Low Dust Density Research Articles

The high dust density regime of dusty plasma: Theory and simulations

It is shown that the dust density regimes in the dusty plasma are characterized by two complementary screening processes: (i) the low dust density regime where the Debye screening is the dominant process and (ii) the high dust density regime where the “Coulomb screening” is the dominant process. The Debye regime is characterized by a state where all dust particles carry an equal and constant charge. The high dust density regime or the “Coulomb plasma” regime is characterized by (a) “Coulomb screening” where the dust charge depends on the spatial location and is screened by other dust particles in the vicinity by charge reduction, (b) “asymptotic freedom” where dust particles, which on an average carry minimal electric charge, are asymptotically free in the high dust density limit, (c) uniform dust charge density and plasma potential, (d) dust charge neutralization by a uniform background of hot ions, and (e) dust is weakly coupled due to strong Coulomb screening. Thus, the dusty plasma is essentially a weakly coupled, one-component plasma with screening in the high dust density limit. Molecular dynamics (MD) simulations verify these properties. The MD simulations are performed, using a recently proposed Hamiltonian formalism to study the dynamics of Yukawa particles carrying variable electric charge. A hydrodynamic model for describing the collective properties of Coulomb plasma and its characteristic acoustic mode called the “Coulomb acoustic mode” arising due to imperfect Coulomb screening is given.

Iceline variations driven by protoplanetary disc gaps

ABSTRACT The composition of forming planets is strongly affected by the protoplanetary disc’s thermal structure. This thermal structure is predominantly set by dust radiative transfer and viscous (accretional) heating and can be impacted by gaps – regions of low dust and gas density that can occur when planets form. The effect of variations in dust surface density on disc temperature has been poorly understood to date. In this work, we use the radiative transfer code MCMax to model the 2D dust thermal structure with individual gaps corresponding to planets with masses of 0.1 MJ –5 MJ and orbital radii of 3, 5, and 10 au. Low dust opacity in the gap allows radiation to penetrate deeper and warm the mid-plane by up to 16 K, but only for gaps located in the region of the disc where stellar irradiation is the dominant source of heating. In viscously heated regions, the mid-plane of the gap is relatively cooler by up to 100 K. Outside of the gap, broad radial oscillations in heating and cooling are present due to disc flaring. These thermal features affect local dust–gas segregation of volatile elements (H2O, CH4, CO2, and CO). We find that icelines experience dramatic shifts relative to gapless models: up to 6.5 au (or 71 per cent) closer to the star and 4.3 au (or 100 per cent) closer to the mid-plane. While quantitative predictions of iceline deviations will require more sophisticated models, which include transport, sublimation/condensation kinetics, and gas–dust thermal decoupling in the disc atmosphere, our results suggest that planet-induced iceline variations represent a potential feedback from the planet on to the composition of material it is accreting.

Semantic Segmentation and Inpainting of Dust with the S-Dust Dataset

In agriculture the performance of camera-based vision systems is strongly affected by environmental factors, such as rain, fog, and dust. This paper presents the first open dataset for training and evaluating image processing algorithms that remove swirling dust from images. It includes images from agricultural fields during tillage, which raises dust that is manually labeled in the images. However, dust is not a classical object that can be delineated based on edges. Moreover, the dust density is not uniformly distributed but varies and is concentrated in the area behind the implement. Therefore, different segmentation approaches were investigated. First, traditional and deep-learning methods for dust segmentation in images were compared. For the neural network, a Unet with a pretrained VGG encoder was chosen. The results show that the network cannot distinguish from the background in areas with low dust density. In contrast, the traditional segmentation method based on dark channel prior (DCP) outperforms it. Subsequently, various inpainting methods to remove dust were investigated. One observation was that dust removal from images cannot be solved with current inpainting methods. The dataset is available at *.*www.agriscapes-dataset.com

New cutoff and resonance for dust acoustic waves in dusty mesosphere

The WKB approximation has been employed to study the impacts of dust density inhomogeneity and dust size variation on dust acoustic (DA) wave propagation in the mesosphere. A local dispersion relation is obtained, which introduces one cutoff and one resonance frequency. It is shown that the growth rate of a gradient instability and the range of the amplifying wave propagation are substantially affected by the dust size distribution and density inhomogeneity. Moreover, in the low dust density region, the amplitude and the steepness of DA increase.

Effect of a low density dust shell on the propagation of gravitational waves

Using the Bondi-Sachs formalism, the problem of a gravitational wave source surrounded by a spherical dust shell is considered. Using linearized perturbation theory, the geometry is found in the regions: in the shell, exterior to the shell, and interior to the shell. It is found that the dust shell causes the gravitational wave to be modified both in magnitude and phase, but without any energy being transferred to or from the dust.

Planet formation by pebble accretion in ringed disks

Context.Pebble accretion is expected to be the dominant process for the formation of massive solid planets, such as the cores of giant planets and super-Earths. So far, this process has been studied under the assumption that dust coagulates and drifts throughout the full protoplanetary disk. However, observations show that many disks are structured in rings that may be due to pressure maxima, preventing the global radial drift of the dust.Aims.We aim to study how the pebble-accretion paradigm changes if the dust is confined in a ring.Methods.Our approach is mostly analytic. We derived a formula that provides an upper bound to the growth of a planet as a function of time. We also numerically implemented the analytic formulæ to compute the growth of a planet located in a typical ring observed in the DSHARP survey, as well as in a putative ring rescaled at 5 AU.Results.Planet Type I migration is stopped in a ring, but not necessarily at its center. If the entropy-driven corotation torque is desaturated, the planet is located in a region with low dust density, which severely limits its accretion rate. If the planet is instead near the ring’s center, its accretion rate can be similar to the one it would have in a classic (ringless) disk of equivalent dust density. However, the growth rate of the planet is limited by the diffusion of dust in the ring, and the final planet mass is bounded by the total ring mass. The DSHARP rings are too far from the star to allow the formation of massive planets within the disk’s lifetime. However, a similar ring rescaled to 5 AU could lead to the formation of a planet incorporating the full ring mass in less than 1/2 My.Conclusions.The existence of rings may not be an obstacle to planet formation by pebble-accretion. However, for accretion to be effective, the resting position of the planet has to be relatively near the ring’s center, and the ring needs to be not too far from the central star. The formation of planets in rings can explain the existence of giant planets with core masses smaller than the so-called pebble isolation mass.

The Radial Distribution of Dust Particles in the HL Tau Disk from ALMA and VLA Observations

Abstract Understanding planet formation requires one to discern how dust grows in protoplanetary disks. An important parameter to measure in disks is the maximum dust grain size present. This is usually estimated through measurements of the dust opacity at different millimeter wavelengths assuming optically thin emission and dust opacity dominated by absorption. However, Atacama Large Millimeter/submillimeter Array (ALMA) observations have shown that these assumptions might not be correct in the case of protoplanetary disks, leading to overestimation of particle sizes and to underestimation of the disk’s mass. Here, we present an analysis of high-quality ALMA and Very Large Array images of the HL Tau protoplanetary disk, covering a wide range of wavelengths, from 0.8 mm to 1 cm, and with a physical resolution of ∼7.35 au. We describe a procedure to analyze a set of millimeter images without any assumption about the optical depth of the emission, and including the effects of absorption and scattering in the dust opacity. This procedure allows us to obtain the dust temperature, the dust surface density, and the maximum particle size at each radius. In the HL Tau disk, we found that particles have already grown to a few millimeters in size. We detect differences in the dust properties between dark and bright rings, with dark rings containing low dust density and small dust particles. Different features in the HL Tau disk seem to have different origins. Planet–disk interactions can explain substructure in the external half of the disk, but the internal rings seem to be associated with the presence of snow lines of several molecules.

Excitation of low-frequency dust density waves in flowing complex plasmas

We study low frequency dust density waves excited in the microparticle streams of the Plasmakristall-4 chamber under microgravity conditions on board the International Space Station. Dispersion relations and instability conditions are analyzed for the dust density modes propagating along the axial electric field and in the reversed direction, revealing some important implications for the dust diagnostics. The results clearly point out to the Havnes parameter as a key factor for the excitation of the dust density modes.

Inferring giant planets from ALMA millimeter continuum and line observations in (transition) disks

Context. Radial gaps or cavities in the continuum emission in the IR-mm wavelength range are potential signatures of protoplanets embedded in their natal protoplanetary disk are. Hitherto, models have relied on the combination of mm continuum observations and near-infrared scattered light images to put constraints on the properties of embedded planets. Atacama Large Millimeter/submillimeter Array (ALMA) observations are now probing spatially resolved rotational line emission of CO and other chemical species. These observations can provide complementary information on the mechanism carving the gaps in dust and additional constraints on the purported planet mass. Aims. We investigate whether the combination of ALMA continuum and CO line observations can constrain the presence and mass of planets embedded in protoplanetary disks. Methods. We post-processed azimuthally averaged 2D hydrodynamical simulations of planet-disk models, in which the dust densities and grain size distributions are computed with a dust evolution code that considers radial drift, fragmentation, and growth. The simulations explored various planet masses (1 MJ ≤ Mp ≤ 15 MJ) and turbulent parameters (10−4 ≤ α ≤ 10−3). The outputs were then post-processed with the thermochemical code DALI, accounting for the radially and vertically varying dust properties. We obtained the gas and dust temperature structures, chemical abundances, and synthetic emission maps of both thermal continuum and CO rotational lines. This is the first study combining hydrodynamical simulations, dust evolution, full radiative transfer, and chemistry to predict gas emission of disks hosting massive planets. Results. All radial intensity profiles of 12CO, 13CO, and C18O show a gap at the planet location. The ratio between the location of the gap as seen in CO and the peak in the mm continuum at the pressure maximum outside the orbit of the planet shows a clear dependence on planet mass and is independent of disk viscosity for the parameters explored in this paper. Because of the low dust density in the gaps, the dust and gas components can become thermally decoupled and the gas becomes colder than the dust. The gaps seen in CO are due to a combination of gas temperature dropping at the location of the planet and of the underlying surface density profile. Both effects need to be taken into account and disentangled when inferring gas surface densities from observed CO intensity profiles; otherwise, the gas surface density drop at the planet location can easily be overestimated. CO line ratios across the gap are able to quantify the gas temperature drop in the gaps in observed systems. Finally, a CO cavity not observed in any of the models, only CO gaps, indicating that one single massive planet is not able to explain the CO cavities observed in transition disks, at least without additional physical or chemical mechanisms.

Light-curve analysis of KOI 2700b: the second extrasolar planet with a comet-like tail

Context. The Kepler object KOI 2700b (KIC 8639908b) was discovered recently as the second exoplanet with a comet-like tail. It exhibits a distinctly asymmetric transit profile, likely indicative of the emission of dusty effluents and reminiscent of KIC 12557548b, the first exoplanet with a comet-like tail. Aim. The scientific goal of this work is to verify the disintegrating-planet scenario of KOI 2700b by modeling its light curve and to put constraints on various tail and planet properties, as was done in the case of KIC 12557548b. Methods. We obtained the phase-folded and binned transit light curve of KOI 2700b, which we subsequently iteratively modeled using the radiative-transfer code SHELLSPEC. We modeled the comet-like tail as part of a ring around the parent star and we also included the solid body of the planet in the model. During the modeling we applied selected species and dust particle sizes. Results. We confirmed the disintegrating-planet scenario of KOI 2700b. Furthermore, via modeling, we derived some interesting features of KOI 2700b and its comet-like tail. It turns out that the orbital plane of the planet and its tail are not edge-on, but the orbital inclination angle is from the interval [85.1, 88.6] deg. In comparison with KIC 12557548b, KOI 2700b exhibits a relatively low dust density decreasing in its tail. We also derived the dust density at the beginning of the ring and the highest optical depth through the tail in front of the star, based on a tail-model with a cross-section of 0.05 × 0.05 R⊙ at the beginning and 0.09 × 0.09 R⊙ at its end. Our results show that the dimension of the planet is Rp∕Rs ≤ 0.014 (Rp ≤ 0.871 R⊕, or ≤5551 km). We also estimated the mass-loss rate from KOI 2700b, and we obtained Ṁ values from the interval [5.05 × 107, 4.41 × 1015] g s−1. On the other hand, we could not draw any satisfactory conclusions about the typical grain size in the dust tail.

Equilibrium and stability of a gravitationally bound uniformly charged dust cloud

An exactly solvable model of equilibrium and stability of a gravitationally bound low dust density cloud embedded in background plasma is described. The dust is distributed almost homogeneously within the cloud. The corresponding eigen value equation is solved using the Frobenius method to calculate the complete set of radial eigen modes and corresponding frequencies. The low dust density cloud is shown to be stable. This model can be useful in benchmarking simulation codes and modelling dust density distribution in the interstellar cloud in the HII region.

Study on incoherent scatter theory of high density dusty plasma

Incoherent scatter radar is one of the most important detection instruments of the space plasma. But because of the low dust density in natural space plasma, the contribution of charged dust to incoherent scatter spectrum can be completely ignored, therefore the incoherent scattering theory has not appeared in dusty plasma. In the solid rocket plume, the propellant combustion can form a large number of nanometer- and micronmeter-sized dusty particles, and produce a high electron density from high temperature ionization, which makes considerable contributionto charged dusty particles with the high density. Therefore, we develop the incoherent scattering theory of dusty plasma in order to calculate the scattering characteristics of high density dusty plasma produced by rocket plume, for example. The theoretical model including electrons, ions and dusty particles is established by combining effects of charged dusty particles. The incoherent scatter spectral lines of ion resonance region and dust resonance regionare calculated. The effects of dusty particle radius, temperature and density on spectral line structure are discussed. With the increases of dusty particle radius and density, the amplitude of power spectrum increases. With the increase of dust temperature, the amplitude of power spectrum decreases. In the dust resonance region, the control mechanism of dust in spectrum is similar to that of the ions. With the increase of particle size (mass) and decrease of the temperature, the spectrum width narrows, and amplitude and area increase with the increase of density. But in the ion resonance region, the dust control mechanism is completely different, and the influence of the dust on ion line is in the way of attracting ions. So with the increase of dust density, ion line characteristics do not show that the area increases, and dust controls ions by adjusting the Debye radius or electrostatic shielding ball size. By comparing the ion lines with and without dust under the same parameters conditions, the amplitude of the ion line with dust is much larger than that without dust, and the resonance frequency of the ion line is greatly changed. With the dust particles of a relatively high density, one can enhance the ion line, hence the incoherent scattering phenomenon can be more easily observed in rocket plume. On the other hand, due to significant changes of frequency and amplitude in the ion line spectrum, the incoherent scattering inversion method based on the traditional theory will cause a large error in the inversion parameter, even a failure of parameter retrieval. The incoherent scattering theory and relevant physical laws of dusty plasma are presented, which are of great significance for establishing the incoherent scattering theory system and studying the rocket plume parameters.

Particle-in-cell simulation of spacecraft/plasma interactions in the vicinity of Enceladus

The Cassini Langmuir Probe of the Radio and Plasma Wave Science instrument has measured an electron depletion in a region extending at least 50 satellite radii away from Saturn’s small but geologically active icy moon Enceladus. The maximal imbalance between the electron and ion densities was observed in the dust loaded plume and to date is attributed to the electron attachment to abundant dust grains. We report the results from a three dimensional particle-in-cell simulation of a plasma structure formed around a charged spacecraft in the conditions relevant inside the Enceladus torus and in the moon’s plume. In addition to the plasma population the plume simulation includes singly charged nanograins detected by the Cassini Plasma Spectrometer. The accompanied spacecraft plasma perturbations can significantly modify an ambient plasma at the Cassini Langmuir probe positions and thus impact the plasma measurements. Our modeling reveals a domination of water group ions over the electron population due to the formation of a conventional plasma sheath at the ram-oriented probe positions in the Enceladus torus and in the plume regions with low dust density (nd0<ne0). In the dust-dominated plume (nd0>ne0) the plasma perturbations are strongly reduced in the ram direction but can significantly compromise the probe measurements in the orbiter wake. Simulation results can qualitatively explain the long profiles of the electron–ion imbalance registered by the Cassini Langmuir probe during the flybys E2, E3 and E5. In either case, the plasma perturbations associated with the moving Cassini orbiter appear to be important for reliable interpretations of the Langmuir probe measurements.

Observation of the Taylor instability in a dusty plasma

Observations of the Taylor instability in a laboratory dusty plasma are presented. The dust cloud, formed in a dc argon glow-discharge plasma, is stratified into regions of high and low dust densities. The instability was triggered by a spontaneous intrusion of the low density dust fluid into the high density dust fluid at the interface. The instability in the dust fluid was phenomenologically similar to the hydrodynamic Taylor instability that occurs when a light fluid is accelerated into a heavy fluid.

Magnetopause displacements: the possible role of dust

Abstract. Large compressions of the magnetopause are proposed to occasionally result from temporary encounters of the magnetosphere with dust streams in interplanetary space. Such streams may have their origin in cometary dust tails or asteroids which cross the inner heliosphere or in meteoroids in Earth's vicinity. Dust ejected from such objects when embedding the magnetosphere for their limited transition time should cause substantial global deformations of the magnetopause/magnetosphere due to the very large dust grain mass and momentum which compensates for the low dust density when contributing to the upstream pressure variation.

Effects of charge depletion in dusty plasmas

The charge reduction effect is studied in dense dust clouds. The saturation currents of Langmuir probes are used to derive the density of ions and electrons, which are calibrated with the plasma oscillation method. The plasma potential inside the dust cloud is measured with an emissive probe, which also yields the floating potential in a heated nonemitting mode. The presence of the dust also affects the density and the plasma potential of the ambient plasma. The ion densities inside the dust cloud and in the ambient plasma are found equal, while the electron density is reduced inside the dust cloud. The measured potentials are compared with current models. Inclusion of the bi-Maxwellian distribution of the electrons leads to an improved description in the limit of low dust density. The strong increase of the floating and cloud potential for high dust density, predicted by the constant ion density model, is not confirmed.

Near-IR bispectrum speckle interferometry, AO imaging polarimetry, and radiative transfer modeling of the proto-planetary nebula Frosty Leonis

Aims. We combined bispectrum speckle interferometry, adaptive optics (AO) imaging polarimetry, and radiative transfer modeling of polarized light to derive various physical properties of the proto-planetary nebula Frosty Leo. Methods. We performed bispectrum K � -band speckle interferometry and H -a ndK-band imaging polarimetry of Frosty Leo using the ESO 3.6 m telescope and the AO-equipped CIAO instrument of the 8 m Subaru telescope, respectively. Two-dimensional radiative transfer modeling was carried out in order to obtain a quantitative interpretation of our observations. Results. Our diffraction-limited speckle image shows distinct hourglass-shaped, point-symmetric bipolar lobes, an equatorial dust lane, and complex clumpy structures in the lobes. Our polarimetric data display a centro-symmetric polarization vector pattern with P ∼ 30–50% in the bipolar lobes and a polarization disk between them. The polarization images also reveal an elongated region with low polarization along a position angle of −45 ◦ . The observations suggest that this region has a low dust density and was carved out by a jet-like outflow. Our radiative transfer modeling can simultaneously explain the observed spectral energy distribution, the intensity distribution of the hourglass-shaped lobes, and our polarization images if we use two grain species with sizes of 0.005 ≤ a ≤ 2.0 μm at latitudes between −2 ◦ and +2 ◦ ,a nd 0.005 ≤ a ≤ 0.7 μm in the bipolar lobes. Assuming a distance of 3 kpc, an expansion velocity of 25 km s −1 , and a gas-to-dust mass ratio of 160, we derive a dust mass of the disk of 2.85 × 10 −3 M� , a gas mass-loss rate of ) ( ) ( *

An energy principle for the stability of charged dust cloud

The stability of a charged dust cloud embedded in a quasineutral plasma background is studied using an energy principle. In this equilibrium, the electric fields arising due to the inhomogeneous dust distribution within the plasma background balances its self-gravity. The perturbed equation for the normal modes is obtained in terms of the Lagrangian displacement of the dust fluid ξ⃗ with a Hermitian operator. Hence, the square of the eigenvalue is real. The total potential energy (gravitational and electrostatic) integral is expressed as a quadratic in ξ⃗. The compression due to the electrostatic field (or equivalently the acoustic modes) and the gravity due to the plasma background is shown to make a stabilizing contribution, while the gravity of the dust makes a destabilizing contribution to the potential energy integral. The stabilizing contribution due to the plasma background, which is missed in analysis based on Jean’s swindle, is a new effect and appears in our stability analysis due to a proper treatment of the equilibrium. This stability theory is applied to the study of the gravitational stability of one-dimensional spherically symmetric dust clouds. The equation for the radial oscillations is shown to be of the Sturm-Liouville form; hence the stability of the fundamental mode decides the overall stability of the configuration. A simple trial function is used to obtain the eigenvalue of the fundamental mode. The low dust density branch of the equilibrium is shown to be robustly stable mainly due to the electric field compression. At high dust densities, the charge reduction effects weaken the field compression. This destabilizes the fundamental mode and the over all configuration. A spherically symmetric collapse of the dust cloud sets in as the eigenvalue of the fundamental mode crosses zero. The radial oscillations of the low density dust cloud, the collapse at high densities, and the resulting mass limit has been predicted earlier and seen in recent numerical simulations.

Theory of charged dust clouds: Equilibrium

Gravitational equilibria of low density dust clouds imbedded in plasma background are studied. These are a new class of astrophysical objects, in which the electric field in quasineutral plasma balances the self-gravity of the dust. Heuristic arguments based on the total potential energy of a uniform dust ball are given for the existence of stable equilibrium and the mass limit for the maximum dust mass MAS, which can be supported against its self-gravity. The gravitational collapse is initiated by the destabilization of the fundamental radial mode as the mass of the dust cloud MD→MAS. The physics of the mass limit is similar to the Chandrasekhar’s mass limit for compact objects. One parameter family of spherically symmetric, static solutions of radius R(≈0.1AU) and the total mass of the dust cloud MD(≈1019g), for given central dust density, are constructed. A finite flux of photoelectrons is shown to decrease R and MD. Two-dimensional rotating dust disk equilibrium, characterized by a sharp and diffuse boundary, is also constructed. These dust equilibria (spherical clouds, rotating disks), which are predicted to be within the interstellar clouds, could be relevant for planet formation, as they provide locations where a significant quantity of dust can be stably trapped. The accretion and coagulation interaction between these trapped dust grains may lead to planet formation.

Cosmic dust physical properties and theICAPS facility on board the ISS

Cometary comae, cometary tails, and the interplanetary dust cloud, are low density dust clouds built of cosmic dust particles. Light scattering observations, from in-situ space probes and remote observatories, are a key to their physical properties. This presentation updates results on cometary and interplanetary dust derived from such observations (with emphasis on polarization), and compares them with results on asteroidal regoliths. The polarization phase curves follow similar trends, with parameters that may vary from one object to another. The wavelength dependence is highly variable, although it is usually linear in the visible domain. It may be suggested (from observations, modeling and laboratory measurements) that these dust particles are irregular, with a size greater than the wavelength, and that cometary dust is highly porous, as compared to asteroidal or interplanetary dust. Sophisticated numerical models and laboratory measurements on dust analogues are indeed required to interpret without any ambiguity the ensemble of results. The opportunity offered by the ICAPS facility (an ESA project selected for the ISS, now in phase B) to deduce the physical properties of cosmic dust particles from their optical properties, as well as their evolution (breaking-off and agglomeration, ices condensation and evaporation), is presented.