines, density bumps... The resulting irregularities in the surface mass density and temperature profiles are key elements to determine the location where planetary embryos can be trapped. These traps provide hints of which planets are most likely to survive, at what distance from the star, and potentially with what composition (Baillié, Charnoz, Pantin, 2015, A&A 577, A65; Baillié, Charnoz, Pantin, 2016, A&A 590, A60). These structures are determined by the viscous spreading of the disk, that is initially formed by the collapse of the molecular cloud.Starting from the numerical hydrodynamical model detailed in Baillié & Charnoz., 2014, ApJ 786, 35 which couples the disk thermodynamics, its photosphere geometry, its dynamics and its dust composition in order to follow its long-term evolution, we now consider the early stages of the central star. We model the joint formation of the disk and the star: their mass are directly derived from the collapse of the molecular cloud while the star temperature, radius and brightness are interpolated over pre-calculated stellar evolutions. Therefore, our simulations no longer depend on the initial profile of the "Minimum Mass Solar Nebula", and allow us to model the influence of the forming star on the protoplanetary disk. In particular, we will present the resulting distribution of the sublimation lines of the main dust species, as well as the locations of the planet traps at various disk ages. In the longer term, we intend to investigate the influence of the star properties on the selection of the surviving planets

Coupling protoplanetary disk formation with early protostellar evolution: influence on planet traps

International audienceProtoplanetary disk structures are known to be shaped by various thermal and compositional effects such as (though not limited to) shadowed regions, sublimation lines, density bumps... The resulting irregularities in the surface mass density and temperature profiles are key elements to determine the location where planetary embryos can be trapped. These traps provide hints of which planets are most likely to survive, at what distance from the star, and potentially with what composition (Baillié, Charnoz, Pantin, 2015, A&A 577, A65; Baillié, Charnoz, Pantin, 2016, A&A 590, A60). These structures are determined by the viscous spreading of the disk, that is initially formed by the collapse of the molecular cloud.Starting from the numerical hydrodynamical model detailed in Baillié & Charnoz., 2014, ApJ 786, 35 which couples the disk thermodynamics, its photosphere geometry, its dynamics and its dust composition in order to follow its long-term evolution, we now consider the early stages of the central star. We model the joint formation of the disk and the star: their mass are directly derived from the collapse of the molecular cloud while the star temperature, radius and brightness are interpolated over pre-calculated stellar evolutions. Therefore, our simulations no longer depend on the initial profile of the "Minimum Mass Solar Nebula", and allow us to model the influence of the forming star on the protoplanetary disk. In particular, we will present the resulting distribution of the sublimation lines of the main dust species, as well as the locations of the planet traps at various disk ages. In the longer term, we intend to investigate the influence of the star properties on the selection of the surviving planets