Evolution Of Dust Particles Research Articles

Structure and photochemistry of a potential precursor of circumstellar dust: The optical spectrum of Si4C2+

Mixed silicon carbide clusters are important components of circumstellar shells and their optical properties are fundamental for modelling the chemistry involved in the evolution of dust particles. Herein, we report the ultraviolet photodissociation (UVPD) spectrum of mass-selected Si4C2+ cations obtained in the 208–330 nm range (3.8–6.0 eV) in a quadrupole/time-of-flight tandem mass spectrometer coupled to a laser desorption source operating with a SiC rod and a broadly tuneable optical parametric oscillator laser. The UVPD spectrum is observed in the two lowest-energy fragment channels, corresponding to dominant loss of atomic Si at low energy and the rather stable Si2C molecule at high energy. The UVPD spectrum is analyzed by quantum chemical calculations at two levels of density functional theory (CAM-B3LYP, M11) to determine the structures, energies, electronic spectra, and fragmentation energies of the lowest-energy isomers. To locate these isomers on the multidimensional potential energy surface, basin hopping is employed as global optimization approach. The UVPD spectrum is consistent with the optical absorption spectrum computed for the two lowest-energy isomers, one of which is reported for the first time.

Nonlinear acoustic-gravity waves and dust particle redistribution in earth's atmosphere

A continuously stratified model of nonadiabatic terrestrial atmosphere with taking into account the temperature profile is developed to study a possibility of instability development of acoustic-gravity (AG-) waves. It is shown that the existence of the regions in the atmosphere where the instability conditions are satisfied is due to the cooperation of thermal flow of solar radiation, infrared emission of the atmosphere, water vapor condensation, as well as thermal conductivity. Large-amplitude vortices in Earth's troposphere and ionosphere and their possible structure as well as redistribution of dust particles in the ionosphere as a result of vortical motions are discussed. The following possibilities for the dust particle redistribution are studied: capture and evolution of dust particles in AG-vortices, formation of dust vortices as a result of involving a great number of dust particles into vortex motions, and formation of vertical and horizontal dust flows (streamers and zonal flows). It is shown that excitation of AG-vortices at the ionospheric altitudes as a result of development of AG-wave instability leads to a substantial transportation of dust particles and their mixing. Layers of dust particles with a thickness of about a kilometer, forming at the altitudes less than 120km, distribute within the region of the existence of AG-vortical structures. As a result, at altitudes of 110–120km, dust vortices can appear, and transportation of particles up to altitudes of 130km becomes possible. One of the ways of transportation of dust particles in the ionosphere is dust flows, which are generated by dust vortices as a result of development of parametric instability.

Planetesimal formation in self-gravitating discs – the effects of particle self-gravity and back-reaction

We study particle dynamics in self-gravitating gaseous discs with a simple cooling law prescription via two-dimensional simulations in the shearing sheet approximation. It is well known that structures arising in the gaseous component of the disc due to a gravitational instability can have a significant effect on the evolution of dust particles. Previous results have shown that spiral density waves can be highly efficient at collecting dust particles, creating significant local over-densities of particles. The degree of such concentrations has been shown to be dependent on two parameters: the size of the dust particles and the rate of gas cooling. We expand on these findings, including the self-gravity of dust particles, to see how these particle over-densities evolve. We use the PENCIL CODE to solve the local shearing sheet equations for gas on a fixed grid together with the equations of motion for solids coupled to the gas through an aerodynamic drag force. We find that the enhancements in the surface density of particles in spiral density wave crests can reach levels high enough to allow the solid component of the disc to collapse under its own self-gravity. This produces many gravitationally bound collections of particles within the spiral structure. The total mass contained in bound structures appears nearly independent of the cooling time, suggesting that the formation of planetesimals through dust particle trapping by self-gravitating density waves may be possible at a larger range of radii within a disc than previously thought. So, density waves due to gravitational instabilities in the early stages of star formation may provide excellent sites for the rapid formation of many large, planetesimal-sized objects.

LAPLACE PLANE MODIFICATIONS ARISING FROM SOLAR RADIATION PRESSURE

The dynamical effects of solar radiation pressure (SRP) in the solar system have been rigorously studied since the early 1900s. This non-gravitational perturbation plays a significant role in the evolution of dust particles in circumplanetary orbits, as well as in the orbital motion about asteroids and comets. For gravitationally dominated orbits, SRP is negligible and the resulting motion is largely governed by the oblateness of the primary and the attraction of the Sun. The interplay between these gravitational perturbations gives rise to three mutually perpendicular planes of equilibrium for circular satellite orbits. The classical Laplace plane lies between the equatorial and orbital planes of the primary, and is the mean reference plane about whose axis the pole of a satellite's orbit precesses. From a previously derived solution for the secular motion of an orbiter about a small body in a SRP dominated environment, we find that SRP acting alone will cause an initially circular orbit to precess around the pole of the primary's heliocentric orbital plane. When the gravitational and non-gravitational perturbations act in concert, the resulting equilibrium planes turn out to be qualitatively different, in some cases, from those obtained without considering the radiation pressure. The warping of the surfaces swept out by the modified equilibria as the semi-major axis varies depends critically on the cross-sectional area of the body exposed. These results, together with an adiabatic invariance argument on Poynting–Robertson drag, provide a natural qualitative explanation for the initial albedo dichotomy of Saturn's moon, Iapetus.

Kinetic-Monte-Carlo-Based Parallel Evolution Simulation Algorithm of Dust Particles

The evolution simulation of dust particles provides an important way to analyze the impact of dust on the environment. KMC-based parallel algorithm is proposed to simulate the evolution of dust particles. In the parallel evolution simulation algorithm of dust particles, data distribution way and communication optimizing strategy are raised to balance the load of every process and reduce the communication expense among processes. The experimental results show that the simulation of diffusion, sediment, and resuspension of dust particles in virtual campus is realized and the simulation time is shortened by parallel algorithm, which makes up for the shortage of serial computing and makes the simulation of large-scale virtual environment possible.

Cyclic powder formation during pulsed injection of hexamethyldisiloxane in an axially asymmetric radiofrequency argon discharge

A new approach of periodic production of dusty plasma consisting of pulsed injection of hexamethyldisiloxane (HMDSO) in argon axially asymmetric radiofrequency (RF) discharge was investigated in this work. The range of plasma operating conditions in which this dusty plasma can exist was closely examined. The obtained results clearly show that a net periodicity in the formation/disappearance of dust particles in the plasma can be maintained on a very large scale of discharge duration. The significance of discharge axial asymmetry to the dust particles behaviour in the plasma is revealed by the development of an asymmetric in shape void shifted towards the powered RF electrode. The key role of the reactive gas and its pulsed injection on each stage of the oscillating process of formation/disappearance of dust particles is disclosed by optical and electrical measurements. It is shown that the period of dusty plasma formation/disappearance is inversely related to the HMDSO injection time. Moreover, the impact of time injection over short period (5 s) is examined. It indicates the conflicting role played by the HMDSO on the reduction of dusty plasma during the reactive gas injection and the reappearance of particles in the plasma during the time off. The electronegative behavior of the plasma in the presence of negatively charged particles seems to explain the energetic modifications in the discharge. A frequency analysis of the floating potential reveals all these cyclic processes. Particularly, in the 10–200 Hz frequency range, the presence and the evolution of dust particles in the plasma over one generation can be observed.

Planetesimal formation in self-gravitating discs

We study particle dynamics in local two-dimensional simulations of self-gravitating accretion discs with a simple cooling law. It is well known that the structure which arises in the gaseous component of the disc due to a gravitational instability can have a significant effect on the evolution of dust particles. Previous results using global simulations indicate that spiral density waves are highly efficient at collecting dust particles, creating significant local over-densities which may be able to undergo gravitational collapse. We expand on these findings, using a range of cooling times to mimic the conditions at a large range of radii within the disc. Here we use the Pencil Code to solve the 2D local shearing sheet equations for gas on a fixed grid together with the equations of motion for solids coupled to the gas solely through aerodynamic drag force. We find that spiral density waves can create significant enhancements in the surface density of solids, equivalent to 1-10cm sized particles in a disc following the profiles of Clarke (2009) around a solar mass star, causing it to reach concentrations several orders of magnitude larger than the particles mean surface density. We also study the velocity dispersion of the particles, finding that the spiral structure can result in the particle velocities becoming highly ordered, having a narrow velocity dispersion. This implies low relative velocities between particles, which in turn suggests that collisions are typically low energy, lessening the likelihood of grain destruction. Both these findings suggest that the density waves that arise due to gravitational instabilities in the early stages of star formation provide excellent sites for the formation of large, planetesimal-sized objects.

Ice sublimation of dust particles and their detection in the outer solar system

The flux of interplanetary dust beyond the Jupiter’s orbit, which supposedly originates from Edgeworth-Kuiper belt, has been measured in situ by instruments on board Voyager and Pioneer spacecraft. The measured flux shows a nearly flat radial profile at 10–50AU for Voyager and at 5–15AU for Pioneer. Because the orbital evolution of dust particles controlled by radiation forces results in the flux that is inversely proportional to distance from the sun, dust particles detected by spacecraft should have suffered from other dynamical effects. We calculate model fluxes on the spacecraft taking into account the effect of ice sublimation as well as radiation forces on the orbital evolution of dust particles. Our results show that the radial profile of the model flux becomes relatively flat near the outer edge of the sublimation zone, where ice substantially sublimes. The expected location of the flat radial profile, which depends on the detection threshold of instruments, is 15–40AU for Voyager and 5–20AU for Pioneer. Because our model fluxes are comparable with the measured ones, we conclude that the flat radial profiles of the dust flux derived from in-situ dust impacts may be caused by ice sublimation.

Vortex motions and transportation of fine disperse dust particles in the ionosphere

Redistribution of dust particles in the ionosphere as a result of vortical motions is discussed. The following possibilities are studied: capture and evolution of dust particles in acoustic-gravitational (AG) vortices, formation of dust vortices as a result of involving a great number of dust particles into vortex motions, and formation of vertical dust flows (streamers). It is shown that excitation of AG-vortices at altitudes of 110–130 km as a result of development of AG-wave instability, associated with non-zero balance of heat fluxes, owing to solar radiation, water vapors condensation, infrared emission of the atmosphere, and thermal conductivity, leads to a substantial transportation of dust particles and their mixing at altitudes of 110–120 km. Layers of dust particles in the ionosphere with a thickness of about a kilometer, forming at altitudes less than 120 km, distribute within the region of existence of AG-vortical structures. As a result, at altitudes of 110–120 km, dust vortices can appear, and transportation of particles up to altitudes of 130 km becomes possible. One of the ways of transportation of dust particles in the ionosphere is vertical flows (streamers), which are generated by dust vortices as a result of development of parametric instability.

Small Mass Ratio Limit of Boltzmann Equations in the Context of the Study of Evolution of Dust Particles in a Rarefied Atmosphere

We propose a model based on the coupling of two Boltzmann-like equations for the study of the evolution of dust particles in a rarefied atmosphere, such as it can be found in the context of safety studies for the ITER project of nuclear fusion.

Modeling the particle mass distribution within 1 AU of the Sun

We propose a method for estimating the particle flux and number density distribution of interplanetary dust particles within 1 AU of the Sun. The method takes into account both the evolution of dust particles by collision and Poynting–Robertson orbital decay. The collisional evolution and the resulting particle mass distribution function both depend strongly on the orbits of collision fragments. Our preliminary results indicate that the flux of hyperbolic particles (β-meteoroids) with masses in the range 10 −15 < m < 10 −13 g at 1 AU will directly lead to the supply of m ⩾ 10 −7 g particles inside 1 AU. Moreover, we show that the slope of the particle mass distribution function inside 1 AU may differ significantly from that at 1 AU if we assume that the flux distribution inside 1 AU does not vary with time.

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.