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1.
In a previous publication (Dominik and Nübold, 2002, Icarus 157, 173-186), we presented analytical expressions and theoretical considerations concerning preplanetary dust aggregation with magnetized grains in the solar nebula. The present work is dedicated to the experimental study of magnetic aggregation in a ground-based laboratory as well as under microgravity conditions on parabolic flights. We conducted aggregation experiments with dust analogues in order to study the temporal evolution and the structural outcome of grain growth processes dominated by or comprising exclusively magnetic grains. Within aggregation times ranging from a couple of seconds to a few minutes only, formation of huge chain-like and/or web-like dust aggregates was observed. After aggregate retrieval we were able to study the sizes and structures of the aggregates in great detail. We established the fractal dimension of the aggregates as Dfs=1.20±0.05 for single chains and Dfc=1.50±0.21 for inter-connected web-like structures. This is considerably lower than for non-magnetic grain growth. The dynamic exponent z for the mass increase with time according to tz was found to be z=2.7 from in-situ video images of the microgravity aggregation runs. The results are compared with the theoretical considerations presented earlier as well as with previous experimental work on the same and on related topics, respectively. 相似文献
2.
Lindsey F. Smith 《Astrophysics and Space Science》1975,34(1):49-55
The upper limit for the absorption cross section σ H ext , of dust in Hii regions in the wave-length range 912–504 Å derived by Mezgeret al. (1974), is compatible with that expected for large dust grains, and a gas-to-dust ratio equal to that in the general interstellar medium. The albedo of the small grains must be high for λ>504 Å. This restriction is lifted if the visual extinction cross section of the grains in Hii regions is less than that for grains in the general interstellar medium. New observations of the Orion Nebula indicate that the visual extinction cross section is within a factor 2 of the value in the general interstellar medium. 相似文献
3.
We investigate the method by which nearby supernovae – within a few tens of pc of the solar system – can penetrate the solar system and deposit live radioactivities on earth. The radioactive isotopic signatures that could potentially leave an observable geological imprint are in the form of refractory metals; consequently, it is likely they would arrive in the form of supernova-produced dust grains. Such grains can penetrate into the solar system more easily than the bulk supernova plasma, which gets stalled and deflected near the solar system due to the solar wind plasma pressure. We therefore examine the motion of charged grains as they decouple from the supernova plasma and are influenced by the solar magnetic, radiation, and gravitational fields. We characterize the dust trajectories with analytical approximations which display the roles of grain size, initial velocity, and surface voltage. These results are verified with full numerical simulations for wide ranges of dust properties. We find that supernova dust grains traverse the inner solar system nearly undeflected, if the incoming grain velocity – which we take to be that of the incident supernova remnant – is comparable to the solar wind speeds and much larger than the escape velocity at 1 AU. Consequently, the dust penetration to 1 AU has essentially 100% transmission probability and the dust capture onto the earth should have a geometric cross section. Our results cast in a new light the terrestrial deposition of radioisotopes from nearby supernovae in the geological past. For explosions beyond ～10 pc from earth, dust grains can still deliver supernova ejecta to earth, and thus the amount of supernova material deposited is set by the efficiency of dust condensation and survival in supernovae. Turning the problem around, we use observations of live 60Fe in both deep-ocean and lunar samples to infer a conservative lower bound iron condensation efficiency of Mdust,Fe/Mtot,Fe ? 4 × 10?4 for the supernova which apparently produced these species 2–3 Myr ago. 相似文献
4.
A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We reformulate the equation of motion so that the radiation pressure and the Poynting-Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows numerical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates.The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence depends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital eccentricity and semimajor axis, the orbital inclination of the particle from being preserved in orbit around the Sun. However, a change in the inclination is most probably controlled by the Lorentz force as a consequence of the interaction between electric charges on the grains and the solar magnetic field. Although rapidly and randomly rotating grains spiral into the Sun under the Poynting-Robertson effect in spite of their shapes and structures, fluffy grains drift inward on time scales longer at submicrometer sizes and shorter at much larger sizes than spherical grains of the same sizes. Numerical calculations reveal that the dynamical lifetimes of fluffy particles are determined by the material composition of the grains rather than by their morphological structures and sizes. The Poynting-Robertson effect alone is nevertheless insufficient for giving a satisfactory estimate of lifetimes for fluffy dust grains since their large ratios of cross section to mass would reduce the lifetimes by enhancing the collisional probabilities. We also show that the radiation pressure on a dust particle varies with the orbital velocity of the particle but that this effect is negligibly small for dust grains in the Solar System. 相似文献
5.
6.
Lightning discharge generated in the protoplanetary nebula is viewed as a temporally isolated surge in the flow of electrically charged particles, similar to that of terrestrial lightning. If the current is intense enough, a powerful circular impulse magnetic field is generated around the instantaneous virtual electric conductor. Such magnetic field is capable of magnetizing dust grains containing ferromagnetic components present in its vicinity to their saturation levels. As a result, dust grains attract one another, forming the aggregates. This magnetically driven attraction suggests an important process possibly operational at an early stage of the planetary accretion. Based on both a classical model for electric conductor, and the theory of Lienard–Wiechert electromagnetic potentials, our calculations show that the magnetic impulse due to a discharge channel of a few cm in diameter transferring a charge of about 104 electrons reaches as high as 10 T. At these magnetic fields, the ferromagnetic dust grains, and possibly the already-formed larger aggregates as well, are easily magnetized to the saturation levels, producing compact clusters exhibiting permanent magnetic moments. 相似文献
7.
Of the formation processes in the solar system, the process of growth and sedimentation of dust grains in the primordial solar nebula is investigated for a region near the Earth's orbit. The growth equation for dust grains, which are sinking as well as being in thermal motion, is solved numerically in the wide mass range between 10?12 and 106 g. Any turbulent motions in the nebula are assumed to have already decayed when the sedimentation begins. The numerical simulation shows that the growth and sedimentation proceed faster than was found by Kusaka et al. (1970) but in accordance with the estimate of Safronov (1969) owing to a cooperative interaction of the growth and the sedimentation; that is, at about 3 × 103 years after the beginning of the growth and sedimentation a dust layer, composed of centimeter-sized grains, is formed at the equator of the solar nebula. Furthermore, the mass density of dust grains floating in the outer layers of the nebula is found to be of the order of 10?5 after 105 years compared with that before the sedimentation. From these results, it can be estimated that at about 5 × 103 years after the beginning of sedimentation the dust layer breaks up owing to the onset of gravitational instability. 相似文献
8.
S.J. Weidenschilling 《Icarus》1980,44(1):172-189
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 103 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 ～103 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. 相似文献
9.
Augusto Carballido 《Icarus》2011,211(1):876-884
Numerical magnetohydrodynamic (MHD) simulations of a turbulent solar nebula are used to study the growth of dust mantles swept up by chondrules. A small neighborhood of the solar nebula is represented by an orbiting patch of gas at a radius of 3 AU, and includes vertical stratification of the gas density. The differential rotation of the nebular gas is replaced by a shear flow. Turbulence is driven by destabilization of the flow as a result of the magnetorotational instability (MRI), whereby magnetic field lines anchored to the gas are continuously stretched by the shearing motion. A passive contaminant mimics small dust grains that are aerodynamically well coupled to the gas, and chondrules are modeled by Lagrangian particles that interact with the gas through drag. Whenever a chondrule enters a region permeated by dust, its radius grows at a rate that depends on the local dust density and the relative velocity between itself and the dust. The local dust abundance decreases accordingly. Compaction and fragmentation of dust aggregates are not included. Different chondrule volume densities ρc lead to varying depletion and rimmed-chondrule size growth times. Most of the dust sweep-up occurs within ～1 gas scale-height of the nebula midplane. Chondrules can reach their asymptotic radius in 10–800 years, although short growth times due to very high ρc may not be altogether realistic. If the sticking efficiency Q of dust to chondrules depends on their relative speed δv, such that Q < 10?2 whenever δv > vstick ≈ 34 cm/s (with vstick a critical sticking velocity), then longer growth times result due to the prevalence of high MRI-turbulent relative velocities. The vertical variation of nebula turbulent intensity results in a moderate dependence of mean rimmed-chondrule size with nebula height, and in a ～20% dispersion in radius values at every height bin. The technique used here could be combined with Monte Carlo (MC) methods that include the physics of dust compaction, in a self-consistent MHD-MC model of dust rim growth around chondrules in the solar nebula. 相似文献
10.
V. M. Reshetnyk Yu. V. Skorov P. Lacerda P. Hartogh L. Rezac 《Solar System Research》2018,52(3):266-281
We consider the estimates of the main forces acting on dust particles near a cometary nucleus. On the basis of these estimates, the motion of dust particles of different structure and mass is analyzed. We consider the following forces: (1) the cometary nucleus gravity, (2) the solar radiation pressure, and (3) the drag on dust particles by a flow of gas produced in the sublimation of cometary ice. These forces are important for modeling the motion of dust particles relative to the cometary nucleus and may substantially influence the dust transfer over its surface. In the simulations, solid silicate spheres and homogeneous ballistic aggregates are used as model particles. Moreover, we propose a technique to build hierarchic aggregates—a new model of quasi-spherical porous particles. A hierarchic type of aggregates makes it possible to model rather large dust particles, up to a millimeter in size and larger, while no important requirements for computer resources are imposed. We have shown that the properties of such particles differ from those of classical porous ballistic aggregates, which are usually considered in the cometary physics problems, and considering the microscopic structure of particles is of crucial significance for the analysis of the observational data. With the described models, we study the dust dynamics near the nucleus of comet 67P/Churyumov–Gerasimenko at an early stage of the Rosetta probe observations when the comet was approximately at 3.2 AU from the Sun. The interrelations between the main forces acting on dust aggregates at difference distances from the nucleus have been obtained. The dependence of the velocity of dust aggregates on their mass has been found. The numerical modeling results and the data of spaceborne observations with the Grain Impact Analyzer and Dust Accumulator (GIADA) and the Cometary Secondary Ion Mass Analyzer (COSIMA) onboard the Rosetta probe are compared at a quantitative level. 相似文献
11.
《Chinese Astronomy and Astrophysics》1983,7(1):11-18
The modified distribution function of dust particles, f(γ), which can be determined from tail brightness profiles on the basis of mechanical theory, is discussed with special regard to its reliability and accuracy. Physical significance of f(γ) is also discussed in terms of dust model parameters, and it is shown that f(γ), if treated carefully, will serve as an effective tool in studying cometary dust. Four isophotes of Comet Arend-Roland, 1957 III, in the orange-red light (λλ 0.53–0.68 micron) obtained by Ceplecha (1958), are analysed by the numerical method described in Paper I (KIMURA and LIU 1975) with some improvements and higher approximations. The distribution f(γ) thus obtained shows a bimodal character with peaks at γ = 0.10 and 0.010 with a relative height ratio of 1 to 0.6. Dust emission rate, which is assumed to follow the inverse square law of heliocentric distance, is estimated to give PdCsca = (1.3±0.5) × 109 cm2/sec, where Pd is the rate of particle emission at 1 a.u. and the Csca is a mean effective cross section of particles for light scattering including the phase effect (the scattering angles of present interest range from about 80 to 100 degrees). 相似文献
12.
Dust is a major environmental factor on the surface and in the atmosphere of Mars. Knowing the electrical charge state of this dust would be of both scientific interest and important for the safety of instruments on the Martian surface. In this study the first measurements have been performed of dust electrification using suspended Mars analogue material. This has been achieved by attracting suspended dust onto electrodes placed inside a Mars simulation wind tunnel. The Mars analogue used was from Salten Skov in Denmark, this contained a high concentration of ferric oxide precipitate. Once suspended, this dust was found to consist of almost equal quantities of negatively (46±6%) and positively (44±15%) charged grains.These grains were estimated to typically carry a net charge of around 105e, this is sufficient to dominate the processes of adhesion and cohesion of this suspended dust. Evidence is presented for electrostatic aggregation of the dust while in suspension. Development of a simple instrument for measuring electrical charging of the suspended dust on Mars will be discussed. 相似文献
13.
Observations of sungrazing comets, all of which belong to the Kreutz family, provide the opportunity of studying the properties of dust in the comae and tails of the comets. On the basis of available information on cometary and interplanetary dust as well as observations of dust in the tails of sungrazers, we model dust in sungrazing comets as fluffy silicate aggregates of submicrometer sizes. To better interpret observational data, we numerically calculate the solar radiation pressure, the equilibrium temperature, and the sublimation and crystallization rates of silicate grains near the Sun. Our results show that the dust tails contain aggregates of submicrometer crystal grains, but not amorphous grains, since amorphous silicates mostly crystallize after release from the comets. The peak in the lightcurves of the dust comae observed either at 11.2 or 12.3 solar radii (R⊙) seems to result from sublimation of fluffy aggregates consisting of crystalline or amorphous olivines, respectively. We attribute an additional enhancement in the lightcurves inside 7 R⊙ to increasing out-flow of crystalline and amorphous pyroxenes composed fluffy aggregates. According to our model, the observed lightcurves indicate a high abundance of olivine and a low abundance of pyroxene in the comets, which may bear implications about the dynamical and thermal history of the sungrazers and their progenitor. 相似文献
14.
We study the stability of charged dust grains orbiting a planet and subject to gravity and the electromagnetic force. Our numerical models cover a broad range of launch distances from the planetary surface to beyond synchronous orbit, and the full range of charge-to-mass ratios from ions to rocks. Treating the spinning planetary magnetic field as an aligned dipole, we map regions of radial and vertical instability where dust grains are driven to escape or crash into the planet. We derive the boundaries between stable and unstable trajectories analytically, and apply our models to Jupiter, Saturn and the Earth, whose magnetic fields are reasonably well represented by aligned dipoles. 相似文献
15.
Two-deuteron radiative capture reaction is one of the deuterium-burning processes that the cross section is not well known at very low-energies. We develop the formalism based on pionless Effective Field Theory (EFT) for deuteron–deuteron (dd) scattering to derive the cross section of two-deuteron radiative capture. Within this formalism the two- and three-body forces, should also be included in the capture cross section calculations. The aim of the present work is to calculate the observables of the low-energy dd photonuclear reactions. The cross section and the astrophysical S-factor for the 2H(d,γ)4He reaction have been calculated. The 4He is studied as four-body bound states in the pionless EFT. The theoretical uncertainties for observables are estimated based on the variation of the cutoffs. The astrophysical S-factor is accurately determined to be 6.9×10?6 keV?b (6.1×10?6 keV?b) for two-body (two- and three-body) at zero energy. A satisfactory agreement between theory and experiment for dd radiative capture observables up to order of calculation has been found by insertion of three-body force. 相似文献
16.
J. I. Davies P. Alton S. Bianchi & M. Trewhella 《Monthly notices of the Royal Astronomical Society》1998,300(4):1006-1014
We have constructed a numerical model of a galaxy that consists of a stellar, gas and dust disc imbedded within a dark halo. We have used this model to assess the radiation, gravitational and viscous forces on dust grains and to trace their motion through the interstellar medium over a period of 109 yr. We conclude that the disc opacity is a crucial factor in understanding the motion of the grains. Large grains (≈0.1 μm) with low disc opacity will lead to dust expulsion from the stellar disc, while high opacity leads to dust retention. Reasonable disc opacities lead to the recycling of the larger grains from the outer to the inner regions of the galaxy. The larger grains travel at higher velocities than small grains (0.01−0.001 μm), and so the smaller grains remain relatively close to their formation sites. Dust can 'leak' out over the entire surface of the disc because of the imbalance of radiation and gravitational forces. The dust is dynamically coupled to the gas and so although the gas lags behind the dust it is carried along with it. This explains the close correlation between the far-infrared emission from dust and the gas column density. We use a simple analytical model to show how the dust mass of a galaxy may evolve with time and how a significant fraction (90 per cent) of the total dust mass produced may have been expelled into the intergalactic medium. 相似文献
17.
Persistent electrostatic charge is a characteristic feature in lunar soil and results in cohesion forces between the grains ranging in the order of 102 dyn. We suggest that this phenomenon is responsible for the initial aggregation of grains in the formative era of the solar system. The effect would become important when the relative velocity between particles at grazing incidence or collision has been reduced to order of 102 m s–1. The electrostatic adhesion mechanism would also explain the suggestion from optical polarization measurements that small asteroids accumulate silicate dust.Electrostatic accretion and bonding would consequently appear to be a process of basic importance in the solar system, both in the formative era and today.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. 相似文献
18.
A. T. Kearsley M. J. Burchell M. C. Price G. A. Graham P. J. Wozniakiewicz M. J. Cole N. J. Foster N. Teslich 《Meteoritics & planetary science》2009,44(10):1489-1509
Abstract— New experimental results show that Stardust crater morphology is consistent with interpretation of many larger Wild 2 dust grains being aggregates, albeit most of low porosity and therefore relatively high density. The majority of large Stardust grains (i.e. those carrying most of the cometary dust mass) probably had density of 2.4 g cm?3 (similar to soda‐lime glass used in earlier calibration experiments) or greater, and porosity of 25% or less, akin to consolidated carbonaceous chondrite meteorites, and much lower than the 80% suggested for fractal dust aggregates. Although better size calibration is required for interpretation of the very smallest impacting grains, we suggest that aggregates could have dense components dominated by μm‐scale and smaller sub‐grains. If porosity of the Wild 2 nucleus is high, with similar bulk density to other comets, much of the pore space may be at a scale of tens of micrometers, between coarser, denser grains. Successful demonstration of aggregate projectile impacts in the laboratory now opens the possibility of experiments to further constrain the conditions for creation of bulbous (Type C) tracks in aerogel, which we have observed in recent shots. We are also using mixed mineral aggregates to document differential survival of pristine composition and crystalline structure in diverse finegrained components of aggregate cometary dust analogues, impacted onto both foil and aerogel under Stardust encounter conditions. 相似文献
19.
The formation of planetesimals in the early Solar System is hardly understood, and in particular the growth of dust aggregates above millimeter sizes has recently turned out to be a difficult task in our understanding (Zsom, A., Ormel, C.W., Güttler, C., Blum, J., Dullemond, C.P. [2010]. Astron. Astrophys., 513, A57). Laboratory experiments have shown that dust aggregates of these sizes stick to one another only at unreasonably low velocities. However, in the protoplanetary disk, millimeter-sized particles are known to have been ubiquitous. One can find relics of them in the form of solid chondrules as the main constituent of chondrites. Most of these chondrules were found to feature a fine-grained rim, which is hypothesized to have formed from accreting dust grains in the solar nebula. To study the influence of these dust-coated chondrules on the formation of chondrites and possibly planetesimals, we conducted collision experiments between millimeter-sized, dust-coated chondrule analogs at velocities of a few cm s?1. For 2 and 3 mm diameter chondrule analogs covered by dusty rims of a volume filling factor of 0.18 and 0.35–0.58, we found sticking velocities of a few cm s?1. This velocity is higher than the sticking velocity of dust aggregates of the same size. We therefore conclude that chondrules may be an important step towards a deeper understanding of the collisional growth of larger bodies. Moreover, we analyzed the collision behavior in an ensemble of dust aggregates and non-coated chondrule analogs. While neither the dust aggregates nor the solid chondrule analogs show sticking in collisions among their species, we found an enhanced sicking efficiency in collisions between the two constituents, which leads us to the conjecture that chondrules might act as “catalyzers” for the growth of larger bodies in the young Solar System. 相似文献
20.
L. C. Garcia de Andrade 《Astrophysics and Space Science》2010,330(2):347-351
Magneto-curvature stresses could deform magnetic field lines giving rise to back reaction and restoring magnetic stresses (Tsagas in Phys. Rev. Lett., 2001). Barrow and Tsagas (Phys. Rev. D, 2008) have shown that in Friedman universe the expansion slows down in its spatial section of negative Riemann curvature. Earlier, Chicone and Latushkin (Proc. Am. Math. Soc. 125(11):3391, 1995) proved that fast dynamos in compact 2D manifold implies negatively constant Riemannian curvature. Here one applies the Barrow-Tsagas ideas to cosmic dynamos of negative curvature. Fast dynamo, covariant stretching of Riemann slices of cosmic Lobachevsky plane is given. Inclusion of advection term on dynamo equations (Clarkson and Marklund in Mon. Not. R. Astron. Soc., 2005) is considered. In advection absence, slow dynamos are also obtained. It is shown the viscous and restoring forces on stretching particles decrease, as magnetic rates increase. From COBE data ( $\frac{{\delta}B}{B}\approx{10^{-5}}$ ), one is able to compute the stretching $\frac{{\delta}V^{y}}{V^{y}}=1.5\frac{{\delta}B}{B}\approx{1.5{\times}10^{-5}}$ . Zeldovich et al. have computed the maximum magnetic growth rate as γ max ≈8.0×10?1 t ?1. From COBE data a lower growth rate as γ COBE ≈6.0×10?6 t ?1, is well-within Zeldovich et al estimate. Instead of Harrison value $B\approx{t^{\frac{4}{3}}}$ one obtains a lower primordial field B≈10?6 t which yields B≈10?6 G at 1 s Big Bang time. 相似文献