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1.
An earlier paper gave solutions for the mean time rates of change of orbital elements of satellite atoms in an exosphere influenced by solar radiation pressure. Each element was assumet to beahve independently. Here the instantaneous rates of change for three elements (e, ω, and θ = ω + Ω) are integrated simultaneously for the case of the inclination i = 0. The results (a) confirm the validity of using mean rates when the orbits are tightly bound to the planet and (b) serve as examples to be reproduced by the complicated numerical solutions required for arbitrary inclination. Strongly bound hydrogen atoms perturbed in Earth orbit by radiation pressure do not seem a likely cause of the geotail extending in the anti-Sun direction. Instead, radiation pressure wil cause those particles' orbits to form a broad fan-shaped tail and to deteriorate into the Earth's atmosphere. Whether loosely bound H atoms are plentiful enough to create the geotail depends on their source function versusr; that question is beyond the scope of this paper.  相似文献   

2.
This paper presents quantitative estimates of orbital lifetimes for small dust particles in the vicinity of a spherical asteroid orbiting the Sun on a circular orbit. In particular, the modeling accounts for gravitational interactions with the asteroid and the Sun and radiation pressure from the Sun. The analysis focuses on slow drift of particles in a state-space neighborhood of weakly unstable periodic orbits corresponding to the most tightly bound orbits about the asteroid. Through a Hamiltonian normal-form computation, Nekhoroshev-type estimates are derived that yield regions in state space of effective stability, that is, regions in which the grains would remain over significant periods of time in the absence of other depletion and replenishment mechanisms.  相似文献   

3.
We describe a novel N -body code designed for simulations of the central regions of galaxies containing massive black holes. The code incorporates Mikkola's 'algorithmic' chain regularization scheme including post-Newtonian terms up to PN2.5 order. Stars moving beyond the chain are advanced using a fourth-order integrator with forces computed on a GRAPE board. Performance tests confirm that the hybrid code achieves better energy conservation, in less elapsed time, than the standard scheme and that it reproduces the orbits of stars tightly bound to the black hole with high precision. The hybrid code is applied to two sample problems: the effect of finite- N gravitational fluctuations on the orbits of the S-stars, and inspiral of an intermediate-mass black hole into the Galactic Centre.  相似文献   

4.
We use an N -body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given present observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years – within the Sun's lifetime. During the interaction, there is a chance that the Sun will be pulled away from its present orbital radius and reside in an extended tidal tail. The likelihood for this outcome increases as the merger progresses, and there is a remote possibility that our Sun will be more tightly bound to Andromeda than to the Milky Way before the final merger. Eventually, after the merger has completed, the Sun is most likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late-forming elliptical galaxies.  相似文献   

5.
The population of binary asteroids numbers over 160 systems, and they can be found amongst near-Earth asteroids (NEAs), Main-Belt asteroids (MBAs), Jupiter Trojans, Centaurs and trans-Neptunian objects (TNOs). The discoveries have been made with space missions, radar observations, photometric lightcurves, and high resolution imaging from the ground and space. The properties of each population are widely different due to varying formation mechanisms and discovery techniques for each group. Future large-aperture telescopes will be capable of imaging both components for nearly all known systems and will drastically improve prospects for discovery of smaller and more tightly bound systems throughout the Solar System. The study of binary asteroids has provided valuable estimates on asteroid density and structure, a better understanding of the radiative YORP-effect, insights on catastrophic collisions, and may prove to be a key diagnostic for understanding the formation and evolution of the Kuiper Belt population.  相似文献   

6.
We calculate the disc and boundary layer luminosities for accreting rapidly rotating neutron stars with low magnetic fields in a fully general relativistic manner. Rotation increases the disc luminosity and decreases the boundary layer luminosity. A rapid rotation of the neutron star substantially modifies these quantities as compared with the static limit. For a neutron star rotating close to the centrifugal mass shed limit, the total luminosity has contribution only from the extended disc. For such maximal rotation rates, we find that well before the maximum stable gravitational mass configuration is reached, there exists a limiting central density, for which particles in the innermost stable orbit will be more tightly bound than those at the surface of the neutron star. We also calculate the angular velocity profiles of particles in Keplerian orbits around the rapidly rotating neutron star. The results are illustrated for a representative set of equation of state models of neutron star matter.  相似文献   

7.
A simple, idealized model for the rapid escape of a hydrogen thermosphere provides some quantitative estimates for the energy-limited flux of escaping particles. The model assumes that the atmosphere is “tightly bound” by the gravitational field at lower altitudes, that diffusion through the lower atmosphere does not limit the flux, and that the main source of heating is solar euv. Rather low thermospheric temperatures are typical of such escape and a characteristic minimum develops in the temperature profile as the escape flux approaches its maximum possible value. The flux is limited by the amount of euv energy absorbed, which is in turn controlled by the radial extent of the thermosphere. Regardless of the amount of hydrogen in the thermosphere, the low temperatures accompanying rapid escape limit its extent, and thus constrain the flux. Applied to the Earth and Venus, the results suggest that the escape of hydrogen from these planets would have been energy-limited if their primordial atmospheres contained total hydrogen mixing ratios exceeding a few percent. Hydrogen and deuterium may have been lost in bulk, but heavier elements would have remained in the atmosphere. These results place constraints on hypotheses for the origin of the planets and their subsequent evolution.  相似文献   

8.
Anderson et al. [Phys. Rev. D 65 (2002) 082004] recently reported new evidence that both Pioneer 10 and 11 are experiencing nearly the same unmodeled anomalous acceleration directed toward the Sun. Numerous mechanisms, both internal and external to the spacecraft, have been proposed to explain this unmodeled acceleration. If we assume that the cause of the anomalous acceleration is (1) external to the spacecraft, (2) isotropic, and (3) acts on bodies of cometary mass, then it would imply that new comets are more tightly bound to the Sun than previously believed. Here we show that the implied higher binding energy is incompatible with the established evidence that the galactic tide is dominant in making Oort cloud comets observable. We conclude that one or more of these assumptions must be invalid.  相似文献   

9.
We present a set of rotational lightcurve measurements of the small main belt Asteroid 3169 Ostro. Our observations reveal an unambiguous, double-peaked rotational lightcurve with a peak-to-peak variation up to 1.2±0.05 mag and a synodic period of 6.509±0.001 h. From the large flux variation and the overall shape of the lightcurves, we suggest that 3169 Ostro could be a tightly bound binary or a contact binary, similar to the Trojan Asteroid 624 Hektor. A shape model of this system is proposed on the assumption that 3169 Ostro is a Roche binary described by a pair of homogeneous elongated bodies, with a size ratio of 0.87, in hydrostatic equilibrium and in circular synchronous motion around each other. The direction of the spin axis is determined modulo 180° by its J2000 ecliptic coordinates λ0=50±10°, β0=±54±5°. The binary interpretation and the pole solution adequately fit the earlier photometric observations made in 1986 and 1988. However, additional supporting lightcurves are highly desirable especially in the next mutual events occurrence of 2008 and 2009 in order to remove the pole ambiguity and to confirm unambiguously the binary nature of 3169 Ostro.  相似文献   

10.
根据球状星团动力学演化理论,本文探讨了球状星团致密度的演化与球状星用质量和位置的关系。结果表明,银心距和球状星团质量都与致密度的演化紧密相关。一般来说,银心距很大时致密度演化极少,球状星团质量愈大致密度演化愈缓慢。对现有球状星团致密度的分布作了统计研究,结果表明,在银心距较小的区域,致密度的分布与理论分析结果一致。在大银心距处发现,质量不同的球状星团其致密度分布有明显不同,它可能反映了球状星团形成阶段其致密度与质量紧密相关。  相似文献   

11.
According to the latest evidence, the Universe is entering an era of exponential expansion, where gravitationally bound structures will get disconnected from each other, forming isolated 'island universes'. In this scenario, we present a theoretical criterion to determine the boundaries of gravitationally bound structures and a physically motivated definition of superclusters as the largest bound structures in the Universe. We use the spherical collapse model self-consistently to obtain an analytical condition for the mean density enclosed by the last bound shell of the structure (2.36 times the critical density in the present Universe, assumed to be flat, with 30 per cent matter and 70 per cent cosmological constant, in agreement with the previous, numerical result of Chiueh & He). N -body simulations extended to the future show that this criterion, applied at the present cosmological epoch, defines a sphere that encloses ≈99.7 per cent of the particles that will remain bound to the structure at least until the scale parameter of the Universe is 100 times its present value. On the other hand, (28 ± 13) per cent of the enclosed particles are in fact not bound, so the enclosed mass overestimates the bound mass, in contrast with the previous, less rigorous criterion of, e.g. Busha and collaborators, which gave a more precise mass estimate. We also verify that the spherical collapse model estimate for the radial infall velocity of a shell enclosing a given mean density gives an accurate prediction for the velocity profile of infalling particles, down to very near the centre of the virialized core.  相似文献   

12.
This paper discusses the role played by the interstellar gas in spiral galaxies,using a two-disk model, one for the stars, one for the gas. The following conclusions are drawn. 1. When stars and gas have different velocity dispersions,the stellar and gaseous arm must separate. 2. Such a separation makes the spiral mode of density waves unstable. 3. The ratio η between the densities of gas and stars must be less than a certain value for density waves to be maintained.4.The smaller η is,the more tightly wound will be the arms.  相似文献   

13.
Using the extensive catalog of Binggelli, Sandage, and Tammann it is shown that dwarf galaxies in the Virgo cluster are more tightly connected with barred rather than with normal spirals. The number of dwarfs is higher, the greater the number of barreds in the given field. There is no preferential direction in the distribution of dwarfs around barred galaxies.Published in Astrofizika, Vol. 38, No. 4, pp. 630–635, October–December, 1995.  相似文献   

14.
We discuss lower and upper bounds on the Higgs mass using different values of the coupling constant λ of the self-interacting scalar field by considering a new inflationary scenario. We use the value of the amplitude of the primordial energy density fluctuations as inferred from the COBE results and the top quark mass as recently measured by the D0 and CDF experiments. The upper bound obtained on the Higgs mass is compatible with the upper bound favoured by the electroweak data.  相似文献   

15.
The problem of placing an upper bound on the mass of a non-rotating neutron star is investigated under minimal assumptions on the equation of state above nuclear densities. In particular causality (dp/dq<1) is not assumed in this region. An upper bound of about five solar masses is found.Supported in part by the National Science Foundation.  相似文献   

16.
We present an analysis of star-forming gas cores in a smooth particle hydrodynamics simulation of a giant molecular cloud. We identify cores using their deep potential wells. This yields a smoother distribution with clearer boundaries than density. Additionally, this gives an indication of future collapse, as bound potential cores (p-cores) represent the earliest stages of fragmentation in molecular clouds. We find that the mass function of the p-cores resembles the stellar initial mass function and the observed clump mass function, although p-core masses  (∼0.7 M⊙)  are smaller than typical density clumps. The bound p-cores are generally subsonic, have internal substructure and are only quasi-spherical. We see no evidence of massive bound cores supported by turbulence. We trace the evolution of the p-cores forward in time, and investigate the connection between the original p-core mass and the stellar mass that formed from it. We find that there is a poor correlation, with considerable scatter suggesting accretion on to the core is dependent on more factors than just the initial core mass. During the accretion process the p-cores accrete from beyond the region first bound, highlighting the importance of the core environment to its subsequent evolution.  相似文献   

17.
Within the context of the restricted problem of three bodies, an analytic upper bound on the three-dimensional regions of stability about the triangular equilibrium points is derived for general initial velocity limits and a wide class of bounding regions. This upper bound is illustrated and compared to numerical investigations for two bounding regions using the Earth-Moon mass ratio.  相似文献   

18.
Stability of a self-gravitating ring about a central body is considered. The purpose is to derive a bound on the mass of the ring in order that the system will be linearly stable. Our bound will, in some cases, be the best possible bound. The bound is also expanded as an asymptotic series. Comparisons of our result are made with respect to previous analyses performed by Tisserand, Pendse and Willerding.  相似文献   

19.
Abstract— The Sahara Desert is a region of high diurnal temperature variation and sporadic rainfall that has recently yielded over 450 meteorites. Eighteen of these Saharan samples are carbonaceous chondrites, of which we have analysed 17 for C content and isotopic composition. Ten of the 18 are paired CR chondrites, of which four have also had N and H contents and compositions determined. A primitive ordinary chondrite (L/LL3.2) found in the region has also been analysed for C, N and H contents and isotopic composition. Saharan samples contain between 21% and 45% of the light elements of their non-Saharan counterparts. Paired Saharan samples show a greater heterogeneity in both C content and isotopic composition than multiple analyses of non-Saharan samples. The cause of the observed isotopic and abundance effects is due to the hot desert weathering processes experienced by these samples. Peak temperatures of meteorites on the desert floor may be in excess of 100 °C, leading to low-temperature hydrous pyrolysis and oxidation reactions, liberating volatile organics and CO2. This may also cause the remaining material to become partially solubilised and ultimately lost during rainfall. The low δD of the CR and ordinary chondrites can be attributed to the destruction and loss of organic material through dehydrogenation and exchange reactions on the desert surface. The increased 13C abundance suggests that the less tightly bound C from the macromolecular organic material is isotopically lighter than the remaining C. Carbon contents and isotopic compositions are also affected by the addition of terrestrial calcitic evaporite deposits, up to 10,000 ppm carbonate has been measured, with a δ13C of between 0 and ?10%0.  相似文献   

20.
The accumulation of giant planets involves processes typical for terrestrial planet formation as well as gasdynamic processes that were previously known only in stars. The condensible element cores of the gas-giants grow by solid body accretion while envelope formation is governed by stellar-like equilibria and the dynamic departures thereof. Two hypotheses for forming Uranus/Neptune-type planets — at sufficiently large heliocentric distances while allowing accretion of massive gaseous envelopes, i.e. Jupiter-type planets at intermediate distances — have been worked out in detailed numerical calculations: (1) Hydrostatic gas-accretion models with time-dependent solid body accretion-rates show a slow-down of core-accretion at the appropriate masses of Uranus and Neptune. As a consequence, gas-accretion also stagnates and a window is opened for removing the solar nebula during a time of roughly constant envelope mass. (2) Gasdynamic calculations of envelope accretion for constant planetesimal accretion-rates show a dynamic transition to new envelope equilibria at the so called critical mass. For a wide range of solar nebula conditions the new envelopes have respective masses similar to those of Uranus and Neptune and are more tightly bound to the cores. The transitions occur under lower density conditions typical for the outer parts of the solar nebula, whereas for higher densities, i.e. closer to the Sun, gasdynamic envelope accretion sets in and is able to proceed to Jupiter-masses.  相似文献   

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