Effect of turbulence on collisions of dust particles with planetesimals in protoplanetary disks

Planetesimals in gaseous protoplanetary disks may grow by collecting dust particles. Hydrodynamical studies show that small particles generally avoid collisions with the planetesimals because they are entrained by the flow around them. This occurs when $St$, the Stokes number, defined as the ratio of the dust stopping time to the planetesimal crossing time, becomes much smaller than unity. However, these studies have been limited to the laminar case, whereas these disks are believed to be turbulent. We want to estimate the influence of gas turbulence on the dust-planetesimal collision rate and on the impact speeds. We used three-dimensional direct numerical simulations of a fixed sphere (planetesimal) facing a laminar and turbulent flow seeded with small inertial particles (dust) subject to a Stokes drag. A no-slip boundary condition on the planetesimal surface is modeled via a penalty method. We find that turbulence can significantly increase the collision rate of dust particles with planetesimals. For a high turbulence case (when the amplitude of turbulent fluctuations is similar to the headwind velocity), we find that the collision probability remains equal to the geometrical rate or even higher for $St\geq 0.1$, i.e., for dust sizes an order of magnitude smaller than in the laminar case. We derive expressions to calculate impact probabilities as a function of dust and planetesimal size and turbulent intensity

Construction digitalisation tools in South African construction industry : an added advantage

Abstract: The execution of the building project requires the assembling and accumulation of works at a large scale for it to become a reality. This involves the exchange of a large amount of data from one party to another. The development of information and communication technology (ICT) has therefore enhanced an active and proficient way of exchanging data among professionals. This research work set out to assess the benefits of using ICT tools by construction organisations in South Africa. A quantitative research technique was used through a well-structured questionnaire to collect data from construction professionals. They include construction project managers, civil engineers, quantity surveyors, construction managers, industrial, mechanical and electrical engineers. Standard deviation (SD) and mean item score (MIS) were used to analyse the retrieved data. The findings indicated that the most eminent benefits of ICT tools to construction organisations are effective communication management, competitive advantage and time-saving. This study recommended that construction organisations should provide lessons on different ICT tools uses to address each problem that will rise during the construction cycle and reap the full benefit of ICT such as improved productivity, increased speed, and diversification

Is asteroid 2002NY40 a rubble pile gravitationally disrupted

The existence of meteoroid streams containing meter-sized meteoroids capable of produc- ing meteorites after atmospheric interaction was pro- posed quite recently [1]. Their existence has important implications because they can be naturally delivering to the Earth different types of rock-forming materials from Potentially Hazardous Asteroids (PHA). The recent identification of Near Earth Object (NEO) asteroid 2002NY40 as source of meteorite- dropping bolides [2] opens new questions on the na- ture of this asteroid, and the physical process that originated the fireballs detected in 2006 August. Two of the fireballs exhibited a clear similarity with the orbit of 2002NY40, while a third meteoroid was hav- ing an orbit close to NEO 2004NL8. In fact, these five bodies would be related by the close similarity among their orbits (Figure 1, and Table 3 of [1]). We try to focus here on the possible origin of this complex of bodies on the basis of our present (little) knowledge on the structure and orbital evolution of asteroids 2002NY40 and 2004NL8

Dynamics of a small neutrally buoyant sphere in a fluid and targeting in Hamiltonian systems

We show that, even in the most favorable case, the motion of a small spherical tracer suspended in a fluid of the same density may differ from the corresponding motion of an ideal passive particle. We demonstrate furthermore how its dynamics may be applied to target trajectories in Hamiltonian systems.Comment: See home page http://lec.ugr.es/~julya

Rapid planetesimal formation in turbulent circumstellar discs

The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee mode), 3 figures. Supplementary Information can be found at 0708.389

Debris disk size distributions: steady state collisional evolution with P-R drag and other loss processes

We present a new scheme for determining the shape of the size distribution, and its evolution, for collisional cascades of planetesimals undergoing destructive collisions and loss processes like Poynting-Robertson drag. The scheme treats the steady state portion of the cascade by equating mass loss and gain in each size bin; the smallest particles are expected to reach steady state on their collision timescale, while larger particles retain their primordial distribution. For collision-dominated disks, steady state means that mass loss rates in logarithmic size bins are independent of size. This prescription reproduces the expected two phase size distribution, with ripples above the blow-out size, and above the transition to gravity-dominated planetesimal strength. The scheme also reproduces the expected evolution of disk mass, and of dust mass, but is computationally much faster than evolving distributions forward in time. For low-mass disks, P-R drag causes a turnover at small sizes to a size distribution that is set by the redistribution function (the mass distribution of fragments produced in collisions). Thus information about the redistribution function may be recovered by measuring the size distribution of particles undergoing loss by P-R drag, such as that traced by particles accreted onto Earth. Although cross-sectional area drops with 1/age^2 in the PR-dominated regime, dust mass falls as 1/age^2.8, underlining the importance of understanding which particle sizes contribute to an observation when considering how disk detectability evolves. Other loss processes are readily incorporated; we also discuss generalised power law loss rates, dynamical depletion, realistic radiation forces and stellar wind drag.Comment: Accepted for publication by Celestial Mechanics and Dynamical Astronomy (special issue on EXOPLANETS

Long optical plateau in the afterglow of the short GRB 150424A with extended emission: Evidence for energy injection by a magnetar?

Short-duration gamma-ray bursts (GRBs) with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also rare (about 30%) because of their lower luminosity. Aims. We present a multiband data set of the short burst with extended emission, GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS, and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8 h. The analysis of this unique GRB afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate. Methods. We present a phenomenological analysis made by applying fireball closure relations and interpret the findings in the context of the fireball model. We discuss the plausibility of a magnetar as a central engine, which would be responsible for additional and prolonged energy injection into the fireball. Results. We find convincing evidence for energy injection into the afterglow of GRB 150424A. We find that a magnetar spin-down as the source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted into radiation. © ESO, 2017.H.J.v.E. was supported by the Alexander von Humboldt foundation at the time of this work. D.A.K. acknowledges financial support from MPE, from TLS, from the Spanish research project AYA 2014-58381-P, and from Juan de la Cierva Incorporacion fellowships IJCI-2015-26153 and IJCI-2014-21669. P.S., T.W.C., J.F.G., M.T. acknowledge support through the Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation of Germany. S.K. and A.N.G. acknowledge support by DFG grant K1 766/16-1. S.S. acknowledges support by the Thuringer Ministerium fur Bildung, Wissenschaft und Kultur under FKZ 12010-514. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to G. Hasinger (DFG grant HA 1850/28-1).Peer Reviewe

Long optical plateau in the afterglow of the short GRB 150424A with extended emission

Context. Short-duration gamma-ray bursts (GRBs) with extended emission form a subclass of short GRBs, comprising about 15% of the short-duration sample. Afterglow detections of short GRBs are also rare (about 30%) because of their lower luminosity. Aims. We present a multiband data set of the short burst with extended emission, GRB 150424A, comprising of GROND observations, complemented with data from Swift/UVOT, Swift/XRT, HST, Keck/LRIS, and data points from the literature. The GRB 150424A afterglow shows an extended plateau phase, lasting about 8 h. The analysis of this unique GRB afterglow might shed light on the understanding of afterglow plateau emission, the nature of which is still under debate. Methods. We present a phenomenological analysis made by applying fireball closure relations and interpret the findings in the context of the fireball model. We discuss the plausibility of a magnetar as a central engine, which would be responsible for additional and prolonged energy injection into the fireball. Results. We find convincing evidence for energy injection into the afterglow of GRB 150424A. We find that a magnetar spin-down as the source for a prolonged energy injection requires that at least 4% of the spin-down energy is converted into radiation

Asteroids. From Observations to Models

We will discuss some specific applications to the rotation state and the shapes of moderately large asteroids, and techniques of observations putting some emphasis on the HST/FGS instrument.Comment: to appear in LNP; 28pages; written in 2003; Winter School "Dynamique des Corps Celestes Non Ponctuels et des Anneaux", Lanslevillard (FRANCE

Effect-Directed Analysis of Municipal Landfill Soil Reveals Novel Developmental Toxicants in the Zebrafish Danio rerio

Effect-directed analysis (EDA) is an approach used to identify (unknown) contaminants in complex samples which cause toxicity, using a combination of biology and chemistry. The goal of this work was to apply EDA to identify developmental toxicants in soil samples collected from a former municipal landfill site. Soil samples were extracted, fractionated, and tested for developmental effects with an embryotoxicity assay in the zebrafish Danio rerio. Gas chromatograph mass selective detection (GC-MSD) chemical screening was used to reveal candidate developmental toxicants in fractions showing effects. In a parallel study, liquid chromatography-hybrid linear ion trap Orbitrap mass spectrometry was also applied to one polar subfraction (Hoogenboom et al. J. Chromatogr. A2009, 1216, 510-519). EDA resulted in the identification of a number of previously unknown developmental toxicants, which were confirmed to be present in soil by GC-MS. These included 11H-benzo[b]fluorene, 9-methylacridine, 4-azapyrene, and 2-phenylquinoline, as well as one known developmental toxicant (retene). This work revealed the presence of novel contaminants in the environment that may affect vertebrate development, which are not subject to monitoring or regulation under current soil quality assessment guidelines. © 2011 American Chemical Society