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Search Results: 1 - 10 of 219586 matches for " C Leinhardt "
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Planetesimals to Protoplanets II: Effect of Debris on Terrestrial Planet Formation
Z. M. Leinhardt,D. C. Richardson,G. Lufkin,J. Haseltine
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.14769.x
Abstract: In this paper we extend our numerical method for simulating terrestrial planet formation from Leinhardt and Richardson (2005) to include dynamical friction from the unresolved debris component. In the previous work we implemented a rubble pile planetesimal collision model into direct N-body simulations of terrestrial planet formation. The new collision model treated both accretion and erosion of planetesimals but did not include dynamical friction from debris particles smaller than the resolution limit for the simulation. By extending our numerical model to include dynamical friction from the unresolved debris, we can simulate the dynamical effect of debris produced during collisions and can also investigate the effect of initial debris mass on terrestrial planet formation. We find that significant initial debris mass, 10% or more of the total disk mass, changes the mode of planetesimal growth. Specifically, planetesimals in this situation do not go through a runaway growth phase. Instead they grow concurrently, similar to oligarchic growth. In addition to including the dynamical friction from the unresolved debris, we have implemented particle tracking as a proxy for monitoring compositional mixing. Although there is much less mixing due to collisions and gravitational scattering when dynamical friction of the background debris is included, there is significant inward migration of the largest protoplanets in the most extreme initial conditions.
Direct N-body Simulations of Rubble Pile Collisions
Z. M. Leinhardt,D. C. Richardson,T. Quinn
Physics , 1999, DOI: 10.1006/icar.2000.6370
Abstract: There is increasing evidence that many km-sized bodies in the Solar System are piles of rubble bound together by gravity. We present results from a project to map the parameter space of collisions between km-sized spherical rubble piles. The results will assist in parameterization of collision outcomes for Solar System formation models and give insight into fragmentation scaling laws. We use a direct numerical method to evolve the positions and velocities of the rubble pile particles under the constraints of gravity and physical collisions. We test the dependence of the collision outcomes on impact parameter and speed, impactor spin, mass ratio, and coefficient of restitution. Speeds are kept low (< 10 m/s, appropriate for dynamically cool systems such as the primordial disk during early planet formation) so that the maximum strain on the component material does not exceed the crushing strength. We compare our results with analytic estimates and hydrocode simulations. Off-axis collisions can result in fast-spinning elongated remnants or contact binaries while fast collisions result in smaller fragments overall. Clumping of debris escaping from the remnant can occur, leading to the formation of smaller rubble piles. In the cases we tested, less than 2% of the system mass ends up orbiting the remnant. Initial spin can reduce or enhance collision outcomes, depending on the relative orientation of the spin and orbital angular momenta. We derive a relationship between impact speed and angle for critical dispersal of mass in the system. We find that our rubble piles are relatively easy to disperse, even at low impact speed, suggesting that greater dissipation is required if rubble piles are the true progenitors of protoplanets.
Writing and Publishing as Conversation
LEINHARDT,GAEA;
Revista Colombiana de Psicología , 2012,
Abstract: this article asserts that the publishing process will be better understood as a conversational task that connects empirical precedents and theoretical debates in a given domain to produce new claims. it proposes that the lack of a conversational focus on the current academic environment and an excessive emphasis on impact indexes have created a citational inflation, in which authors and journals try to artificially increase the impact indexes without contributing to disciplinary progress. based on current literature in psychology and conversational analysis, the article suggests that conversations are collaborative efforts that, in the case of publishing, must respond to two principles: quantity and quality. quantity refers to making contributions as informative as possible, and quality refers to providing strong support for every claim. to explain this perspective, the author presents two case studies regarding the elaboration of seminal papers on educational psychology and education.
WRITING AND PUBLISHING AS CONVERSATION/ LA ESCRITURA Y LA PUBLICACIóN COMO CONVERSACIóN/ A ESCRITA E A PUBLICA O COMO CONVERSA O
Gaea Leinhardt
Revista Colombiana de Psicología , 2012,
Abstract: This article asserts that the publishing process will be better understood as a conversational task that connects empirical precedents and theoretical debates in a given domain to produce new claims. It proposes that the lack of a conversational focus on the current academic environment and an excessive emphasis on impact indexes have created a citational inflation, in which authors and journals try to artificially increase the impact indexes without contributing to disciplinary progress. Based on current literature in psychology and conversational analysis, the article suggests that conversations are collaborative efforts that, in the case of publishing, must respond to two principles: quantity and quality. Quantity refers to making contributions as informative as possible, and quality refers to providing strong support for every claim. To explain this perspective, the author presents two case studies regarding the elaboration of seminal papers on educational psychology and education.
Modelling circumbinary protoplanetary disks: I. Fluid simulations of the Kepler-16 and 34 systems
S. Lines,Z. M. Leinhardt,C. Baruteau,S. -J. Paardekooper,P. J. Carter
Physics , 2015, DOI: 10.1051/0004-6361/201526295
Abstract: The Kepler mission's discovery of a number of circumbinary planets orbiting close (a_p < 1.1 au) to the stellar binary raises questions as to how these planets could have formed given the intense gravitational perturbations the dual stars impart on the disk. The gas component of circumbinary protoplanetary disks is perturbed in a similar manner to the solid, planetesimal dominated counterpart, although the mechanism by which disk eccentricity originates differs. This is the first work of a series that aims to investigate the conditions for planet formation in circumbinary protoplanetary disks. We present a number of hydrodynamical simulations that explore the response of gas disks around two observed binary systems: Kepler-16 and Kepler-34. We probe the importance of disk viscosity, aspect-ratio, inner boundary condition, initial surface density gradient, and self-gravity on the dynamical evolution of the disk, as well as its quasi steady-state profile. We find there is a strong influence of binary type on the mean disk eccentricity, e_d, leading to e_d = 0.02 - 0.08 for Kepler-16 and e_d = 0.10 - 0.15 in Kepler-34. The value of alpha-viscosity has little influence on the disk, but we find a strong increase in mean disk eccentricity with increasing aspect-ratio due to wave propagation effects. The choice of inner boundary condition only has a small effect on the surface density and eccentricity of the disk. Our primary finding is that including disk self-gravity has little impact on the evolution or final state of the disk for disks with masses less than 12.5 times that of the minimum-mass solar nebula. This finding contrasts with the results of self-gravity relevance in circumprimary disks, where its inclusion is found to be an important factor in describing the disk evolution.
Mycobacterial species causing pulmonary tuberculosis At the korle bu teaching hospital, Accra,
K.K Addo, K Owusu-darko, D Yeboah-manu, P Caulley, M Minamikawa, F Bonsu, C Leinhardt, P Akpedonu, D Ofori-adjei
Ghana Medical Journal , 2007,
Abstract: Objective: Characterize mycobacterial species causing pulmonary tuberculosis (PTB) at the Korle-Bu Teaching Hospital in Ghana. Design: Sputum smear positive samples, two (2) from 70 patients diagnosed as having tuberculosis, after they had consented, were collected from the Korle-Bu Teaching Hospital Chest Clinic between January and July 2003. Setting: Korle-Bu Teaching Hospital Chest Clinic, Accra. Results: Sixty-four mycobacterial isolates were obtained and confirmed as members of Mycobacterium tuberculosis complex by colonial morphology and conventional biochemical assays. Fortyseven (73%) were M. tuberculosis, the human strain, 2 (3%) M. bovis, the bovine strain, 13 (20%) M. africanum I (West Africa type), and 2 (3%) M. africanum II (East Africa type). Conclusion: The results indicate that, there are various strains causing PTB at the Korle-Bu Teaching Hospital and of great concern is M. bovis, which mostly causes extra-PTB in humans but found to cause PTB in this study. This calls for the need to conduct a nationwide survey using both conventional and molecular techniques to characterize various mycobacterial species causing TB in Ghana. This will result in better understanding of the various strains circulating in the country and inform individual TB treatment regimen especially the inclusion or exclusion of pyrazinamide.
The Validity of the Super-Particle Approximation during Planetesimal Formation
Hanno Rein,Geoffroy Lesur,Zoe M. Leinhardt
Physics , 2009, DOI: 10.1051/0004-6361/200912870
Abstract: The formation mechanism of planetesimals in protoplanetary discs is hotly debated. Currently, the favoured model involves the accumulation of meter-sized objects within a turbulent disc, followed by a phase of gravitational instability. At best one can simulate a few million particles numerically as opposed to the several trillion meter-sized particles expected in a real protoplanetary disc. Therefore, single particles are often used as super-particles to represent a distribution of many smaller particles. It is assumed that small scale phenomena do not play a role and particle collisions are not modeled. The super-particle approximation can only be valid in a collisionless or strongly collisional system, however, in many recent numerical simulations this is not the case. In this work we present new results from numerical simulations of planetesimal formation via gravitational instability. A scaled system is studied that does not require the use of super-particles. We find that the scaled particles can be used to model the initial phases of clumping if the properties of the scaled particles are chosen such that all important timescales in the system are equivalent to what is expected in a real protoplanetary disc. Constraints are given for the number of particles needed in order to achieve numerical convergence. We compare this new method to the standard super-particle approach. We find that the super-particle approach produces unreliable results that depend on artifacts such as the gravitational softening in both the requirement for gravitational collapse and the resulting clump statistics. Our results show that short range interactions (collisions) have to be modelled properly.
Planetesimal collisions in binary systems
S. -J. Paardekooper,Z. M. Leinhardt
Physics , 2010, DOI: 10.1111/j.1745-3933.2010.00816.x
Abstract: We study the collisional evolution of km-sized planetesimals in tight binary star systems to investigate whether accretion towards protoplanets can proceed despite the strong gravitational perturbations from the secondary star. The orbits of planetesimals are numerically integrated in two dimensions under the influence of the two stars and gas drag. The masses and orbits of the planetesimals are allowed to evolve due to collisions with other planetesimals and accretion of collisional debris. In addition, the mass in debris can evolve due to planetesimal-planetesimal collisions and the creation of new planetesimals. We show that it is possible in principle for km-sized planetesimals to grow by two orders of magnitude in size if the efficiency of planetesimal formation is relatively low. We discuss the limitations of our two-dimensional approach.
Collisions Between Gravity-Dominated Bodies: 1. Outcome Regimes and Scaling Laws
Zo? M. Leinhardt,Sarah T. Stewart
Physics , 2011, DOI: 10.1088/0004-637X/745/1/79
Abstract: Collisions are the core agent of planet formation. In this work, we derive an analytic description of the dynamical outcome for any collision between gravity-dominated bodies. We conduct high-resolution simulations of collisions between planetesimals; the results are used to isolate the effects of different impact parameters on collision outcome. During growth from planetesimals to planets, collision outcomes span multiple regimes: cratering, merging, disruption, super-catastrophic disruption, and hit-and-run events. We derive equations (scaling laws) to demarcate the transition between collision regimes and to describe the size and velocity distributions of the post-collision bodies. The scaling laws are used to calculate maps of collision outcomes as a function of mass ratio, impact angle, and impact velocity, and we discuss the implications of the probability of each collision regime during planet formation. The analytic collision model presented in this work will significantly improve the physics of collisions in numerical simulations of planet formation and collisional evolution. (abstract abridged)
Collisions between Gravity-Dominated Bodies: 2. The Diversity of Impact Outcomes during the End Stage of Planet Formation
S. T. Stewart,Z. M. Leinhardt
Physics , 2011, DOI: 10.1088/0004-637X/745/1/79
Abstract: Numerical simulations of the stochastic end stage of planet formation typically begin with a population of embryos and planetesimals that grow into planets by merging. We analyzed the impact parameters of collisions leading to the growth of terrestrial planets from recent $N$-body simulations that assumed perfect merging and calculated more realistic outcomes using a new analytic collision physics model. We find that collision outcomes are diverse and span all possible regimes: hit-and-run, merging, partial accretion, partial erosion, and catastrophic disruption. The primary outcomes of giant impacts between planetary embryos are approximately evenly split between partial accretion, graze-and-merge, and hit-and-run events. To explore the cumulative effects of more realistic collision outcomes, we modeled the growth of individual planets with a Monte Carlo technique using the distribution of impact parameters from $N$-body simulations. We find that fewer planets reached masses $>0.7 M_{\rm Earth}$ using the collision physics model compared to simulations that assumed every collision results in perfect merging. For final planets with masses $>0.7 M_{\rm Earth}$, 60% are enriched in their core-to-mantle mass fraction by >10% compared to the initial embryo composition. Fragmentation during planet formation produces significant debris ($\sim15$% of the final mass) and occurs primarily by erosion of the smaller body in partial accretion and hit-and-run events. In partial accretion events, the target body grows by preferentially accreting the iron core of the projectile and the escaping fragments are derived primarily from the silicate mantles of both bodies. Thus, the bulk composition of a planet can evolve via stochastic giant impacts.
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