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1.
In this paper, we analyze the results of ground-based and space-born photometric observations of the major satellites of Uranus—Miranda, Ariel, Umbriel, Titania, and Oberon. All sets of photometric observations of the satellites available in the literature were examined for uniformity and systematic differences and summarized to a unified set by wavelength ranging from 0.25 to 2.4 μm. This set covers the interval of phase angles from 0.034° to 35°. The compound phase curves of brightness of the satellites in the spectral bands at 0.25, 0.41, 0.48, 0.56, 0.75, 0.91, 1.4, and 1.8 μm, which include a pronounced opposition surge and linear part, were constructed. For each satellite, the geometric albedo was found in different spectral bands taking into account the brightness opposition effect, and its spectral dependence was studied. It has been shown that the reflectance of the satellites linearly depends on the wavelength at different phase angles, but has different spectral gradients. The parameters of the phase functions of brightness, including the amplitude and the angular width of the brightness opposition surge, the phase coefficient, and the phase angle at which the nonlinear increase in brightness starts, were determined and their dependences on wavelength and geometric albedo were analyzed. Our investigations show that, in their optical properties, the satellites Miranda and Ariel, Titania and Oberon, and Umbriel present three types of surfaces. The observed parameters of the brightness opposition effect for the Uranian satellites, some ice satellites of Jupiter and Saturn, and the E-and S-type asteroids are analyzed and compared within the framework of the coherent backscattering and mutual shadowing mechanisms. 相似文献
2.
The family of (490) Veritas is a young, dynamically heterogeneous asteroid family, located in the outer main belt. As such, it represents a valuable example for studying the effects of chaotic diffusion on the shape of asteroid families. The Veritas family can be decomposed into several groups, in terms of the principal mechanisms that govern the local dynamics, which are analyzed here. A relatively large spread in proper eccentricity is observed, for the members of two chaotic groups. We show that different types of chaos govern the motion of bodies within each group, depending on the extent of overlap among the components of the corresponding resonant multiplets. In particular, one group appears to be strongly diffusive, while the other is not. Studying the evolution of the diffusive group and applying statistical methods, we estimate the age of the family to be τ=(8.7±1.7) Myr. This value is statistically compatible with that of 8.3 Myr previously derived by Nesvorný et al. [Nesvorný, D., Bottke, W.F., Levison, H.F., Dones, L., 2003. Astrophys. J. 591, 486-497], who analyzed the secular evolution of family members on regular orbits. Our methodology, applied here in the case of the Veritas family, can be used to reconstruct the orbital history of other, dynamically complex, asteroid families and derive approximate age estimates for young asteroid families, located in diffusive regions of the main belt. Possible refinements of the method are also discussed. 相似文献
3.
Natural, low-frequency, hydromagnetic oscillations of an isolated, nonrotating neutron star, which are localized in the peripheral
crust, the structure of which is determined by the electron-nuclear plasma (the Ae phase), are studied. The plasma medium
of the outer crust is treated as a homogeneous, infinitely conducting, incompressible continuum, the motions of which are
determined by the equations of magnetohydrodynamics. In the approximation of a constant magnetic field inside the crust (the
magnetic field outside the star is assumed to have a dipole structure), the spectrum of normal poloidal and toroidal hydromagnetic
oscillations, due to presumed residual fluctuations of flow and their associated fluctuations in magnetic field strength,
is calculated. Numerical estimates given for the periods of MHD oscillations fall in the range of periods of radio pulsar
emission, indicating a close connection between the residual hydromagnetic oscillations and the electromagnetic activity of
neutron stars.
Translated from Astrofizika, Vol. 40, No. 1, pp. 77–86, January–March, 1997. 相似文献
4.
In our previous papers, we showed that at the final phases of the dynamical evolution of an open cluster, an extended population of stars elongated along its Galactic orbit, the stellar tail of the cluster, is formed. The tail stars that escaped from the cluster at different times move in a common orbit with low relative velocities. Experiencing a weak interaction with Galactic field stars, these objects, the relics of open clusters, can exist for a fairly long time. In this paper, we investigate the structures of such stellar tails in the nearest open clusters: Hyades, Pleiades, Praesepe, Alpha Persei, Coma, IC 2391, and IC 2602. To this end, we performed several numerical simulations of the dynamical evolution of these clusters in the tidal field of the Galaxy. Our computations of the dynamical evolution were based on known cluster age estimates and real Galactic orbits. The initial conditions were chosen in such a way that the parameters of the simulated clusters corresponded to their observed parameters. As a result, we obtained models of the stellar tails for the nearest open clusters and estimated such parameters of the tails as their sizes, densities, locations relative to the solar neighborhood, and others. 相似文献
5.
We analyze the conditions for the formation and time evolution of peripheral comet structures of solar-type planetary systems. In the Solar system, these include the Kuiper belt, the Oort cloud, the comet spear, and the Galactic comet ring that marks the Galactic orbit of the Sun. We consider the role of the viscosity of a protoplanetary gas–dust disk, major planets, field stars, globular clusters, giant molecular clouds, and the Galactic gravitational field in the formation of these peripheral structures marked by comets and asteroids. We give a list of the closest past and future passages of neighboring stars through the solar Oort cloud that perturb the motion of its comets and, thus, contribute to the enhancement of its cometary activity, on the one hand, and to the replenishment of the solar comet spear with new members, on the other hand. 相似文献
6.
J. C. Raymond J. L. Kohl G. Noci E. Antonucci G. Tondello M. C. E. Huber L. D. Gardner P. Nicolosi S. Fineschi M. Romoli D. Spadaro O. H. W. Siegmund C. Benna A. Ciaravella S. Cranmer S. Giordano M. Karovska R. Martin J. Michels A. Modigliani G. Naletto A. Panasyuk C. Pernechele G. Poletto Peter L. Smith R. M. Suleiman L. Strachan 《Solar physics》1997,175(2):645-665
The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers the 940–1350 Å range as well as the 470–630 Å range in second order. It has detected coronal emission lines of H, N, O, Mg, Al, Si, S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resonance scattering of emission lines from the solar disk dominates the intensities of a few lines, but electron collisional excitation produces most of the lines observed. Resonance, intercombination and forbidden lines are seen, and their relative line intensities are diagnostics for the ionization state and elemental abundances of the coronal gas. The elemental composition of the solar corona and solar wind vary, with the abundance of each element related to the ionization potential of its neutral atom (First Ionization Potential–FIP). It is often difficult to obtain absolute abundances, rather than abundances relative to O or Si. In this paper, we study the ionization state of the gas in two coronal streamers, and we determine the absolute abundances of oxygen and other elements in the streamers. The ionization state is close to that of a log T = 6.2 plasma. The abundances vary among, and even within, streamers. The helium abundance is lower than photospheric, and the FIP effect is present. In the core of a quiescent equatorial streamer, oxygen and other high-FIP elements are depleted by an order of magnitude compared with photospheric abundances, while they are depleted by only a factor of 3 along the edges of the streamer. The abundances along the edges of the streamer (‘legs’) resemble elemental abundances measured in the slow solar wind, supporting the identification of streamers as the source of that wind component. 相似文献
7.
8.
We describe the present state of massive star research seen from the viewpoint of stellar evolution, with special emphasis
on close binaries. Statistics of massive close binaries are reasonably complete for the Solar neighbourhood. We defend the
thesis that within our knowledge, many scientific results where the effects of binaries are not included, have an academic
value, but may be far from reality. In chapter I, we summarize general observations of massive stars where we focus on the
HR diagram, stellar wind mass loss rates, the stellar surface chemistry, rotation, circumstellar environments, supernovae.
Close binaries can not be studied separately from single stars and vice versa. First, the evolution of single stars is discussed
(chapter I). We refer to new calculations with updated stellar wind mass loss rate formalisms and conclusions are proposed
resulting from a comparison with representative observations. Massive binaries are considered in chapter II. Basic processes
are briefly described, i.e. the Roche lobe overflow and mass transfer, the common envelope process, the spiral-in process
in binaries with extreme mass ratio, the effects of mass accretion and the merging process, the implications of the (asymmetric)
supernova explosion of one of the components on the orbital parameters of the binary. Evolutionary computations of interacting
close binaries are discussed and general conclusions are drawn. The enormous amount of observational data of massive binaries
is summarized. We separately consider the non-evolved and evolved systems. The latter class includes the semi-detached and
contact binaries, the WR binaries, the X-ray binaries, the runaways, the single and binary pulsars. A general comparison between
theoretical evolution and observations is combined with a discussion of specially interesting binaries: the evolved binaries
HD 163181, HD 12323, HD 14633, HD 193516, HD 25638, HD 209481, Per and Sgr; the WR+OB binary V444 Cyg; the high mass X-ray binaries Vela X-1, Wray 977, Cyg X-1; the low mass X-ray binaries Her
X-1 and those with a black hole candidate; the runaway Pup, the WR+compact companion candidates Cyg X-3, HD 50896 and HD 197406. We finally propose an overall evolutionary model
of massive close binaries as a function of primary mass, mass ratio and orbital period. Chapter III deals with massive star
population synthesis with a realistic population of binaries. We discuss the massive close binary frequency, mass ratio and
period distribution, the observations that allow to constrain possible asymmetries during the supernova explosion of a massive
star. We focuss on the comparison between observed star numbers (as a function of metallicity) and theoretically predicted
numbers of stellar populations in regions of continuous star formation and in starburst regions. Special attention is given
to the O-type star/WR star/red supergiant star population, the pulsar and binary pulsar population, the supernova rates.
Received 17 July 1998 相似文献
9.
Interpretation of photometric and polarimetric observations of atmosphereless celestial bodies faces the problems connected with both the insufficient accuracy and level of details in groundbased observations and the current state of the theory of the multiple scattering of light. In application to sparse media, where the electromagnetic waves, propagating between the scatterers, can be considered as spherical (the socalled far-field approximation), this theory is rather well developed for both the diffuse and coherent components of the scattered radiation. In this paper, we show that this theory can be also successfully applied to the measurements of polarization of light scattered by densely packed, though nonabsorbing or weakly absorbing, media. For this purpose, we calculated the models for a semi-infinite layer of the medium composed of randomly oriented clusters of spherical particles and compared them with the data of laboratory and astronomical measurements. The potential of the present approach is illustrated by an example of the interpretation of the polarization measurements of the ice satellites of Saturn—Rhea and Enceladus—which allowed some properties of the surface of these celestial bodies to be estimated. In particular, the ratio of the surface area that makes no contribution to the negative polarization of light reflected at small phase angles to the area producing the negative polarization branch was found. Under the assumption of the same albedo of these areas, this ratio turned out to be 3.31–3.66 and 1.7–3.8 for Rhea and Enceladus, respectively. For Enceladus, it is difficult to obtain a sufficiently narrow range of the estimated parameters, since the number of measurement points in the phase dependence of polarization of this satellite is small. For the surface of Rhea, the estimated packing density of particles, participating in the opposition effects, is approximately 15%, while their smallest size is of the order of the wavelength of visible light. 相似文献
10.
The Interstellar Heliopause Probe: Heliospheric Boundary Explorer Mission to the Interstellar Medium
The Sun, driving a supersonic solar wind, cuts out of the local interstellar medium a giant plasma bubble, the heliosphere. ESA, jointly with NASA, has had an important role in the development of our current understanding of the Suns’ immediate neighborhood. Ulysses is the only spacecraft exploring the third, out-of-ecliptic dimension, while SOHO has allowed us to better understand the influence of the Sun and to image the glow of interstellar matter in the heliosphere. Voyager 1 has recently encountered the innermost boundary of this plasma bubble, the termination shock, and is returning exciting yet puzzling data of this remote region. The next logical step is to leave the heliosphere and to thereby map out in unprecedented detail the structure of the outer heliosphere and its boundaries, the termination shock, the heliosheath, the heliopause, and, after leaving the heliosphere, to discover the true nature of the hydrogen wall, the bow shock, and the local interstellar medium beyond. This will greatly advance our understanding of the heliosphere that is the best-known example for astrospheres as found around other stars. Thus, IHP/HEX will allow us to discover, explore, and understand fundamental astrophysical processes in the largest accessible plasma laboratory, the heliosphere. 相似文献
11.
在大科学时代,大科学工程的建设日益复杂,设计者需综合考虑设备性能、技术储备、经费、风险、环境等因素才能进行合理决策.在国内外天文学发展的背景中,梳理了500 m口径球面射电望远镜(Five-hundredmeter Aperture Spherical radio Telescope, FAST)概念形成的过程以及FAST团队在其中所做的决策,包括大望远镜中国方案的设想、先导单元的提出、主动反射面技术的采用等,并对决策过程进行了分析探讨. FAST诞生于我国与国际天文学发展的互动与融合进程,实现了从跟进到占据先机的转变,可为在相关基础薄弱的领域建设大科学工程提供参考. 相似文献
12.
The problem of the efficiency of the ion-synchrotron maser proposed by Hoshino and Arons is analyzed in a linear approximation.
A hot, relativistic, electron-positron plasma penetrated by a relativistic ion beam is considered. At the front of the magnetosonic
shock wave an electromagnetic wave is generated, which should be damped on positrons of the plasma. This should, in turn,
result in synchrotron emission from energetic positrons in the high-frequency range, far above the natural frequencies of
the plasma. It is shown that one must allow simultaneously for the conditions of resonance at a high harmonic of the ion-cyclotron
frequency and at the fundamental of the electron-cyclotron frequency. Natural transverse waves are generated in the process,
but within the framework of the linear theory there is no positron acceleration due to the kinetic energy of ions.
Translated from Astrofizika, Vol. 43, No. 3, pp. 389-396, July–September, 2000. 相似文献
13.
14.
15.
This paper presents the approximate general solution of the triple well, double oscillator non-linear dynamical system. This
system is non-integrable and the approximate general solution is calculated by application of the Last Geometric Theorem of
Poincaré (Birkhoff, 1913, 1925). The original problem, known as the Duffing one, is a 1 degree of freedom system that, besides
the conservative force component, includes dumping and external forcing terms (see details in the web site: http://www.uncwil.edu/people/hermanr/chaos/ted/chaos.html).
The problem considered here is a 2 degree of freedom, autonomous and conservative one, without dumping, and of axisymmetric
potential. The space of permissible motions is scanned for identification of all solutions re-entering after from one to nine
oscillations and the precise families of periodic solutions are computed, including their stability parameter, covering all
cases with periods T corresponding to 4osc/T. Seven sub-domains of the space of solutions were investigated in detail by zooming, an operation that proved the possibility
to advance the accuracy of the approximate general solution to the level permitted by the integration routine. The approximation
of the general solution, although impressive, provides clear evidence of the complexity of the problem and the need to proceed
to larger period families. Nevertheless, it allows prediction of the areas where chaos and order regions in the Poincaré surfaces
of section are to be expected. Examples of such surfaces of sections, as well as of types of closed solutions, are given.
Two peculiar points of the space of solutions were identified as crossing, or source points from which infinite families of
periodic solutions emanate. The morphology and stability of solutions of the problem are studied and discussed. 相似文献
16.
Linkin V Harri AM Lipatov A Belostotskaja K Derbunovich B Ekonomov A Khloustova L Kremnev R Makarov V Martinov B Nenarokov D Prostov M Pustovalov A Shustko G Jarvinen I Kivilinna H Korpela S Kumpulainen K Lehto A Pellinen R Pirjola R Riihela P Salminen A Schmidt W McKay CP 《Planetary and Space Science》1998,46(6-7):717-737
A mission to Mars including two Small Stations, two Penetrators and an Orbiter was launched at Baikonur, Kazakhstan, on 16 November 1996. This was called the Mars-96 mission. The Small Stations were expected to land in September 1997 (Ls approximately 178 degrees), nominally to Amazonis-Arcadia region on locations (33 N, 169.4 W) and (37.6 N, 161.9 W). The fourth stage of the Mars-96 launcher malfunctioned and hence the mission was lost. However, the state of the art concept of the Small Station can be applied to future Martian lander missions. Also, from the manufacturing and performance point of view, the Mars-96 Small Station could be built as such at low cost, and be fairly easily accommodated on almost any forthcoming Martian mission. This is primarily due to the very simple interface between the Small Station and the spacecraft. The Small Station is a sophisticated piece of equipment. With the total available power of approximately 400 mW the Station successfully supports an ambitious scientific program. The Station accommodates a panoramic camera, an alpha-proton-x-ray spectrometer, a seismometer, a magnetometer, an oxidant instrument, equipment for meteorological observations, and sensors for atmospheric measurement during the descent phase, including images taken by a descent phase camera. The total mass of the Small Station with payload on the Martian surface, including the airbags, is only 32 kg. Lander observations on the surface of Mars combined with data from Orbiter instruments will shed light on the contemporary Mars and its evolution. As in the Mars-96 mission, specific science goals could be exploration of the interior and surface of Mars, investigation of the structure and dynamics of the atmosphere, the role of water and other materials containing volatiles and in situ studies of the atmospheric boundary layer processes. To achieve the scientific goals of the mission the lander should carry a versatile set of instruments. The Small Station accommodates devices for atmospheric measurements, geophysical and geochemical studies of the Martian surface and interior, and cameras for descent phase and panoramic views. These instruments would be able to contribute remarkably to the process of solving some of the scientific puzzles of Mars. 相似文献
17.
《天文和天体物理学研究(英文版)》2020,(3)
The detection of clumps(cores) in molecular clouds is an important issue in sub-millimetre astronomy. However, the completeness of the identification and the accuracy of the returned parameters of the automated clump identification algorithms are still not clear. In this work, we test the performance and bias of the GaussClumps, ClumpFind, FellWalker, Reinhold, and Dendrograms algorithms in identifying simulated clumps. By designing the simulated clumps with various sizes, peak brightness, and crowdedness, we investigate the characteristics of the algorithms and their performance. In the aspect of detection completeness, FellWalker, Dendrograms, and GaussClumps are the first, second, and third best algorithms, respectively. The numbers of correct identifications of the six algorithms gradually increase as the size and signal-to-noise ratio(SNRs) of the simulated clumps increase and they decrease as the crowdedness increases. In the aspect of the accuracy of retrieved parameters, FellWalker and Dendrograms exhibit better performance than the other algorithms. The average deviations in clump parameters for all algorithms gradually increase as the size and SNR of clumps increase. Most of the algorithms except FellWalker exhibit significant deviation in extracting the total flux of clumps. Taken together, FellWalker, GaussClumps,and Dendrograms exhibit the best performance in detection completeness and extracting parameters. The deviation in virial parameter for the six algorithms is relatively low. When applying the six algorithms to the clump identification for the Rosette molecular cloud, ClumpFind1994, ClumpFind2006, GaussClumps,FellWalker, and Reinhold exhibit performance that is consistent with the results from the simulated test. 相似文献
18.
Horton E. Newsom Melissa J. Nelson Charles K. Shearer Burkhard O. Dressler 《Meteoritics & planetary science》2006,41(12):1929-1945
Abstract— The concentrations of the fluid mobile trace elements lithium, beryllium, boron, and barium were measured in samples of the altered matrix of several impactite breccias of the Yaxcopoil‐1 drill core using secondary ion mass spectrometry (SIMS) to determine the extent of transport due to aqueous or hydrothermal processes. Three of the elements, Li, Be, and B, have higher concentrations in the upper suevite impact breccias than in the lower impact melt deposits by factors of 3.5, 2.2, and 1.5, respectively. Lithium and B are the most enriched elements up section, and appear to have had the greatest mobility. The similar fractionation of Li and B is consistent with fluid transport and alteration under low‐temperature conditions of less than 150 °C based on published experimental studies. In contrast to Li, Be, and B, the concentration of Ba in the altered matrix materials decreases upward in the section, and the concentration of Ba in the matrix is an order of magnitude less than the bulk concentrations, likely due to the presence of barite. The origin of the elemental variations with depth may be related to different protolith compositions in the upper versus the lower impactite units. A different protolith in the altered matrix is suggested by the Mg‐rich composition of the lower units versus the Al‐rich composition of the upper units, which largely correlates with the mobile element variations. The possibility that vertical transport of mobile elements is due to a postimpact hydrothermal system is supported by published data showing that the sediments immediately overlying the impactites are enriched in mobile elements derived from a hydrothermal system. However, the mobile elements in the sediments do not have to originate from the underlying impactites. In conclusion, our data suggests that the impactites at this location did not experience extensive high‐temperature hydrothermal processing, and that only limited transport of some elements, including Li, Be, and B, occurred. 相似文献
19.
This paper deals with Oort cloud formation while the Sun was in an embedded cluster and surrounded by its primordial nebula. This work is a continuation of Brasser et al. [Brasser, R., Duncan, M., Levison, H., 2006. Icarus 184, 59-82], building on the model presented therein, and adding the aerodynamic drag and gravitational potential of the primordial solar nebula. Results are presented of numerical simulations of comets subject to the gravitational influence of the Sun, Jupiter, Saturn, star cluster and primordial solar nebula; some of the simulations included the gravitational influence of Uranus and Neptune as well. The primordial solar nebula was approximated by the minimum-mass Hayashi model [Hayashi, C., Nakozawa, K., Nakagawa, Y., 1985. In: Black, D.C., Matthews, M.S. (Eds.). Protostars and Planets II. Univ. of Arizona Press, Tucson, AZ] whose inner and outer radii have been truncated at various distances from the Sun. A comet size of 1.7 km was used for most of our simulations. In all of our simulations, the density of the primordial solar nebula decayed exponentially with an e-folding time of 2 Myr. It turns out that when the primordial solar nebula extends much beyond Saturn or Neptune, virtually no material will end up in the Oort cloud (OC) during this phase. Instead, the majority of the material will be on circular orbits inside of Jupiter if the inner edge of the disk is well inside Jupiter's orbit. If the disk's inner edge is beyond Jupiter's orbit, most comets end up on orbits in exterior mean-motion resonances with Saturn when Uranus and Neptune are not present. In those cases where the outer edge of the disk is close to Saturn or Neptune, the fraction of material that ends up in the subsequently formed OC is much less than that found in Brasser et al. [Brasser, R., Duncan, M., Levison, H., 2006. Icarus 184, 59-82] for the same cluster densities. This implies that for comets of roughly 2 km in size, the presence of the primordial solar nebula hinders OC formation. A byproduct of some of our simulations are endresults with a substantial fraction of the comets in the Uranus-Neptune scattered disk. A subsequent followup of this material is planned for the near future. In order to determine the effect of the size of the comets on OC formation efficiency, a set of runs with the same initial conditions but different cometary radii have been performed as well, from which it is determined that the threshold comet size to begin producing significant Oort clouds is roughly 20 km. This implies that the presence of the primordial solar nebula acts as a size-sorting mechanism, with large bodies unaffected by the gas drag and ending up in the OC while small bodies remain trapped in the planetary region, in the models studied. 相似文献
20.
A. A. Saharian 《Astrophysics》1997,40(4):335-346
Investigations of string cosmology with a nonperturbative dilaton potential, begun in the first part of this work, are continued.
The picture of cosmological evolution of an isotropic, gravi-dilaton model for different cases of the behavior of the potential
in the region of strong coupling is analyzed by methods of the qualitative theory of dynamical systems. Features of models
with potentials that take negative values in certain ranges of values of the dilaton field are discussed. A specific mechanism
of generation of the nonperturbative potential, based on gaugino condensation in a hidden sector of the gauge group, is considered.
Translated from Astrofizika, Vol. 40, No. 4, pp. 517–534, October-December, 1997. 相似文献