共查询到20条相似文献，搜索用时 32 毫秒
1.
S.J. Weidenschilling 《Icarus》1974,22(4):426-435
One possible origin of the terrestrial planets involves their formation by gravitational accretion of particles originally in Keplerian orbits about the sun. Some implications of this theory are considered. A formal expression for the rate of mass accretion by a planet is developed. The formal singularity of the gravitational collision cross-section for low relative velocities is shown to be without physical significance when the accreting bodies are in heliocentric orbits. The distribution of particle velocities relative to an accreting planet is considered; the mean velocity increases with time. The internal temperature of an accreting planet is shown to depend simply on the accretion rate. A simple and physically reasonable approximate expression for a planetary accretion rate is proposed. 相似文献
2.
Blowing in the wind: I. Velocities of chondrule-sized particles in a turbulent protoplanetary nebula
Small but macroscopic particles—chondrules, higher temperature mineral inclusions, metal grains, and their like—dominate the fabric of primitive meteorites. The properties of these constituents, and their relationship to the fine dust grains which surround them, suggest that they led an extended existence in a gaseous protoplanetary nebula prior to their incorporation into their parent primitive bodies. In this paper we explore in some detail the velocities acquired by such particles in a turbulent nebula. We treat velocities in inertial space (relevant to diffusion), velocities relative to the gas and entrained microscopic dust (relevant to accretion of dust rims), and velocities relative to each other (relevant to collisions). We extend previous work by presenting explicit, closed-form solutions for the magnitude and size dependence of these velocities in this important particle size regime, and we compare these expressions with new numerical calculations. The magnitude and size dependence of these velocities have immediate applications to chondrule and CAI rimming by fine dust and to their diffusion in the nebula, which we explore separately. 相似文献
3.
The formation of massive galaxies at the centre of a cluster is discussed here. The protogalaxies move with both rotation and random velocities through the gaseous medium pervading the cluster. Each galaxy is supposed to move through a resisting medium under the general gravitational field produced by the cluster as a whole. Also, the mass of the galaxy increases by accretion all the time as it moves through the medium. Using plausible laws for density of the medium and accretion of matter and solving equation of motion, we find that the galaxy loses angular momentum in the course of the time. The loss of angular momentum drives the galaxy towards the centre of the cluster. Thus over a sufficiently long time-scale several galaxies may merge in the central region of the cluster resulting in a single massive galaxy. The process can drive rise to several massive galaxies in the central region of the cluster. 相似文献
4.
We present a model that relates the width of the broad emission lines of active galactic nuclei (AGNs) to the Keplerian velocity of an accretion disk at a critical distance from the central black hole. This critical distance falls in a region bounded on the inward side by the transition radius between the radiation pressure- and the gas pressure-dominated region of the accretion disk and on the outward side by the maximum radius below which a stabilizing, radially accreting and vertically outflowing corona exists. We show that in the framework of this picture, the observed range of Hbeta FWHMs from broad-line to narrow-line type 1 AGNs is well reproduced as a function of the accretion rate. This interval of velocities is the only permitted range and goes from approximately 20,000 km s-1 for sub-Eddington accretion rates to approximately 1000 km s-1 for Eddington accretion rates. 相似文献
5.
Richard Greenberg 《Earth, Moon, and Planets》1980,22(1):63-66
Safronov's (1972) demonstration that relative velocities of planetesimals would be comparable to the dominant size bodies' escape velocities, combined with a plausible size distribution that has most mass in the largest bodies, yielded his evolution model with limited growth of the largest planetesimal with respect to its next largest neighbors. A numerical simulation of planetesimal accretion (Greenberget al., 1978) suggests that at least over one stage of collisional accretion, velocities were much lower than the escape velocity of the largest bodies, because the bulk of the mass still resided in km-scale bodies. The low velocities at this early stage may conceivably have permitted early runaway growth, which, in turn, would have kept the velocities low and permitted continued runaway growth of the largest bodies.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy. 相似文献
6.
S.J. Weidenschilling 《Icarus》1976,27(1):161-170
Some aspects and consequences of the theory of gravitational accretion of the terrestrial planets are examined. The concept of a “closed feeding zone” is somewhat unrealistic, but provides a lower bound on the accretion time. Safronov's relative velocity relation for planetesimals is not entirely consistent with the feeding zone model. A velocity relation which includes an initial velocity component is suggested. The orbital parameters of the planetesimals and the dimensions of the feeding zone are related to their relative velocities. The assumption of an initial velocity does not seriously change the accretion time.Mercury, Venus, and the Earth have accretion times on the order of 108yr. Mars requires well over 109yr to accrete by the same assumptions. Currently available data do not rule out a late formation of Mars, but the lunar cratering history makes it unlikely. If Mars is as old as the Earth, nongravitational forces or a violation of the feeding zone concept is required. One such possibility is the removal of matter from the zone of Mars by Jupiter's influence. The final sweeping up by Mars after this event would result in the scattering of a considerable mass among the other terrestrial planets. The late postaccretional bombardments infrerred for the Moon and Mercury may have had this source. 相似文献
7.
8.
Amitabha Ghosh Stuart J. Weidenschilling Harry Y. McSween 《Meteoritics & planetary science》2003,38(5):711-724
Abstract— Widespread evidence exists for heating that caused melting, thermal metamorphism, and aqueous alteration in meteorite parent bodies. Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high‐grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories. 相似文献
9.
G. P. Horedt 《Earth, Moon, and Planets》1979,21(1):63-121
In Sections 1–6, we determine an approximate analytical model for the density and temperature distribution in the protoplanetary could. The rotation of the planets is discussed in Section 7 and we conclude that it cannot be determined from simple energy conservation laws.The velocity of the gas of the protoplanetary cloud is found to be smaller by about 5×103 cm s–1 in comparison to the Keplerian circular velocity. If the radius of the planetesimals is smaller than a certain limitr
1, they move together with the gas. Their vertical and horizontal motion for this case is studied in Sections 8 and 9.As the planetesimals grow by accretion their radius becomes larger thanr
1 and they move in Keplerian orbits. As long as their radius is betweenr
1 and a certain limitr
2 their gravitational interaction is negligible. In Section 10, we study the accretion for this case.In Section 11, we determine the change of the relative velocities due to close gravitational encounters. The principal equations governing the late stages of accretion are deduced in Section 12, In Section 13 there are obtained approximate analytical solutions.The effect of gas drag and of collisions is studied in Sections 14 and 15, respectively. Numerical results and conclusions concerning the last and principal stage of accretion are drawn in Section 16. 相似文献
10.
T.A. Heppenheimer 《Icarus》1974,22(4):436-447
A theory is presented for determining regions where planets may form in binary star systems. Planet formation by accretion is assumed possible if mean planetesimal collision velocities do not exceed a critical value. Collision velocities are increased by perturbations due to the companion star, treated by secular perturbation theory. Collision velocities are damped by aerodynamic drag within the solar nebula, taken as the linear case of Cameron and Pine.A general feature of planetary systems in binary stars is the existence of two zones. The inner zone has enough damping to permit unimpeded growth by accretion; in the outer zone, growth proceeds to a limited diameter, beyond which damping is insufficient. It is shown that the asteroids could not have failed to coalesce due to Jupiter perturbations in the primitive solar nebula. Binary star systems with semimajor axis < 30AU are not likely to have planets; these include Alpha Centauri and 70 Ophiuchi. Systems possibly possessing planets include Eta Cassiopeiae, 40 Eridani, and Σ 2398. Epsilon Eridani is a marginal case. 相似文献
11.
Safronov's statement that relative velocities of planetesimals are on the order of the escape velocity of the largest body of the population is shown to be correct only when a major part of the total mass resides in several large bodies. In the first stage of accumulation, runaway accretion produces large bodies separated by mass form the remaining population. At this stage, relative velocities of planetesimals are much smaller than those adopted earlier. This requires a modification of Schmidt's scheme of accumulation of the Earth and other terrestrial planets from material in their feeding zones. This also leads to removal of the author's arguments (Levin 1972c) in favor of a protoplanetary nebula with an extended, massive periphery.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.
Kazem Faghei 《Astrophysics and Space Science》2013,345(1):125-132
The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A? and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction. 相似文献
13.
Results from simultaneous spectral and photometric monitoring of the Herbig Ae star CQ Tau in the neighborhood of the Hα and
resonance sodium doublet Na I D lines are presented. It is shown that the inner structure of the accretion disk of CQ Tau
is nonuniform and consists of two regions with quite different kinematic characteristics. Region I is characterized by relative
stability and a smooth long-term variation in the velocity of the gas along the line of sight. Region II is distinguished
by the highest velocities and a variability in their maximum values over time scales from a few days to 700 days. The dust
clouds which produse the star’s brightness minima may also be the source of cold gas and contribute to the observed spectral
variability. We assume that region I of the disk coincides with the accretion disk of the star. The kinematic differences
in region II may be caused by dissipation of circumstellar dust clouds which, moving in elongated orbits, are able to approach
the star quite closely.
__________
Translated from Astrofizika, Vol. 50, No. 1, pp. 39–55 (February 2007). 相似文献
14.
Early evolution of trans-Neptunian objects,commonly known as Kuiper Belt objects (KBOs),is the result of heating due to radioactive decay, the most important sourcebeing 26Al. Several studiesare reviewed, dealing with the long-termevolution of KBO models, calculatedby means of 1-D numerical codesthat solve the heat and mass balanceequations on a fixed spherically symmetric grid. It is shown that, depending on parameters, the interior may reachquite high temperatures. The modelsthus suggest that KBOs are likely to lose the ices of very volatile species during early evolution; ices of less volatile species are retained in the cold subsurface layer. As the initially amorphous ice isshown to crystallize in the interior, some objects may also lose part of the volatiles trapped in amorphous ice. Generally, the outer layers are far less affected than the inner part, resulting in a stratified composition and altered porosity distribution. It is further shown that the thermal evolution of KBOs cannot be treated separately from their accretional evolution, as the processes occur in parallel. A systematic attempt to calculate accretion and thermal evolution simultaneously is presented, based on a numerical moving grid scheme. The accretion rate is obtained from the solution of the coupled coagulation equations for gravitationally interacting planetesimals. The effect of planetesimal velocities on the accretion scheme is included by a simplified equipartition argument. The time dependent accretion rates serve as input for the numerical solution of the heat transport equation for growing bodies mainly heated by radioactive decay of 26Al, allowing for phase transitions. Calculations carried out over the parameter space [heliocentric distance; final radius; ice fraction] lead to conclusions regarding the structure of KBOs, such as melt fraction, or extent of crystalline ice region. 相似文献
15.
Maria E. De Rossi Patricia B. Tissera Gabriella De Lucia † Guinevere Kauffmann 《Monthly notices of the Royal Astronomical Society》2009,395(1):210-217
We analyse a sample of 52 000 Milky Way (MW) type galaxies drawn from the publicly available galaxy catalogue of the Millennium Simulation with the aim of studying statistically the differences and similarities of their properties in comparison to our Galaxy. Model galaxies are chosen to lie in haloes with maximum circular velocities in the range 200–250 km s−1 and to have bulge-to-disc ratios similar to that of the MW. We find that model MW galaxies formed 'quietly' through the accretion of cold gas and small satellite systems. Only ≈12 per cent of our model galaxies experienced a major merger during their lifetime. Most of the stars formed ' in situ ', with only about 15 per cent of the final mass gathered through accretion. Supernovae (SNe) and active galactic nuclei (AGN) feedback play an important role in the evolution of these systems. At high redshifts, when the potential wells of the MW progenitors are shallower, winds driven by SNe explosions blow out a large fraction of the gas and metals. As the systems grow in mass, SNe feedback effects decrease and AGN feedback takes over, playing a more important role in the regulation of the star formation activity at lower redshifts. Although model MW galaxies have been selected to lie in a narrow range of maximum circular velocities, they nevertheless exhibit a significant dispersion in the final stellar masses and metallicities. Our analysis suggests that this dispersion results from the different accretion histories of the parent dark matter haloes. Statistically, we also find evidences to support the MW as a typical Sb/Sc galaxy in the same mass range, providing a suitable benchmark to constrain numerical models of galaxy formation. 相似文献
16.
17.
We present evidence for cosmological gas accretion on to spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extraplanar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extraplanar gas is produced by supernova feedback (galactic fountain) and only 10–20 per cent comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extraplanar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona. 相似文献
18.
W. Kundt 《Astrophysics and Space Science》1990,172(2):285-291
Magnetic tensions are likely to be the dominant shear force in accretion disks, larger when integrated than turbulent viscosity by an order of magnitude or more. In galactic disks, they guarantee the mass-accretion rate required by the quasar phenomenon. In fast-revolving, clumpy disks, magnetic pressures can exceed static pressures and be amplified towards ram pressures. The inner, near-rigidly rotating parts of galactic disks are suggestive candidates. The gas velocities in such magnetically controlled disks mimic higher central masses than present. 相似文献
19.
Repeated explosions in the nuclei of galaxies are now accepted as observationally established phenomena. Each explosion leads to the ejection of gas from the central region of a galaxy with velocities depending on the strength of the explosive event. In the process the nucleus temporarily becomes gas-deficient. It is suggested that the mass los is replenished by the accretion of the mass which is shed by those evolved stars in the galactic bulge that possess relatively low rotational velocities. The gas to be accreted is assumed to be magnetized. In the present model, the accretion rate has been assumed to be a function of both radial distance and time. The cross-radial equation of motion has been solved to derive the expression for the rotational velocity which is found to bealmost linear with the radial distance from the centre. The radial equation has been solved to calculate the time-scale over which the nucleus accumulates sufficient mass to undergo instability and suffer explosion. The calculated time-scale range from few multiples of 107 to a few multiples of 108 yr. This range agrees very well with that as has been suggested on the basis of observation in the case of our own Galaxy. 相似文献
20.
A. Merloni T. Di Matteo † A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2000,318(1):L15-L19
The simultaneous presence of a strong quasi-periodic oscillation, of period ∼10 s, in the optical and X-ray light curves of the X-ray transient XTE J1118+480 suggests that a significant fraction of the optical flux originates from the inner part of the accretion flow, where most of the X-rays are produced. We present a model of magnetic flares in an accretion disc corona where thermal cyclo-synchrotron emission contributes significantly to the optical emission, while the X-rays are produced by inverse Compton scattering of the soft photons produced by dissipation in the underlying disc and by the synchrotron process itself. Given the observational constraints, we estimate the values for the coronal temperature, optical depth and magnetic field intensity, as well as the accretion rate for the source. Within our model we predict a correlation between optical and hard X-ray variability and an anticorrelation between optical and soft X-rays. We also expect optical variability on flaring time-scales (∼tens of ms), with a power-density spectrum similar to that observed in the X-ray band. Finally, we use both the available optical/extreme-ultraviolet/X-ray spectral energy distribution and the low-frequency time variability to discuss limits on the inner radius of the optically thick disc. 相似文献