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1.
René Duffard 《Earth, Moon, and Planets》2009,105(2-4):221-226
2.
Darwin is a cornerstone mission of the Horizons 2000+ program of the European Space Agency. It has the express purpose of
carrying out the first direct search for terrestrial exoplanets, and to achieve unprecedented spatial resolution in the infrared wavelength region. The
detection and study of terrestrial exoplanets promises to usher in a new era in science and will affect a broad spectrum of
disciplines. Further, the time line for implementation of such an instrument is now likely to be of the close order of 10
years, leading to possible answers to one of mankind's most fundamental questions in the second decade of the 21st century.
It has been found that in order to realize am interferometer in space in the next 10–15 years, it is necessary to prepare
the way through a number of intermediary steps – both on the ground and in space. In this context, we here describe GENIE
– a ground based nulling interferometry experiment to be implemented at the VLTI in a partnership between ESA and ESO.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
In the present work, we investigate phase correlations by recourse to the Shannon entropy. Using theoretical arguments, we show that the entropy provides an accurate measure of phase correlations in any dynamical system, in particular when dealing with a chaotic diffusion process. We apply this approach to different low-dimensional maps in order to show that indeed the entropy is very sensitive to the presence of correlations among the successive values of angular variables, even when it is weak. Later on, we apply this approach to unveil strong correlations in the time evolution of the phases involved in the Arnold’s Hamiltonian that lead to anomalous diffusion, particularly when the perturbation parameters are comparatively large. The obtained results allow us to discuss the validity of several approximations and assumptions usually introduced to derive a local diffusion coefficient in multidimensional near-integrable Hamiltonian systems, in particular the so-called reduced stochasticity approximation. 相似文献
4.
The Fokker-Planck equation for small stochastic changes to particles in Kepler orbits has to be formulated in terms of the integrals of motion. We generalize the modelling of proton and electron collisional perturbations to gas particles on trajectories through the solar system in order to include both spatial and velocity diffusion. The general solution is obtained in terms of a 4-dimensional normal distribution. Treatment of the singularity in the Fokker-Planck operator reduces the dimensionality by one. In addition to extending earlier results for anisotropic collisional heating in the thermal approximation, the present formulation gives the changes in density due to the mean repulsive force and to perturbations of trajectories (spatial diffusion). The net diffusion is almost everywhere towards the sun and the density increase is significant in the downstream hydrogen wake, particularly where destructive depletion is strong and gravitational focussing weak. 相似文献
5.
In our previous investigation of Jupiter models, subject to the constraint that the hydrogen-to-helium ratio is solar, we found a need to include considerable additional mass in the form of volatilized condensates in the atmosphere and excess mass in a central core. The stricter constraints imposed by the measurement of the gravitational moments of the planet as a result of the Pioneer 10 flyby indicate that the mass of the excess volatilized condensates, assumed to be water, relative to the core mass, assumed to be rock, exceeds the ratio of the relevant elements in the solar composition. In this paper we have tested the sensitivity of this conclusion to varying assumptions about the constraints and the equation of state: in particular, to variations in the hydrogen-to-helium ratio, in the softness of the equation of state for water, in the treatment of the internal adiabat, in departures from an internal adiabat, and to variations in the temperature at the 1 bar level. The above conclusion is not changed if any of these alterations are made within reasonable limits. 相似文献
6.
Stefano Casotto 《Celestial Mechanics and Dynamical Astronomy》1995,62(1):1-22
A new method for calculating the perturbation spectrum in the framework of Kaula's linear satellite theory (LST) is introduced. The novelty of this approach consists in using recent results on the spectral decomposition of the perturbation frequencies in LST to provide a closed formulation for the amplitude and the phase of each line in the perturbation spectrum. The theory presented here can be applied to perturbations in the elements or in the radial and transverse directions due to the geopotential or to the tides. Separate algorithms are developed for application to orbits with circulating or frozen perigee. 相似文献
7.
Randolph H. Levine 《Solar physics》1979,62(2):277-303
The spatial organization of the observed photospheric magnetic field, as well as its relation to the polarity of the interplanetary field, have been studied using high resolution magnetograms from Kitt Peak National Observatory. Systematic patterns in the large scale field have been found to be due to contributions from both concentrated flux and more diffuse flux. It is not necessary to assume, as has often been done in previous studies, that there is a weak background solar magnetic field causing the large-scale patterns in the photosphere, although the existence of such a field cannot be excluded. The largest scale structures in the photosphere correspond to the expected pattern at the base of a warped heliomagnetic equator.The polarity of the photospheric field, determined on various spatial scales, correlates with the polarity of the interplanetary field, with the most significant correlation due to mid-latitude fields. However, because the interplanetary field is likely to be rooted in concentrated photospheric regions, rather than across an entire polarity region, both the strength and polarity of the field are important in determining the interplanetary field. Thus studies of the interplanetary field which are based on either instrumental or numerical averaging of fields in the solar photosphere are subject to serious inherent limitations.Analyses based on several spatial scales in the photosphere suggest that new flux in the interplanetary medium is often due to relatively small photospheric features which appear in the photosphere up to one month before they are manifest at the Earth. The evolution of the over-all photospheric pattern may be due to individual sub-patterns which have slightly different rotation properties and which alternate in their relative dominance of the interplanetary medium. 相似文献
8.
David Brown 《Astrophysics and Space Science》2010,329(1-2):33-39
One of the ways by which subdwarf B stars are thought to form is through binary star interactions. The metallicity of the sdB progenitor stars in such binary systems should not seem to be a major factor in the formation of sdB stars. However, given the different environments in which sdB stars are found, binary population synthesis simulations have been conducted in order to examine how metallicity might be a subtle factor in the formation of sdB stars in such environments. This is then applied to clusters of stars and to the UV Upturn phenomenon. 相似文献
9.
G. E. Romero 《Astrophysics and Space Science》1995,234(1):49-55
Several versions of the shock-in-jet model of the blazars' variability have been proposed in recent years. This family of models resorts to the interaction between a relativistic shock and a feature in the parsec-scale jet associated to an AGN in order to explain the origin of the rapid variability recently observed in many extragalactic radio sources. The nature of the involved features varies from one particular model to another. Here I discuss a possible origin for this variety of subparsec-sized features in the frame of the two-fluid model of extragalactic jets. Macroscopic Kelvin-Helmholtz instabilities are shown to be capable of producing the required plurality of morphologies in the innermost part of the beams, in such a way that the different models have particular ranges of applicability according to the differencies in the parameters of the flows. 相似文献
10.
The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant
HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies
is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight
radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions.
It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions
are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the
hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could
have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that
tidal interaction is likely to be an effective mechanism for transforming spirals to S0s. 相似文献
11.
A simple one-dimensional model is presented to describe the energy budget of the tropical atmosphere. Heating of the atmosphere is associated primarily with latent energy released due to precipitation in localized regions of intense cumulonimbus activity. Air transported to upper regions of the troposphere by cumulonimbus systems is returned to the surface over a large region of descent. Heat released by subsidence is balanced primarily by emission of radiation in the infrared. The model accounts for this energy balance, exploring specifically the constraints on permissible fluxes of mass and energy.
Results suggest that the strength of the background subsidence field in the tropics may be sensitive to surface temperature and to changes in atmospheric composition, specifically variations in the altitude distribution of H2O and changes in the abundance of greenhouse gases such as CO2. The mean level of detrainment of deep cumulonimbus clouds is found to increase with increasing surface temperature. This behavior is shown to be sensitive to atmospheric composition. The surface temperature for an atmosphere containing twice the present level of CO2 is predicted to increase by 1.4K, about 25% less than the change obtained with models in which the lapse rate of temperature is specified at lower altitudes, where assumptions of radiative equilibrium would lead otherwise to a statically unstable condition.
Buoyancy considerations suggest that there may be an upper limit to the range of permissible values for surface temperature in the tropics. Models in which the subsidence mass flux is assumed constant with respect to altitude are found to be unable to maintain buoyancy for rising air in the face of heat released by descent when surface temperatures exceed about 312K. 相似文献
12.
Paul Goudfrooij Diane Gilmore Markus Kissler-Patig † Claudia Maraston 《Monthly notices of the Royal Astronomical Society》2006,369(2):697-704
We compare the conclusions reached using the coarse-graining technique employed by Henriksen for a one degree of freedom (per particle) collisionless system to those presented in a paper by Binney based on an exact one degree of freedom model. We find agreement in detail, but in addition we show that the isolated 1D system is self-similar and therefore unrelaxed. Fine graining of this system recovers much less prominent wave-like structure than in a spherically symmetric isotropic 3D system. The rate of central flattening is also reduced in the 1D system. We take this to be evidence that relaxation of collisionless systems proceeds ultimately by way of short wavelength Landau damping. N -body systems, both real and simulated, can be trapped in an incompletely relaxed state because of a break in the cascade of energy to small scales. This may be due to the rapid dissipation of the small-scale oscillations in an isolated system to the existence of conserved quantities such as angular momentum, or to the failure in simulations to resolve sub-Jeans length scales. Such a partially relaxed state appears to be the Navarro, Frenk and White (NFW) state and is to be expected especially in young systems. The NFW core is shown to be isolated. In non-isolated systems, continuing coarse-grained relaxation should be towards a density core in solid body rotation. 相似文献
13.
W.G. Tifft 《Astrophysics and Space Science》2003,285(2):429-449
The Lehto-Tifft redshift quantization model is used to predict the redshift distribution for certain classes of quasars, and
for galaxies in the neighborhood of z = 0.5. In the Lehto-Tifft model the redshift is presumed to arise from time dependent decay from an origin at the Planck
scale; the decay process is a form of period doubling. Looking back in time reveals earlier stages of the process where redshifts
should correspond to predictable fractions of the speed of light. Quasar redshift peaks are shown to correspond to the earliest
simple fractions of c as predicted by the model. The sharp peaks present in deep field galaxy redshifts surveys are then shown to correspond to
later stages in such a decay process. Highly discordant redshift associations are expected to occur and shown to be present
in the deep field surveys. Peaks in redshift distributions appear to represent the spectrum of possible states at various
stage of the decay process rather than physical structures.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
A quasi-one-dimensional hydrodynamic model for the collapse of a rotating iron stellar core is used to determine the neutrino
spectra in the limiting case of total transparency to neutrino emission (without any deposition effect). The derived spectra
allow the previously constructed spectra used to theoretically estimate the number of events in the LSD underground neutrino
detector from SN 1987A to be refined. At typical iron stellar core parameters, including those that characterize the core
rotation specified in the initial conditions of the model, this number has turned out to be 1.6, which is close in order of
magnitude to its experimental value of 5. Here, we compare in detail these results by assuming that the transparency of the
collapsing iron core itself could be attributable to the development of its three-dimensional dynamical instability—the subject
of future theoretical studies. The physical formulation of the problem coincides closely with the collapse model proposed
in our previous paper, where the above number of events turned out to be 0.5. We have confirmed the previously published results
with regard to the neutrino spectra, including the significant superiority of electron neutrinos over electron antineutrinos
in them. The hydrostatically equilibrium configuration (a rotating collapsar) obtained in our model calculation is discussed
in comparison with self-similar solutions that are close in physical formulation of the problem. This result seems a nontrivial
consequence of the included rotation effects that hinder nonstop collapse established in the mentioned self-similar solutions. 相似文献
15.
Hui Zhao Dean-Yi Chou Ming-Hsu Yang Zhi-Chao Liang Ming-Tsung Sun 《Solar physics》2011,268(2):363-376
The cross-correlation function is a useful tool in helioseismology. The magnitude of the cross-correlation function has been used to represent the power of wave packets. Dispersion causes a decrease in amplitude and an increase in width of wave packets. This leads to a decrease in magnitude and an increase in width of cross-correlation functions. The effect of dispersion on the magnitude of cross-correlation functions needs to be adequately corrected for in order to use the magnitude of cross-correlation functions to represent the power of wave packets. In this study, we investigate how the magnitude of cross-correlation functions changes with the number of skips owing to dispersion and the method to correct it. Our study, using simulated and observational data, indicates that the correction should be three dimensional instead of the one-dimensional correction adopted in previous studies. Using the three-dimensional correction, the measured dissipation rate in the quiet Sun is smaller than the value of previous studies. 相似文献
16.
We consider the motions of particles in the one-dimensional Newtonian three-body problem as a function of initial values.
Using a mapping of orbits to symbol sequences we locate the initial values leading to triple collisions. These turn out to
form curves which give clear structure to the region in which the motions depend sensitively on initial conditions. In addition
to finding the triple collision orbits we also locate orbits which end up to a triple collision in both directions of time,
that is, orbits which are finite both in space and time.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
17.
J. C. B. Papaloizou 《Celestial Mechanics and Dynamical Astronomy》2003,87(1-2):53-83
We consider orbital resonances in multiplanet systems. These are expected to arise during or just after formation in a gaseous disc. Disc–planet interaction naturally produces orbital migration and circularization through the action of tidal torques which in turn may lead to an orbital resonance. The mass and angular momentum content of the disc is likely to be comparable to that in the planets so that it is essential to fully incorporate the disc in the analysis.We study the orbital evolution of two planets locked in 2:1 commensurability through migration tidally induced by the disc using both analytic methods and numerical hydrodynamic simulations. The planets are assumed to orbit in an inner cavity containing at most only a small amount of disc material. Results are found to be sensitive to initial surface density profile, planet masses and disc parameters. The evolution may range between attaining and subsequently maintaining a resonance lock with two angles librating to divergent migration with no commensurability formed. In the former case eccentricities increase monotonically with time while the system undergoes inward migration. If the migration is halted by loss of the disc leaving the planets in a final configuration, there is likely to be a low probability of seeing resonant planets at small radii as well as a sensitive dependence on past history.We have also considered a multiplanet system in secular apsidal resonance. We consider the system as being in just one secular normal mode and include the effects of a gaseous disc. It is suggested that a normal mode may be selected by adding in some weak dissipative process in the disc and that it may remain, involving only the planets, when the disc is slowly removed. 相似文献
18.
D. V. Nikonchuk 《Astronomy Letters》2012,38(12):813-828
Anonlinear analytical theory of secular perturbations in the problem of the motion of a systemof small bodies around a major attractive center has been developed. Themutual perturbations of the satellites and the influence of the oblateness of the central body are taken into account in the model. In contrast to the classical Laplace-Lagrange theory based on linear equations for Lagrange elements, the third-degree terms in orbital eccentricities and inclinations are taken into account in the equations. The corresponding improvement of the solution turns out to be essential in studying the evolution of orbits over long time intervals. A program inC has been written to calculate the corrections to the fundamental frequencies of the solution and the third-degree secular perturbations in orbital eccentricities and inclinations. The proposed method has been applied to investigate the motion of the major Uranian satellites. Over time intervals longer than 100 years, allowance for the nonlinear terms in the equations is shown to give corrections to the coordinates of Miranda on the order of the orbital eccentricity, which is several thousand kilometers in linear measure. For other satellites, the effect of allowance for the nonlinear terms turns out to be smaller. Obviously, when a general analytical theory of motion for the major Uranian satellites is constructed, the nonlinear terms in the equations for the secular perturbations should be taken into account. 相似文献
19.
M. Yu. Reshetnyak 《Astronomy Letters》2016,42(7):482-487
The inverse problem in a spherical shell to find the two-dimensional spatial distributions of the α-effect and differential rotation in a mean-field dynamo model has been solved. The derived distributions lead to the generation of a magnetic field concentrated inside the convection zone. The magnetic field is shown to have no time to rise from the region of maximum generation located in the lower layers to the surface in the polarity reversal time due to magnetic diffusion. The ratio of the maximum magnetic energy in the convection zone to its value at the outer boundary reaches two orders of magnitude or more. This result is important in interpreting the observed stellar and planetary magnetic fields. The proposed method of solving the inverse nonlinear dynamo problem is easily adapted for a wide class of mathematical-physics problems. 相似文献
20.
测定天文大气折射和建立电磁波折射延迟实测模型(Ⅲ)-专用测量仪器 总被引：1，自引：0，他引：1
针对用天文大气折射测定值,建立随观测站和随方位而异的电磁波折射延迟改正模型的高精度要求,提出了新的仪器误差理论,其主要内容是允许仪器误差存在,并看成是不断变化的,采用相应的测量方法作实时的测定和修正,同时消除仪器的各种变形和误差的影响,排除观测数据中的各种系统误差来源,并达到提高单次测定精度目的;文中还针对不同纬度的观测站、多方位、从天顶直到低空的观测需要,给出了仪器总体结构的安排,和采用视频CCD作为接收器的终端设计方案,也给出了各种仪器误差的测定方法和测量装置的设计要求。 相似文献