Divide-and-Conquer Method for Instanton Rate Theory

Ring-polymer instanton theory has been developed to simulate the quantum dynamics of molecular systems at low temperatures. Chemical reaction rates can be obtained by locating the dominant tunneling pathway and analyzing fluctuations around it. In the standard method, calculating the fluctuation terms involves the diagonalization of a large matrix, which can be unfeasible for large systems with a high number of ring-polymer beads. Here we present a method for computing the instanton fluctuations with a large reduction in computational scaling. This method is applied to three reactions described by fitted, analytic and on-the-fly ab initio potential-energy surfaces and is shown to be numerically stable for the calculation of thermal reaction rates even at very low temperature

Coorbital Satellites of Saturn: Congenital Formation

Saturn is the only known planet to have coorbital satellite systems. In the present work we studied the process of mass accretion as a possible mechanism for coorbital satellites formation. The system considered is composed of Saturn, a proto-satellite and a cloud of planetesimals distributed in the coorbital region around a triangular Lagrangian point. The adopted relative mass for the proto-satellite was 10^-6 of Saturn's mass and for each planetesimal of the cloud three cases of relative mass were considered, 10^-14, 10^-13 and 10^-12 masses of Saturn. In the simulations each cloud of planetesimal was composed of 10^3, 5 x 10^3 or 10^4 planetesimals. The results of the simulations show the formation of coorbital satellites with relative masses of the same order of those found in the saturnian system (10^-13 - 10^-9). Most of them present horseshoe type orbits, but a significant part is in tadpole orbit around L_4 or L_5. Therefore, the results indicate that this is a plausible mechanism for the formation of coorbital satellites.Comment: 10 pages, 9 figures, 4 table

A Raman anemometer for component-selective velocity measurements of particles in a flow

An anemometer for the measurement of the velocity of particles of different components in a flow, separate and apart from that of the flow itself, is described. As a component-selective mechanism Raman scattering is used. The velocity is measured by relating the autocorrelated scattering signal to the known laser beam profile

Terrestrial Planet Formation in a protoplanetary disk with a local mass depletion: A successful scenario for the formation of Mars

Models of terrestrial planet formation for our solar system have been successful in producing planets with masses and orbits similar to those of Venus and Earth. However, these models have generally failed to produce Mars-sized objects around 1.5 AU. The body that is usually formed around Mars' semimajor axis is, in general, much more massive than Mars. Only when Jupiter and Saturn are assumed to have initially very eccentric orbits (e $\sim$ 0.1), which seems fairly unlikely for the solar system, or alternately, if the protoplanetary disk is truncated at 1.0 AU, simulations have been able to produce Mars-like bodies in the correct location. In this paper, we examine an alternative scenario for the formation of Mars in which a local depletion in the density of the protosolar nebula results in a non-uniform formation of planetary embryos and ultimately the formation of Mars-sized planets around 1.5 AU. We have carried out extensive numerical simulations of the formation of terrestrial planets in such a disk for different scales of the local density depletion, and for different orbital configurations of the giant planets. Our simulations point to the possibility of the formation of Mars-sized bodies around 1.5 AU, specifically when the scale of the disk local mass-depletion is moderately high (50-75%) and Jupiter and Saturn are initially in their current orbits. In these systems, Mars-analogs are formed from the protoplanetary materials that originate in the regions of disk interior or exterior to the local mass-depletion. Results also indicate that Earth-sized planets can form around 1 AU with a substantial amount of water accreted via primitive water-rich planetesimals and planetary embryos. We present the results of our study and discuss their implications for the formation of terrestrial planets in our solar system.Comment: Accepted for publication in The Astrophysical Journa

Semileptonic decays of the Higgs boson at the Tevatron

We examine the prospects for extending the Tevatron reach for a Standard Model Higgs boson by including the semileptonic Higgs boson decays h --> WW --> l nu jj for M_h >~ 2 M_W, and h --> W jj --> l nu jj for M_h <~ 2 M_W, where j is a hadronic jet. We employ a realistic simulation of the signal and backgrounds using the Sherpa Monte Carlo event generator. We find kinematic selections that enhance the signal over the dominant W+jets background. The resulting sensitivity could be an important addition to ongoing searches, especially in the mass range 120 <~ M_h <~ 150 GeV. The techniques described can be extended to Higgs boson searches at the Large Hadron Collider.Comment: 68 pages, 19 figure

Asset versus consumption poverty and poverty dynamics in the presence of multiple equilibria in rural Ethiopia

Effective poverty reduction programs require careful measurement of poverty status. Several studies have shown conceptually that assets reflecting productive capacity form a more robust basis for identifying the poor than do flow variables such as expenditures or income. Nonetheless, little work has empirically compared poverty measurements based on assets and expenditures. This paper uses panel data from Ethiopia to generate an asset-based poverty classification scheme. Regression results are used to estimate an asset index and classify households into categories of structural poverty. Asset index dynamics are also explored to test for the existence of multiple asset index equilibria; evidence of potential poverty traps. Results provide evidence of multiple equilibria in the study sample as a whole as well as convergence at different levels over space, depending on commercialization opportunities and agroecological factors. The asset-based poverty classifications consistently predict future poverty status more accurately than do income-based measures, confirming that the asset-based measure could be used to more carefully target poverty interventions in rural areas and to more accurately assess the impact of those interventions.asset index, asset poverty, Commercialization, expenditures, income-based measures, index equilibria, Poverty dynamics, Poverty reduction, regression,

A Compound model for the origin of Earth's water

One of the most important subjects of debate in the formation of the solar system is the origin of Earth's water. Comets have long been considered as the most likely source of the delivery of water to Earth. However, elemental and isotopic arguments suggest a very small contribution from these objects. Other sources have also been proposed, among which, local adsorption of water vapor onto dust grains in the primordial nebula and delivery through planetesimals and planetary embryos have become more prominent. However, no sole source of water provides a satisfactory explanation for Earth's water as a whole. In view of that, using numerical simulations, we have developed a compound model incorporating both the principal endogenous and exogenous theories, and investigating their implications for terrestrial planet formation and water-delivery. Comets are also considered in the final analysis, as it is likely that at least some of Earth's water has cometary origin. We analyze our results comparing two different water distribution models, and complement our study using D/H ratio, finding possible relative contributions from each source, focusing on planets formed in the habitable zone. We find that the compound model play an important role by showing more advantage in the amount and time of water-delivery in Earth-like planets.Comment: Accepted for publication in The Astrophysical Journa

Stable retrograde orbits around the triple system 2001 SN263

The NEA 2001 SN263 is the target of the ASTER MISSION - First Brazilian Deep Space Mission. Araujo et al. (2012), characterized the stable regions around the components of the triple system for the planar and prograde cases. Knowing that the retrograde orbits are expected to be more stable, here we present a complementary study. We now considered particles orbiting the components of the system, in the internal and external regions, with relative inclinations between $90^{\circ}< I \leqslant180^{\circ}$, i.e., particles with retrograde orbits. Our goal is to characterize the stable regions of the system for retrograde orbits, and then detach a preferred region to place the space probe. For a space mission, the most interesting regions would be those that are unstable for the prograde cases, but stable for the retrograde cases. Such configuration provide a stable region to place the mission probe with a relative retrograde orbit, and, at the same time, guarantees a region free of debris since they are expected to have prograde orbits. We found that in fact the internal and external stable regions significantly increase when compared to the prograde case. For particles with $e=0$ and $I=180^{\circ}$, we found that nearly the whole region around Alpha and Beta remain stable. We then identified three internal regions and one external region that are very interesting to place the space probe. We present the stable regions found for the retrograde case and a discussion on those preferred regions. We also discuss the effects of resonances of the particles with Beta and Gamma, and the role of the Kozai mechanism in this scenario. These results help us understand and characterize the stability of the triple system 2001 SN263 when retrograde orbits are considered, and provide important parameters to the design of the ASTER mission.Comment: 11 pages, 8 figures. Accepted for publication in MNRAS - 2015 March 1