Accretion Of Particles Research Articles

Explosive volcanic eruptions on Mars: Tephra and accretionary lapilli formation, dispersal and recognition in the geologic record

Explosive volcanic eruptions, potentially involving large amounts of magmatic and entrained water, are thought to have been very important in the Noachian and Hesperian periods earlier in Mars history, and basaltic plinian eruptions are likely to have occurred throughout martian history. Previous treatments of explosive volcanic plumes on Mars have simply extrapolated plume models for the Earth's atmosphere to Mars. Taking account of known limitations to the applicability of this approach, which suggest that convective plumes may be capable of reaching only ∼ 20 km height on Mars, we introduce the concept of inertial plumes, capable of carrying clasts to much greater heights and dropping them into the lower atmosphere over a wide area. Atmospheric circulation patterns guarantee wide dispersal and thick deposits of tephra resulting from both types of eruption. The presence of large amounts of water in convecting explosive eruption plumes can also lead to condensation of water on small particles and the consequent accretion of other particles as the smaller particles fall through the plume, producing accretionary lapilli. Formation of accretionary lapilli significantly alters the spatial distribution and grain sizes of pyroclastic fall deposits from those involving discrete juvenile clasts with negligible clast interactions. We model the eruption and dispersal of tephra, and the formation of accretionary lapilli on Mars under current atmospheric conditions and explore the consequences of this for the geometry and grain size of deposits formed from explosive eruption plumes. We show that explosive eruptions can produce thick widespread deposits of ash and lapilli similar to those thought to have produced mantling deposits in several regions of Mars. We develop a detailed example that shows that local hydrovolcanic explosive eruptions and solely magmatic eruptions originating from the nearby Apollinaris Patera could have emplaced tephra and accretionary lapilli in the Columbia Hills region of the Mars Exploration Rover Gusev site. If the atmospheric pressure was higher early in Mars' history than now, eruptions would have led to somewhat more extensive pyroclast dispersal than under current conditions.

What can the braking indices tell us about the nature of pulsars?

As a result of observational difficulties, braking indices of only six rotation-powered pulsars are obtained with some certainty. All these values are remarkably smaller than the value ( n = 3) expected for pure magnetodipole radiation model. Various models have been proposed to explain this phenomenon such as introducing additional torques by particle outflow or accretion disk, changes in inclination angle or magnetic field strength, and field-reconnection process in pulsar magnetosphere. We re-examined the braking index based on previous research and suggest a constant gap potential drop in the open-field line region for pulsars with magnetospheric activity. New constraints on model parameters from observed braking indices are presented.

Dust accretion onto high-mass planets

Aims. We study the accretion of dust particles of various sizes onto embedded massive gas giant planets, where we take into account the structure of the gas disk due to the presence of the planet. The accretion rate of solids is important for the structure of giant planets: it determines the growth rate of the solid core that may be present as well as their final enrichment in solids. Methods. We use the RODEO hydrodynamics solver to solve the flow equations for the gas, together with a particle approach for the dust. The solver for the particles' equations of motion is implicit with respect to the drag force, which allows us to treat the whole dust size spectrum. Results. We find that dust accretion is limited to the smallest particle sizes. The largest particles get trapped in outer mean-motion resonances with the planet, while particles of intermediate size are pushed away from the orbit of the planet by the density structure in the gas disk. Only particles' smaller than approximately s max = 10 μm may accrete on a planet with the mass of Jupiter. For a ten times less massive planet s max = 100 μm. The strongly reduced accretion of dust makes it very hard to enrich a newly formed giant planet in solids.

Spherical Accretion

We compare different examples of spherical accretion onto a gravitating mass. Limiting cases include the accretion of a collisionally dominated fluid and the accretion of collisionless particles. We derive expressions for the accretion rate and density profile for semi-collisional accretion which bridges the gap between these limiting cases. Particle crossing of the Hill sphere during the formation of the outer planets is likely to have taken place in the semi-collisional regime.

Forming supermassive black holes by accreting dark and baryon matter

Given a large-scale mixture of self-interacting dark matter (SIDM) particles and baryon matter distributed in the early Universe, we advance here a two-phase accretion scenario for forming supermassive black holes (SMBHs) with masses around ∼109 M⊙ at high redshifts z(⋆6). The first phase is conceived to involve a rapid quasi-spherical and quasi-steady Bondi accretion of mainly SIDM particles embedded with baryon matter on to seed black holes (BHs) created at redshifts z≲ 30 by the first generation of massive Population III stars; this earlier phase rapidly gives birth to significantly enlarged seed BH masses of during z∼ 20-15, where σ0 is the cross-section per unit mass of SIDM particles and Cs is the velocity dispersion in the SIDM halo referred to as an effective ‘sound speed'. The second phase of BH mass growth is envisaged to proceed primarily via baryon accretion, eventually leading to SMBH masses of MBH∼ 109 M⊙; such SMBHs may form either by z∼ 6 for a sustained accretion at the Eddington limit or later at lower z for sub-Eddington mean accretion rates. In between these two phases, there is a transitional yet sustained diffusively limited accretion of SIDM particles which in an eventual steady state would be much lower than the accretion rates of the two main phases. We intend to account for the reported detections of a few SMBHs at early epochs, e.g. Sloan Digital Sky Survey (SDSS) 1148+5251 and so forth, without necessarily resorting to either super-Eddington baryon accretion or very frequent BH merging processes. Only extremely massive dark SIDM haloes associated with rare peaks of density fluctuations in the early Universe may harbour such early SMBHs or quasars. Observational consequences are discussed. During the final stage of accumulating a SMBH mass, violent feedback in circumnuclear environs of a galactic nucleus leads to the central bulge formation and gives rise to the familiar empirical MBH−σb correlation inferred for nearby normal galaxies with σb being the stellar velocity dispersion in the galactic bulge; in our scenario, the central SMBH formation precedes that of the galactic bulge.

Particle accretion and dissipation simulator: Collisional aggregation of icy particles

As a part of a project to study the cometary particle environment, a self-consistent ballistic rigid body particle colliding simulator, capable of producing fractal aggregates of icy particles modelled by hard or soft (metaball) spheres has been developed, to be used for creating test particles in future statistical studies of the cometary coma–particle interaction. Instead of a predefined sticking probability, actual surface interactions are modelled and a method for calculating internal stresses and fragmentation of an aggregate during the accretion phase is described. Simulations suggest that taking fragmentation into account has two major consequences, increase in the fractal dimension of resulting particles and a noticeable change in the mass spectrum of an ensemble.

One-step generation of highly selective hydrogenation catalysts involving sub-nanometric Cu 2O supported on mesoporous alumina: strategies to control their size and dispersion

A series of binary Cu–Al surfactant-assisted mesoporous precursors have been prepared through direct synthesis and catalytically evaluated in the selective hydrogenation of conjugated α,β-unsaturated aldehyde (cinnamaldehyde) to the corresponding unsaturated alcohol in the liquid phase. The resulting material consists in a mesoporous alumina substrate involving a ripple sheet type-texture onto which “Cu hydroxynitrate” domains of uniform size were dispersed. After calcination, these particles yield sub-nanometer (<1 nm) and larger (20–40 nm) CuO particles. An original model implying a homogeneous temperature-induced particle accretion is proposed to explain this bimodal dispersion. A specific re-evaluation of some synthesis parameters allowed us to optimize the relative size, dispersion and final stabilization of these particles. Upon reduction, CuO yields sub-nanometric Cu 2O and metallic Cu particles of relatively uniform size. The remarkable selectivity (70%) of the binary mesoporous samples towards the hydrogenation of the carbonyl group of cinnamaldehyde is due to the presence of Cu 2O particles that are probably in relatively strong interaction with the alumina pore walls. By contrast, solids involving Cu nitrate impregnates on either mesoporous and commercial (bulky) γ-alumina performed poorly towards the formation of cinnamyl alcohol, arguing for a fundamental difference in the electronic states of the various supported Cu particles with respect to those generated on samples by direct synthesis.

Does interplanetary dust control 100 kyr glacial cycles?

The cause of the 100kyr glacial–interglacial cycles during the past 800kyr is one of the fundamental puzzles in paleoclimatology. The widely accepted Milankovitch theory, relating earth's climate cycles to variations in insolation caused by periodic changes in orbital parameters, has difficulties to explain the predominant 100kyr rhythm. Although earth's eccentricity varies with a period of 100kyr, the resulting change in insolation is too small to produce the corresponding climate cycle by direct forcing (Imbrie et al., Paleoceanography 8 (1993) 699). In order to solve the `100kyr problem', Muller and MacDonald (Nature 377 (1995) 107; Science 277 (1997a) 215; Proc. Nat. Acad. Sci. USA 94 (1997b) 8329) proposed an alternative orbital but non-Milankovitch mechanism attributing the glacial cycles to regular variations in the accretion of interplanetary dust particles (IDP) caused by 100kyr cycles in the orbital inclination of the earth. To test this controversial hypothesis, we study the IDP accumulation in deep-sea sediments from a period in the early Pleistocene. We find apparent 41kyr cycles but no 100kyr periodicity in the IDP accumulation rate. As there is no known mechanism to produce 41kyr cycles in IDP supply from space, we conclude that the 41kyr cycles are caused by the dynamics of sediment accumulation, and that changes in the IDP flux do not drive the Pleistocene glacial cycles.

Gravitational accretion of particles in Saturn's rings

Gravitational accretion in the rings of Saturn is studied with local N-body simulations, taking into account the dissipative impacts and gravitational forces between particles. Common estimates of accretion assume that gravitational sticking takes place beyond a certain distance (Roche distance) where the self-gravity between a pair of ring particles exceeds the disrupting tidal force of the central object, the exact value of this distance depending on the ring particles' internal density. However, the actual physical situation in the rings is more complicated, the growth and stability of the particle groups being affected also by the elasticity and friction in particle impacts, both directly via sticking probabilities and indirectly via velocity dispersion, as well as by the shape, rotational state and the internal packing density of the forming particle groups. These factors are most conveniently taken into account via N-body simulations. In our standard simulation case of identical 1 m particles with internal density of solid ice, ρ = 900 kg m −3 , following the Bridges et al., 1984 elasticity law, we find accretion beyond a = 137 , 000 – 146 , 000 km , the smaller value referring to a distance where transient aggregates are first obtained, and the larger value to the distance where stable aggregates eventually form in every experiment lasting 50 orbital periods. Practically the same result is obtained for a constant coefficient of restitution ε n = 0.5 . In terms of r p parameter, the sum of particle radii normalized by their mutual Hill radius, the above limit for perfect accretion corresponds to r p < 0.84 . Increased dissipation ( ε n = 0.1 ), or inclusion of friction (tangential force 10% of normal force) shifts the accretion region inward by about 5000 km. Accretion is also more efficient in the case of size distribution: with a q = 3 power law extending over a mass range of 1000, accretion shifts inward by almost 10 , 000 km . The aggregates forming in simulations via gradual accumulation of particles are synchronously rotating.

Detrital ooids of Holocene age in glaciomarine Champlain Sea sediments, Gatineau, Quebec, Canada

Detrital ooids, from 0.1 to 0.5 mm diameter, almost completely dominate two 0.3 m thick layers between one and two metres depth at a site in clay-size-rich, isostatically uplifted, glaciomarine, Champlain Sea sediments north of Gatineau, Quebec. The ooids, composed of layers of tangentially oriented, platy particles, are physical aggregations of glacially ground, rock flour with the identical mineral suite that constitutes the regional Champlain Sea sediments. The ooids are not cemented and there is no evidence of diagenetic formation of minerals. A hydrodynamic "hail-stone" model, which requires a somewhat turbulent sedimentary environment within the water column, probably created by wind–wave action that kept ooid nuclei in suspension while the flocculating effect of brackish to marine conditions induced particle accretion onto their surface, is proposed for their formation.

A numerical method for coupled filtration equations in porous media in presence of the inverse compaction factor

In this paper a mathematical model of the transport and accretion of colloidal type particles in porous media is examined. Different from previously considered ones, the model also includes an inverse compaction factor. We present a numerical method for the solution of the coupled nonlinear differential system of equations, which models the above phenomenon. Accuracy of the presented method is tested on concrete examples.

Symbiotic Relations of Sediment‐Agglutinating Nematodes and Bacteria in Detrital Habitats: The Enzyme‐Sharing Concept

Abstract. A new concept (‘enzyme sharing’) concerning the interaction of marine nematodes and microbes in the degradation of sedimentary detritus is presented. Elements of this concept are (1) the notorious tendency of many aquatic nematodes to agglutinate detrital particles by mucus secretions, (2) new observations on the stimulation of microbial growth by nematodes in agar plates, and (3) literature data on limited endogenous proteolytic capacities of aquatic nematodes. Observations on nematode‐microbe associations in agar plates prompted the conceptual synthesis. In agar medium without the addition of any nutrients a spectacular growth of bacteria was visible on the sinusoidal crawling trails of nematodes only 2 – 3 days after introduction of the worms (species of Adoncholaimus, Anoplostoma and Sabatieria). Juveniles of Anoplostoma that hatched in the agar cultures left their minute trails in the medium and these were rapidly occupied by bacteria. The nematodes repeatedly visited their bacterial trails, which persisted as a peculiar biotic structure for more than one year and survived the nematodes. In sterile agar preparations containing the fluorogenic methylumbelliferyl‐β‐glucoside in the presence of the nematode Adoncholaimus, an enhanced fluorescence of the medium was visible, indicating β‐glucosidase activity. We therefore assume that oncholaimid nematodes discharge enzymes that alone, or in concert with microbial activities, contribute to the hydrolytic cleavage of refractory polysaccharides containing β‐glucosidic bonds such as agar components and cellulose. The sugars thus produced may then be taken up by the nematodes and concomitantly support the conspicuous growth of microbes. Since we did not observe any feeding of the nematodes on the associated microbes in agar plates, we question the nutritive potential of intact microbial cells for a number of nematodes abounding in detrital habitats, and call attention to the significance of ambient dissolved or adsorbed organic monomeric nutrients. Consequently, we perceive the puzzling perpetual accretion of detrital organic particles to sediment agglutinations by nematodes as an adaptation for operating an ‘enzymatic reactor’ for the production of dissolved nutrients. We hypothesise a relationship of mutual commensalism of nematodes and heterotrophic microbes in detrital habitats and propose the term ‘enzyme sharing’ for this relationship. Both parties invest in a common enzyme pool that decomposes organic detritus for their nutrition. We present here evidence that nematodes contribute β‐glucosidase, which is involved in the cellulase system. Data from the literature suggest that microbial enzymatic processing of detrital proteins yields amino acids available to nematodes, which apparently have no efficient proteolytic enzyme system in their intestines.

Particle accretion and release in flows of suspensions

We consider a pressure-driven, one-dimensional, advective flow of a suspension through a porous domain. Suspended particles are filtered out, changing the porosity. These changes affect the filtration velocity through a porosity-dependent permeability factor. The reaction mechanism includes an accretion term and a term which allows trapped particles to return to the mobile phase. We obtain numerical solutions on a bounded domain and illustrate both contamination and remediation effects, and we consider the existence of traveling wave fronts in an infinite domain. For the latter, accretion rates are characterized that permit such fronts.

Hydrodynamic Drag in Cosmological Simulations

We present a study of hydrodynamic drag forces in smoothed particle hydrodynamic (SPH) simulations. In particular, the deceleration of a resolution-limited cold clump of gas moving through a hot medium is examined. The drag at subsonic velocities exceeds that expected from simple dynamics-based arguments. The excess is shown to be a result of the hydrodynamic method. SPH encourages the accretion of particles from the hot medium on to a shell around the cold clump, effectively increasing the radius of the clump. For sonic and supersonic velocities, the effective area of the clump is shown to be set by the symmetrized smoothing length. The relationship of the effective area to the smoothing length breaks the expected dependency of drag on density since the symmetrized smoothing length increases with decreasing density. The consequences for cosmological simulations are discussed.

New mechanism of dust growth and gravitation-like instabilities in astrophysical plasmas

Dust growth by agglomeration due to a new mechanism of dust attraction is shown to compete in space plasmas with the standard accretion mechanism by neutral particles. We demonstrate that dust growth by accretion of neutrals and plasma particles is aided by a pressure force due to a shadowing effect first proposed (Bingham & Tsytovich [CITE]) to explain rapid growth of dust in laboratory discharges. Consequences of this new force promoting dust growth by agglomeration in astrophysics are discussed including the role of a new gravitational-like instability which produces clumping enhancing the rate of gravitational collapse.

Contamination and remediation waves in a filtration model

We propose a model for the filtration of suspended particles in porous media and we examine some of its mathematical properties. The model includes a variable porosity that depends on the volume of particles retained through filtration and a kinetics law that allows both a positive and negative rate of particle accretion. We characterize the properties of accretion rates that lead to contamination and remediation wave fronts in the model.

Stability considerations associated with the meniscoid particle band at advancing interfaces in hele-shaw suspension flows

In the free surface flow of dense suspensions in confined domains, spontaneous particle enrichment occurs in the region adjacent to the free surface with an attendant increase in the effective viscosity. We present experimental evidence that this particle accretion introduces stability considerations that would otherwise not exist if the suspension remained uniform. For conditions favoring global stability, a local fingering instability can occur in the accretion band. For conditions favoring global instability, accretion can suppress the global growth of viscous fingers at the interface.

Maximal acceleration effects in Reissner–Nordström space

The dynamics of a relativistic particle in a Reissner–Nordström background is studied using Caianiello model with maximal acceleration. The behaviour of the particle, embedded in a new effective geometry, changes with respect to the classical scenario because of the formation of repulsive potential barriers near the horizon. Black hole formation by accretion of massive particles is not therefore a viable process in the model. At the same time, the naked singularity remains largely unaffected by maximal acceleration corrections.

Models of the in Situ Formation of Detected Extrasolar Giant Planets,

We present numerical simulations of the formation of the planetary companions to 47 UMa, ρ CrB, and 51 Peg. They are assumed to have formed in situ according to the basic model that a core formed first by accretion of solid particles, then later it captured substantial amounts of gas from the protoplanetary disk. In most of the calculations we prescribe a constant accretion rate for the solid core. The evolution of the gaseous envelope is calculated according to the following assumptions: (1) it is in quasi-hydrostatic equilibrium, (2) the gas accretion rate is determined by the requirement that the outer radius of the planet is the place at which the thermal velocity of the gas allows it to reach the boundary of the planet's Hill sphere, (3) the gas accretion rate is limited, moreover, by the prescribed maximum rate at which the nebula can supply the gas, and (4) the growth of the planet stops once it obtains approximately the minimum mass determined from radial velocity measurements (in one case the planet is allowed to grow to twice this limit). Calculations are carried out through an initial phase during which solid accretion dominates, past the point of crossover when the masses of solid and gaseous material are equal, through the phase of rapid gas accretion, and into the final phase of contraction and cooling at constant mass. Alternative calculations are presented for the case of 47 UMa in which the solid accretion rate is calculated, not assumed, and the dissolution of planetesimals within the gaseous envelope is considered. In all cases there is a short phase of high luminosity (10 −3–10 −2 L ⊙) associated with rapid gas accretion. The height and duration of this peak depend on uncertain model parameters. The conclusion is reached that in situ formation of all of these companions is possible under some conditions. However, it is more likely that orbital migration was an important component of the evolution, at least for the planets around ρ CrB and 51 Peg.

Baryogenesis from primordial black holes after the electroweak phase transition

Incorporating a realistic model for accretion of ultrarelativistic particles by primordial black holes (PBHs), we study the evolution of an Einstein--de Sitter universe consisting of PBHs embedded in a thermal bath from the epoch $\ensuremath{\sim}{10}^{\ensuremath{-}33}\mathrm{sec}$ to $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\mathrm{sec}.$ In this paper we use the ansatz of Barrow et al. to model black hole evaporation in which the modified Hawking temperature goes to zero in the limit of the black hole attaining a relic state with a mass $\ensuremath{\sim}{m}_{\mathrm{Pl}}.$ Both the single mass PBH case as well as the case in which black hole masses are distributed in the range $8\ifmmode\times\else\texttimes\fi{}{10}^{2}--3\ifmmode\times\else\texttimes\fi{}{10}^{5}\mathrm{g}$ have been considered in our analysis. Black holes with a mass larger than $\ensuremath{\sim}{10}^{5}\mathrm{g}$ appear to survive beyond the electroweak phase transition and, therefore, successfully manage to create baryon excess via $X\ensuremath{-}\overline{X}$ emissions, averting the baryon number washout due to sphalerons. In this scenario, we find that the contribution to the baryon-to-entropy ratio by PBHs of initial mass m is given by $\ensuremath{\sim}\ensuremath{\epsilon}\ensuremath{\zeta}(m/1\mathrm{g}{)}^{\ensuremath{-}1},$ where \ensuremath{\epsilon} and \ensuremath{\zeta} are the CP-violating parameter and the initial mass fraction of the PBHs, respectively. For \ensuremath{\epsilon} larger than $\ensuremath{\sim}{10}^{\ensuremath{-}4},$ the observed matter-antimatter asymmetry in the universe can be attributed to the evaporation of PBHs.