共查询到20条相似文献，搜索用时 38 毫秒
1.
The suggestion that the Moon's magnetic field is due to adiabatic magnetohydrodynamic convection of a molten core has been made by a number of recent authors. Considerations based on petrology, mass and rotational inertia limit the size of this hypothetical core to a few 100 km at the most. A proposal has been made that this core is either molten iron or iron sulfide. Fortunately, we know the properties of both molten iron and iron-sulfide at lunar core pressures. We can find no way of maintaining circulation in a hypothetical lunar core, as circulation is contingent upon a temperature gradient being greater than the adiabatic gradient, or an internal heat source. 相似文献
2.
The mineralogical composition of grains produced in supernova ejecta is explored via chemical equilibrium condensation computations. These calculations are carried out for chemical compositions characteristic of each of several supernova zones, taking into account the pressure decrease due to adiabatic expansion and condensation. The distributions of the major elements among the various gaseous species and solid phases are graphically displayed. These computations reveal that many of the major condensates from supernova ejecta are also stable against evaporation in a gas of solar composition at high temperatures. This is especially true for minerals containing the elements O, Mg, Al, Si, Ca, Fe and Ti. Grains which form in supernova ejecta are less likely to become homogenized with solar nebular gas than SN gas and are thus potential sources of exotic isotopic compositions in the early solar system. The calculated elemental distributions of supernova condensates are applied to problems concerning isotopic anomalies and large mass-dependent isotopic fractionations discovered in the meteorite Allende. The order in which the major elements become totally condensed is found to be nearly independent of the supernova zone considered, being the same as that for a solar gas. The consequence of this may be that some of the observed depletions of heavy elements in the interstellar gas are due to supernova-produced dust.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
3.
A theoretical investigation has been made of propagating electrostatic waves in a four-component adiabatic dusty plasma, whose
constituents are adiabatic electrons, adiabatic ions, adiabatic positively and as well as negatively charged warm dust. The
basic features of the solitary structures in such a four-component adiabatic dusty plasma are studied by the reductive perturbation
method. It is found that the presence of the positive dust component does not only significantly modify the basic properties
of the solitary waves, but also causes the existence of the positive solitary potential structures, which is an interesting
feature shown in an adiabatic dusty plasma with the dust of opposite polarity. It is also observed that the basic properties
(polarity, speed, amplitude and width) of the DA SWs are significantly modified by the effects of adiabaticity (γ>1) of electrons, ions, negatively as well as positively charged warm dust. The present investigation can be of relevance
to the electrostatic solitary structures observed in various dusty space plasma environments (viz. cometary tails, upper mesosphere,
Jupiter’s magnetosphere, etc.). 相似文献
4.
H. Alinejad 《Astrophysics and Space Science》2011,334(2):331-336
The properties of arbitrary amplitude dust ion-acoustic (DIA) solitary waves (SWs) in a dusty plasma containing warm adiabatic ions, electrons following flat-topped velocity distribution, and arbitrarily (positively or negatively) charged immobile dust is studied by the pseudo-potential approach. The effects of ion temperature, resonant electrons, and dust number density are found to significantly modify the basic features of the DIA-SWs as well modify the parametric regime for the existence of compressive DIA-SWs. The pseudo-potential for small but finite amplitude limit is also analytically analyzed. 相似文献
5.
First results are presented of a calculation describing the collapse of a rotating dusty protostellar cloud. The dust and gas components are calculated separately, although their interaction (e.g., radiation transport, friction, etc.) is taken into account. In the early stages of the collapse the dust is dynamically unimportant. The evolution of the dust cloud is strongly influenced by dust-dust collisions: rapid sedimentation into an equatorial dust disc is found to take place as a result of accumulative dust-dust collisions and the corresponding grain growth. Treating the dust separately from the gas allows us to compare our results with solar-system cosmochemical measurements, with celestial mechanics information and to draw conclusions about the time and place of planet formation in the collapsing cloud.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
6.
A theory is presented for the dynamics of dust particles in an incompressible turbulent fluid. Grain-gas coupling occurs through friction forces that are proportional to the mean grain velocity relative to the gas. This test particle theory is applied to the case of a Kolmogoroff spectrum in a protostellar cloud. The mean turbulence induced grain velocity and the mean turbulent relative velocity of two grains are calculated. Whereas the former should determine the dust scale height, grain-grain collisions are influenced by the latter. For a resonable strength of the turbulence, the mean induced relative velocity of two particles turns out to be at least as large as the corresponding terminal velocity difference during gravitational settling.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
7.
E. I. Robson L. L. Leeuw J. A. Stevens & W. S. Holland 《Monthly notices of the Royal Astronomical Society》1998,301(4):935-940
We present millimetre photometry and submillimetre imaging of the central core and two hotspots in the radio lobes of the galaxy Cygnus A. For both hotspots and the central core, the synchrotron spectrum continues smoothly from the radio to a frequency of 677 GHz. The spectral index of the hotspots is constant over our frequency range, with a spectral index of α ≈ −1.0 ( S ν ∝ να ), which is steeper than at lower frequencies and represents the emission from an aged population of electrons. The core is significantly flatter, with α = −0.6 ± 0.1, suggestive of an injected spectrum with no ageing, but some evidence for steepening exists at our highest observing frequency. Although IRAS data suggest the presence of dust in Cygnus A, our 450-μm data show no evidence of cold dust, therefore the dust component must have a temperature lying between 85 and 37 K, corresponding to dust masses of 1.4 × 106 and 1.0 × 108 M⊙ respectively. 相似文献
8.
The Zakharov-Kuznetsov (ZK) equation is derived for electrostatic wave in a rotating magnetoplasma with anisotropic ion pressure and in the presence of stationary charged dust particles. The anisotropic ion pressure is defined using double adiabatic Chew-Golberger-Low (CGL) theory. The reductive perturbation method is employed to study the dynamics of obliquely propagating low frequency ion acoustic wave with adiabatic ions. It is found that the ion pressure anisotropy, polarity, density of the dust particles and rotational frequency have significant effects on the formation nonlinear structures in rotating magnetized dusty plasmas. The numerical results are also presented for illustration. 相似文献
9.
Judith A. Irwin J.M. Stil T.J. Bridges 《Monthly notices of the Royal Astronomical Society》2001,328(2):359-369
Deep SCUBA observations of NGC 1275 at 450 and 850 μm along with the application of deconvolution algorithms have permitted us to separate the strong core emission in this galaxy from the fainter extended emission around it. The core has a steep spectral index and is likely caused primarily by the active galactic nucleus. The faint emission has a positive spectral index and is clearly caused by extended dust in a patchy distribution out to a radius of ∼20 kpc from the nucleus. These observations have now revealed that a large quantity of dust, ∼ (two orders of magnitude larger than that inferred from previous optical absorption measurements), exists in this galaxy. We estimate the temperature of this dust to be ∼20 K (using an emissivity index of and the gas/dust ratio to be 360. These values are typical of spiral galaxies. The dust emission correlates spatially with the hot X-ray emitting gas, which may be a result of collisional heating of broadly distributed dust by electrons. As the destruction time-scale is short, the dust cannot be replenished by stellar mass loss and must be externally supplied, via either the infalling galaxy or the cooling flow itself. 相似文献
10.
The insulating effect of an evolving dust mantle is examined. The role of this mantle in determining the surface temperature of the ice core is studied as a function of the mass fraction of the dust in the ice-dust mixture and the thermal conductivity of the nucleus. Using the so-called “looselattice” model of D.A. Mendis and G.D. Brin (1977, Moon17, 359–372) (which was also extended to include cracks and pores in the mantle), it was found that both high dust to ice ratios and high core conductivities inhibit mantle blowoff. Indeed, it is often possible to build an essentially permanent dust mantle around an ice nucleus, so that the nucleus will take on an asteroidal appearance. 相似文献
11.
Karl-Heinz A. Winkler 《Earth, Moon, and Planets》1978,19(2):237-244
A consistent hydrodynamical and radiative transfer calculation in spherical symmetry for a 1M
protostar is presented. The calculation starts with Larson's initial conditions and continues until almost all the material has fallen onto a hydrostatic core with a large outer convection zone. The innermost percent of the mass is partially degenerate. Due to the numerical technique used, the radius of the hydrostatic core is determined with a high degree of accuracy.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978. 相似文献
12.
M. E. L. Hopwood A. Evans A. Penny & S. P. S. Eyres 《Monthly notices of the Royal Astronomical Society》1998,301(2):30L-32L
We report the 3.5σ detection of dust in the core of the metal-rich ([Fe/H] =−0.4) globular cluster NGC 6356. The dust mass in the core is ∼ 4–17 × 10−3 M⊙, depending on the dust equilibrium temperature. We rule out sputtering as a grain destruction mechanism in NGC 6356, unless grains are very small (∼30 Å). We also place the upper limit of 6.6 × 10−3 on the CO-to-dust ratio in this cluster. This value is significantly smaller than the CO-to-dust ratio in the general Galactic interstellar medium, and in the outflows of evolved stars. 相似文献
13.
Heat flowing out of the core must flow into the mantle. If the Earth's magnetic field is owing to adiabatic magnetohydrodynamic circulation of the outer core, whole mantle convection or melting at the core mantle boundary is required to keep the inner core from becoming isothermal, thereby preventing adiabatic circulation.Alternatively, the outer core fluid must have some unexpected and exotic property such as an extremely low coefficient of thermal expansion and resultant low Gruneisen's parameter.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. Publication No. 1046 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif., U.S.A. 相似文献
14.
H. Alinejad 《Astrophysics and Space Science》2012,339(2):249-254
The effects of dust polarity and superthermal electrons are incorporated in the study of dust ion-acoustic (DIA) solitary
waves (SWs) as well double layers (DLs) in a dusty plasma containing warm adiabatic ions, superthermal electrons, and arbitrarily
(positively or negatively) charged immobile dust. Based on the energy-like integral equation, a new relationship between the
localized electrostatic disturbances and dust polarity is derived. It is shown that there exists rarefactive SWs and DLs with
qualitatively different structures in a way that depends on the population of superthermal electrons. As the electrons evolve
their thermodynamic equilibrium, the localized structures are found with larger amplitude. It is also found that their amplitude
increases (decreases) with the increase in the negative (positive) dust number density. 相似文献
15.
Recent probes of the planet Venus reveal a probable surface temperature exceeding 700K and a pressure exceeding 100 atm. A very dusty lower atmosphere may exist which is composed of micron-sized particles kept airborne by mild turbulence and a gentle circulation of deep adiabatic currents. A study of surface conditions responsible for generation and persistence of surface dust clouds is of fundamental importance in the radiative and dynamic properties of the atmosphere. Also spurious radar echoes may be caused by suspended particulate matter, thus explaining the high relief reported by radar altimeters.Equations describing transportation and deposition of dust and sand have been solved for the surface conditions of Venus. It is concluded that the minimum wind velocity for initiating grain movement is about one order of magnitude smaller than on Earth. In addition, this minimum wind velocity occurs for smaller particles on Venus than on Earth. Once the particles are raised, they can be maintained aloft for longer periods of time and over a larger size range on Venus.Surface structures such as ripples evolved from aeolian deposition are likely to be of smaller vertical dimensions but larger horizontally when compared with equivalent structures on Earth. 相似文献
16.
Bart J. Bok 《Earth, Moon, and Planets》1978,19(2):153-155
The principal properties of dark globules are reviewed. Optical and radio (CO) data suggest masses in the range 20 to 750M
, radii 0.15 to 1.2 parsecs. There is no evidence for fragmentation. The dust density inside near-spherical globules varies inversely as the cube of the distance from the center. There is evidence to show that several globules are in a state of collapse.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.Much of the work was done while the author was a Visiting Scientist at Cerro Tololo Interamerican Observatory in Chile, supported by the National Science Foundation under Contract No. NSF-C866. 相似文献
17.
Antonio C. da Silva rea Catalano Ludovic Montier Etienne Pointecouteau Joseph Lanoux Martin Giard 《Monthly notices of the Royal Astronomical Society》2009,396(2):849-859
We investigate the effect of dust on the scaling properties of galaxy clusters based on hydrodynamic N -body simulations of structure formation. We have simulated five dust models plus radiative cooling and adiabatic models using the same initial conditions for all runs. The numerical implementation of dust was based on the analytical computations of Montier & Giard. We set up dust simulations to cover different combinations of dust parameters that make evident the effects of size and abundance of dust grains. Comparing our radiative plus dust cooling runs with a purely radiative cooling simulation, we find that dust has an impact on cluster scaling relations. It mainly affects the normalization of the scalings (and their evolution), whereas it introduces no significant differences in their slopes. The strength of the effect critically depends on the dust abundance and grain size parameters as well as on the cluster scaling. Indeed, cooling due to dust is effective in the cluster regime and has a stronger effect on the 'baryon driven' statistical properties of clusters such as L X – M , Y – M , S – M scaling relations. Major differences, relative to the radiative cooling model, are as high as 25 per cent for the L X – M normalization, and about 10 per cent for the Y – M and S – M normalizations at redshift zero. On the other hand, we find that dust has almost no impact on the 'dark matter driven' T mw – M scaling relation. The effects are found to be dependent in equal parts on both dust abundances and grain size distributions for the scalings investigated in this paper. Higher dust abundances and smaller grain sizes cause larger departures from the radiative cooling (i.e. with no dust) model. 相似文献
18.
Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ～ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ～ 10?5 g/cm2/sec. 相似文献
19.
Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core-mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present. 相似文献
20.
Jayanta Kar 《Astrophysics and Space Science》1987,133(1):29-44
The unrestricted second-order virial tensor formalism has been used to calculate the characteristic frequencies of linear adiabatic oscillations of a composite stellar model having an isothermal core and a polytropic envelope in presence of a weak poloidal magnetic field. The frequencies of the transverse shear mode and the nonradial pulsation mode for both a radiative and a convective envelope (corresponding to polytropic index 3 and 1.5, respectively) alongwith that of the toroidal mode for the radiative envelope get increased in presence of the magnetic field. However, the frequency of the toroidal mode for the convective envelope registers a decrease in presence of the field. The corrections to the various frequencies decrease with increasing values of the parameter characterizing the lowering of the core temperature in presence of the magnetic field. 相似文献