Coagulation Of Grains Research Articles

Synthesis and characterization of Ti-WO3 films for electrochromic applications

In this work, tungsten oxide thin films with titanium dopant were deposited at various temperatures of substrate and oxygen partial pressures on silicon wafer and indium doped tin oxide glasses using reactive dc-magnetron sputtering. The main objective of the study is to investigate in detail and to discuss the effect of titanium and deposition parameters on morphological and electrochromic studies of the films. With the alteration of deposition parameters, the morphology changed from granular to coarse with coagulation of grains, and the titanium has significant impact in increasing the grain size. The titanium films incorporated, which are analyzed by an energy dispersive spectroscopy. The needle-like grains with columnar topography was observed, and having some voids over the surface analyzed with atomic force microscopy. The films exhibited excellent electrochromic properties. The films coloration efficiency was altered between 31.8 cm2/C to 101.70 cm2/C.

Fast methods for tracking grain coagulation and ionization

Dust grains play a major role in many astrophysical contexts. They affect the chemical, magnetic, dynamical, and optical properties of their environment, from galaxies down to the interstellar medium, star-forming regions, and protoplanetary disks. Their coagulation leads to shifts in their size distribution and ultimately to the formation of planets. However, although the coagulation process is reasonably uncomplicated to numerically implement by itself, it is difficult to couple it with multidimensional hydrodynamics numerical simulations because of its high computational cost. We propose here a simple method for tracking the coagulation of grains at far lower cost. Given an initial grain size distribution, the state of the distribution at time t is solely determined by the value of a single variable integrated along the trajectory, independently of the specific path taken by the grains. Although this method cannot account for processes other than coagulation, it is mathematically exact, fast, inexpensive, and can be used to evaluate the effect of grain coagulation in most astrophysical contexts. It is applicable to all coagulation kernels in which local physical conditions and grain properties can be separated. We also describe another method for calculating the average electric charge of grains and the density of ions and electrons in environments that are shielded from radiation fields, given the density and temperature of the gas, the cosmic-ray ionization rate, and the average mass of the ions. The equations we provide are fast to integrate numerically and can be used in multidimensional numerical simulations to self-consistently calculate on the fly the local resistivities that are required to model nonideal magnetohydrodynamics.

Core-assisted gas capture instability: a new mode of giant planet formation by gravitationally unstable discs

Giant planet formation in the core accretion (CA) paradigm is predicated by the formation of a core, assembled by the coagulation of grains and later by planetesimals within a protoplanetary disc. In contrast, in the disc instability paradigm, giant planet formation is believed to be independent of core formation: massive self-gravitating gas fragments cool radiatively and collapse as a whole. We show that giant planet formation in the disc instability model may be also enhanced by core formation for reasons physically very similar to the CA paradigm. In the model explored here, efficient grain sedimentation within an initial fragment (rather than the disc) leads to the formation of a core composed of heavy elements. We find that massive atmospheres form around cores and undergo collapse as a critical core mass is exceeded, analogous to CA theory. The critical mass of the core to initiate such a collapse depends on the fragment mass and metallicity, as well as core luminosity, but ranges from less than 1 to as much as $\sim80$ Earth masses. We therefore suggest that there are two channels for the collapse of a gaseous fragment to planetary scales within the disc instability model: (i) H$_2$ dissociative collapse of the entire gaseous clump, and (ii) core-assisted gas capture, as presented here. We suggest that the first of these two is favoured in metal-poor environments and for fragments more massive than $\sim 5-10$ Jupiter masses, whereas the second is favored in metal-rich environments and fragments of lower mass. [Abridged]

Dust coagulation processes as constrained by far-infrared observations

Aims. We develop a simple model of coagulated dust particles of two sizes (3.5 and 60 nm radius) to understand the nature and the effects of coagulation, which could explain the evolution of the far-infrared (FIR) dust opacity observed in the transition between the diffuse and the dense interstellar medium (ISM) (n H > 103 cm-3 ).Methods. Using the discrete-dipole approximation (DDA) method, we have calculated the absorption coefficient, directly proportional to the opacity, of coagulated grains with varying numbers of sub-grains and of different grain composition.Results. We show that, in the transition from diffuse to dense clouds, an increase in the FIR opacity by a factor of about 2.7 is possible and a decrease in the grain temperature by up to 3−4 K can be explained by the presence of coagulated aggregates composed of four big grains and 4000 very small grains (40% of the volume of the BGs). The coagulation of very small grains into the aggregates leads to a decrease in the 60 μ m emission. Conclusions. This model can explain the observed increase in opacity at long wavelengths, the decrease in temperature from the diffuse ISM to denser regions with the coagulation of grains into aggregates and the absence of the 60 μ m emission with the coagulation of very small grains onto the surface of the big grains.

Formation of Jupiter using opacities based on detailed grain physics

Numerical simulations, based on the core-nucleated accretion model, are presented for the formation of Jupiter at 5.2 AU in three primordial disks with three different assumed values of the surface density of solid particles. The grain opacities in the envelope of the protoplanet are computed using a detailed model that includes settling and coagulation of grains and that incorporates a recalculation of the grain size distribution at each point in time and space. We generally find lower opacities than the 2% of interstellar values used in previous calculations (Hubickyj, O., Bodenheimer, P., Lissauer, J.J. [2005]. Icarus 179, 415–431; Lissauer, J.J., Hubickyj, O., D’Angelo, G., Bodenheimer, P. [2009]. Icarus 199, 338–350). These lower opacities result in more rapid heat loss from and more rapid contraction of the protoplanetary envelope. For a given surface density of solids, the new calculations result in a substantial speedup in formation time as compared with those previous calculations. Formation times are calculated to be 1.0, 1.9, and 4.0 Myr, and solid core masses are found to be 16.8, 8.9, and 4.7 M ⊕, for solid surface densities, σ, of 10, 6, and 4 g cm −2, respectively. For σ = 10 and σ = 6 g cm −2, respectively, these formation times are reduced by more than 50% and more than 80% compared with those in a previously published calculation with the old approximation to the opacity.

CRITICAL CORE MASSES FOR GAS GIANT FORMATION WITH GRAIN-FREE ENVELOPES

We investigate the critical core mass and the envelope growth timescale, assuming grain-free envelopes, to examine how small cores are allowed to form gas giants in the framework of the core-accretion model. This is motivated by a theoretical dilemma concerning Jupiter formation: modelings of Jupiter's interior suggest that it contains a small core of <10 M⊕, while many core-accretion models of Jupiter formation require a large core of >10 M⊕ to finish its formation by the time of disk dissipation. Reduction of opacity in the accreting envelope is known to hasten gas giant formation. Almost all the previous studies assumed grain-dominated opacity in the envelope. Instead, we examine cases of grain-free envelopes in this study. Our numerical simulations show that an isolated core of as small as 1.7 M⊕ is able to capture disk gas to form a gas giant on a timescale of million years if the accreting envelope is grain free; that value decreases to 0.75 M⊕ if the envelope is metal free, namely, composed purely of hydrogen and helium. It is also shown that alkali atoms, which are known to be one of the dominant opacity sources near 1500 K in the atmospheres of hot Jupiters, have little contribution to determine the critical core mass. Our results confirm that sedimentation and coagulation of grains in the accreting envelope is a key to resolve the dilemma about Jupiter formation.

Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths

Received ?? ; Accepted ?? Abstract. Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study.

Accretion of the gaseous envelope of Jupiter around a 5–10 Earth-mass core

New numerical simulations of the formation and evolution of Jupiter are presented. The formation model assumes that first a solid core of several M ⊕ accretes from the planetesimals in the protoplanetary disk, and then the core captures a massive gaseous envelope from the protoplanetary disk. Earlier studies of the core accretion–gas capture model [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85] demonstrated that it was possible for Jupiter to accrete with a solid core of 10–30 M ⊕ in a total formation time comparable to the observed lifetime of protoplanetary disks. Recent interior models of Jupiter and Saturn that agree with all observational constraints suggest that Jupiter's core mass is 0–11 M ⊕ and Saturn's is 9–22 M ⊕ [Saumon, G., Guillot, T., 2004. Astrophys. J. 609, 1170–1180]. We have computed simulations of the growth of Jupiter using various values for the opacity produced by grains in the protoplanet's atmosphere and for the initial planetesimal surface density, σ init , Z , in the protoplanetary disk. We also explore the implications of halting the solid accretion at selected core mass values during the protoplanet's growth. Halting planetesimal accretion at low core mass simulates the presence of a competing embryo, and decreasing the atmospheric opacity due to grains emulates the settling and coagulation of grains within the protoplanet's atmosphere. We examine the effects of adjusting these parameters to determine whether or not gas runaway can occur for small mass cores on a reasonable timescale. We compute four series of simulations with the latest version of our code, which contains updated equation of state and opacity tables as well as other improvements. Each series consists of a run without a cutoff in planetesimal accretion, plus up to three runs with a cutoff at a particular core mass. The first series of runs is computed with an atmospheric opacity due to grains (hereafter referred to as ‘grain opacity’) that is 2% of the interstellar value and σ init , Z = 10 g / cm 2 . Cutoff runs are computed for core masses of 10, 5, and 3 M ⊕ . The second series of Jupiter models is computed with the grain opacity at the full interstellar value and σ init , Z = 10 g / cm 2 . Cutoff runs are computed for core masses of 10 and 5 M ⊕ . The third series of runs is computed with the grain opacity at 2% of the interstellar value and σ init , Z = 6 g / cm 2 . One cutoff run is computed with a core mass of 5 M ⊕ . The final series consists of one run, without a cutoff, which is computed with a temperature dependent grain opacity (i.e., 2% of the interstellar value for T < 350 K ramping up to the full interstellar value for T > 500 K ) and σ init , Z = 10 g / cm 2 . Our results demonstrate that reducing grain opacities results in formation times less than half of those for models computed with full interstellar grain opacity values. The reduction of opacity due to grains in the upper portion of the envelope with T ⩽ 500 K has the largest effect on the lowering of the formation time. If the accretion of planetesimals is not cut off prior to the accretion of gas, then decreasing the surface density of planetesimals lowers the final core mass of the protoplanet, but increases the formation timescale considerably. Finally, a core mass cutoff results in a reduction of the time needed for a protoplanet to evolve to the stage of runaway gas accretion, provided the cutoff mass is sufficiently large. The overall results indicate that, with reasonable parameters, it is possible that Jupiter formed at 5 AU via the core accretion process in 1 Myr with a core of 10 M ⊕ or in 5 Myr with a core of 5 M ⊕ .

X-Ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars

Recent analysis of high-resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars and for probing the measure of grain depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya—the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material that is significantly depleted in O relative to Ne. In contrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age ~0.6 Myr), dust is still present near the disk corotation radius and can be ionized and accreted, rereleasing elements depleted onto grains. In the more evolved TW Hya disk (age ~10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and the accreting gas is consequently depleted of grain-forming elements.

Grain-Grain Coagulation in the ISM

We have developed a dust emission model in order to reproduce the evolution of the far-infrared (FIR) to sub-millimetre (sub-mm) dust emission properties which is observed in the transition between the diffuse and the dense interstellar medium (ISM). We propose that the coagulation of grains into fluffy aggregates could be the process responsible for the observed dust evolution from the diffuse to the dense ISM in the Galaxy. In dense clouds, dust coagulation can explain the increase in grain size, the decrease in the grain temperature in excess of that predicted by extinction, the sub-mm emissivity excess and the decrease in the small grain abundance.

Dust Evolution in Protoplanetary Disks

AbstractThe evolution of dust particles in circumstellar disk-like structures around protostars and young stellar objects is discussed. We especially consider the coagulation of grains due to collisional aggregation and the influence of this process on the optical properties of the particles. These dust opacities are important for both the derivation of the circumstellar dust mass from submillimetre continuum observations and the dynamical behaviour of the disks.We present first results of a numerical study of the coagulation of dust grains in a turbulent protoplanetary accretion disk described by a time-dependent one-dimensional (radial) “alpha” model. The influence of grain opacity changes due to dust coagulation on the dynamical evolution of a protostellar disk is investigated. In addition, we consider the grain motion in two-dimensional disks.

Dust opacities in dense regions

In this review paper, we discuss dust opacities relevant for dense regions in molecular clouds and for protoplanetary, accretion disks. We consider the formation of dust aggregates by the coagulation of grains. A main part of the paper deals with the different methods which are available for the calculation of the optical properties of fluffy particles. Recent grain models for the regions considered here will be presented.

Coagulation of grains and gas-grain interactions

Three-dimensional off-lattice Monte Carlo simulations have been used to simulate the production of interstellar dust analogues on the computer. At first, the two rather well-known extreme cases of ballistic growth, i.e. particle-cluster (BPCA) and cluster-cluster aggregation (BCCA) were investigated. In a second step, we included rotation of the clusters during the growth process. Following this, gas particles were deposited on these aggregates taking variable sticking probabilities due to the growth of monolayers into account. This is of interest to the problem of molecular abundances in dark clouds.

Coagulation of grains in static and collapsing protostellar clouds

view Abstract Citations (81) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Coagulation of Grains in Static and Collapsing Protostellar Clouds Weidenschilling, S. J. ; Ruzmaikina, T. V. Abstract We simulate collisional evolution of grains in dense turbulent molecular cloud cores (or Bok globules) in static equilibrium and free-fall collapse, assuming spherical symmetry. Relative velocities are due to thermal motions, differential settling, and turbulence, with the latter dominant for sonic turbulence with an assumed Kolmogorov spectrum. Realistic criteria are used to determine outcomes of collisions (coagulation vs. destruction) as functions of particle size and velocity. Results are presented for a variety of cloud parameters (radial density profile, turbulent velocity) and particle properties (density, impact strength). Results are sensitive to the assumed mechanical properties (density and impact strength) of grain aggregates. Particle growth is enhanced if aggregates have low density or fractal structures. On a timescale of a few Myr, an initial population of 0.1 micrometers grains may produce dense compact particles approximately 1 micrometer in size, or fluffy aggregates approximately 100 micrometers. For impact strengths less than or equal to 106 ergs/g, a steady state is reached between coagulation of small grains and collisional disruption of larger aggregates. Formation of macroscopic aggregates requires high mechanical strengths and low aggregate densities. We assume sonic turbulence during collapse, with varied eddy size scales determining the dissipation rate or turbulence strength. The degree of collisional evolution during collapse is sensitive to the assumed small-scale structure (inner sc ale) of the turbulence. Weak turbulence results in few collisions and preserves the precollapse particle size distribution with little change. Strong turbulence tends to produce net destruction, rather than particle growth, during infall, unless inpact strengths are greater than 106ergs/g. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174443 Bibcode: 1994ApJ...430..713W Keywords: Cosmic Dust; Interstellar Extinction; Interstellar Matter; Molecular Clouds; Protostars; Star Formation; Coagulation; Collisional Plasmas; Cosmology; Particle Size Distribution; Plasma Turbulence; Astrophysics; ISM: CLOUDS; ISM: DUST; EXTINCTION; STARS: FORMATION full text sources ADS | data products SIMBAD (3)

FORMATION OF THE EARTH

The origin of the earth is discussed in the context of the formation of the sun and the planets, and a standard model for such a formation assuming gravitational instability in a dense interstellar molecular cloud is outlined, along with the most significant variant of the model in which the loss of the nebular gas occurred after the formation of the earth. The formation of the sun and solar nebulae is addressed, and the coagulation of grains and the formation of small planetesimals are covered, along with the gravitational accumulation of planetesimals into planetary embryos and final stages of accumulation - embryos of planets. It is pointed out that the final stage of accumulation consists of the collision of these embryos; because of their large size, particularly after their further growth, these collisions represent giant impacts. It is concluded that the earth was initially an extremely hot and melted planet, surrounded by a fragile atmosphere and subject to violent impacts by bodies of the size of Ceres and even the moon.

Early stages of accumulation in the solar nebula

Coagulation of grains in the solar nebula should result in formation of low-density aggregates with fractal structure. The opacity of the nebula remains much higher than computed for the assumption that particles are compact. Timescales for particle growth, settling to the central plane of the nebula, and formation of planetesimals are lengthened significantly.

Grain growth in the turbulent accretion disk solar nebula

We have studied the coagulation of grains in the turbulent accretion disk solar nebula, considering turbulent diffusion, radial drift, and supply of interstellar grains by accreting interstellar cloud. The space-time dependent mass spectrum of grains and the Rosseland mean grain opacity have been calculated. Before the accretional effect becomes noticeable, the mass spectrum n( m) is well-described by n( m) ∝ m −1.31 for large grains and n( m) ∝ m −1.03 for small grains. The boundary is at the grains with the friction times comparable to the inner time scale of the turbulence. If the turbulence is fully developed, the growth time of grains is about 20 to 40 times shorter than that for the laminar nebula. In case of weak turbulence with velocity fluctuation of 0.03 × sound speed, the time scale is the same order as that in the laminar nebula. The accretion builds up the reservoir of interstellar grains, from which new generations of grains grow. Finally the spectrum reaches a steady state, crudely approximated by n( m) ∝ m −2. The grain opacity keeps constant as long as the mean grain radius of the first generation is smaller than the wavelength of radiation. The rapid increase is found at the end of this Rayleigh limit stage, and subsequently the opacity decreases as nearly (time) −2 owing to the grain growth. But the opacity increases again when the grains of the second generation grow as large as the wavelength and approaches a steady value. From these results, we have tried to describe the evolution of the nebula and the formation of planetesimals.

Dust to planetesimals: Settling and coagulation in the solar nebula

The behavior of solid particles in a low-mass solar nebula during settling to the central plane and the formation of planetesimals is examined. Gravitational instability in a dust layer and collisional accretion are considered as possible mechanisms of planetesimal formation. Non-Keplerian rotation of the nebula results in shear between the gas and a dust layer. This shear produces turbulence within the layer which inhibits gravitational instability, unless the mean particle size exceeds a critical value, ∼1 cm at 1 AU. The size requirement is less stringent at larger heliocentric distances, suggesting a possible difference in planetesimal formation mechanisms between the inner and outer nebula. Coagulation of grains during settling is expected in the solar nebula environment. Van der Waals forces appear adequate to produce centimeter-sized aggregates. Growth is primarily due to sweepup of small particles by larger ones due to size-dependent settling velocities. A numerical model for computing simultaneous coagulation and settling is described. Relative velocities are determined by gas drag and the non-Keplerian rotation of the nebula. The settling is very nonhomologous. Most of the solid matter reaches the central plane as centimeter-sized aggregates in a few times 10 3 revolutions, but some remains suspended in the form of fine dust. Drag-induced relative velocities result in collisions. The growth of bodies in the central plane is initially rapid. After sizes reach ∼10 3 cm, relative velocities decrease and the growth rate declines. Gas drag rapidly damps the out-of-plane motions of these intermediate-sized bodies. They settle into a thin layer which is subject to gravitational instability. Kilometer-sized planetesimals are formed by this composite process.