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1.
F. Patru N. Tarmoul D. Mourard O. Lardière 《Monthly notices of the Royal Astronomical Society》2009,395(4):2363-2372
In the future, optical stellar interferometers will provide true images thanks to larger number of telescopes and to advanced cophasing subsystems. These conditions are required to have sufficient resolution elements (resel) in the image and to provide direct images in the hypertelescope mode. It has already been shown that hypertelescopes provide snapshot images with a significant gain in sensitivity without inducing any loss of the useful field of view for direct imaging applications. This paper aims at studying the properties of the point spread functions of future large arrays using the hypertelescope mode. Numerical simulations have been performed and criteria have been defined to study the image properties. It is shown that the choice of the configuration of the array is a trade-off between the resolution, the halo level and the field of view. A regular pattern of the array of telescopes optimizes the image quality (low halo level and maximum encircled energy in the central peak), but decreases the useful field of view. Moreover, a non-redundant array is less sensitive to the space aliasing effect than a redundant array. 相似文献
2.
Using one-minute cadence time-series full disk magnetograms taken by the SOHO/MDI, we have studied the magnetic field elements
at high latitude (poleward of 65° in latitude). It is found that an average lifetime of the magnetic field elements is 16.5
h during solar minimum, much longer than that during solar maximum (7.3 h). During solar minimum, number of the magnetic field
elements with the dominant polarity is about 3 times as that of the opposite polarity elements. Their lifetime is 21.0 h on
average, longer than that of the opposite polarity elements (2.3 h). It is also found that the lifetime of the magnetic field
elements is related with their size, consistent with the magnetic field elements in the quiet sun at low latitude found by
Hagenaar et al. (Astrophys. J. 511:932, 1999). During solar maximum, the polar regions are equally occupied by magnetic field elements with both polarities, and their
lifetimes are roughly the same on average. No evidence shows there is a correlation between the lifetime and size of the magnetic
field elements. Using an image cross-correlation method, we also measure the solar rotation rate at high latitude, up to 85°
in latitude. The rate is ω=2.914−0.342sin 2
φ−0.482sin 4
φ μrad s−1 sidereal. It agrees with previous studies using the spectroscopic and image cross-correlation methods, and also agrees with
the results using the element tracking method when the sample of the tracked magnetic field elements is large. The consistency
of those results strongly suggests that this rate at high latitude is reliable. 相似文献
3.
We present a novel numerical method that allows the calculation of nonlinear force-free magnetostatic solutions above a boundary
surface on which only the distribution of the normal magnetic field component is given. The method relies on the theory of
force-free electrodynamics and applies directly to the reconstruction of the solar coronal magnetic field for a given distribution
of the photospheric radial field component. The method works as follows: we start with any initial magnetostatic global field
configuration (e.g. zero, dipole), and along the boundary surface we create an evolving distribution of tangential (horizontal) electric fields
that, via Faraday’s equation, give rise to a respective normal-field distribution approaching asymptotically the target distribution.
At the same time, these electric fields are used as boundary condition to numerically evolve the resulting electromagnetic
field above the boundary surface, modeled as a thin ideal plasma with non-reflecting, perfectly absorbing outer boundaries.
The simulation relaxes to a nonlinear force-free configuration that satisfies the given normal-field distribution on the boundary.
This is different from existing methods relying on a fixed boundary condition – the boundary evolves toward the a priori given
one, at the same time evolving the three-dimensional field solution above it. Moreover, this is the first time that a nonlinear
force-free solution is reached by using only the normal field component on the boundary. This solution is not unique, but
it depends on the initial magnetic field configuration and on the evolutionary course along the boundary surface. To our knowledge,
this is the first time that the formalism of force-free electrodynamics, used very successfully in other astrophysical contexts,
is applied to the global solar magnetic field. 相似文献
4.
Questions of the equilibrium, stability, and observational manifestations of strange stars are considered, in which electrical
neutralization of the quark matter is provided by positrons, as occurs for some sets of bag parameters resulting in a stiffer
equation of state. Such models consist entirely of self-contained, strange quark matter and their maximum mass reaches 2.4–2.5
M⊙ with a radius of 13–14 km. The cooling of such strange quark stars both in the absence and in the presence of mass accretion
is investigated. It is shown that in the absence of mass accretion onto the strange star, the dependence of temperature (T,
K) on age (t, yr) depends very little on the mass of the configuration and has the form T ≈ 2.3·108r−1/5. If the star’s initial temperature is sufficiently high (T0≥2·1010K), then the total number of electron-positron pairs emitted does not depend on it and is determined only by the total mass
of the configuration. In the case of accretion, the annihilation of electrons of the infalling fatter with positrons of the
strange quark matter results in the emission of γ-rays with an energy of∼0.5 MeV, by observing which one can distinguish candidates
for strange stars. The maximum temperature of strange stars with mass accretion is calculated.
Translated from Astrofizika, Vol. 42, No. 4, pp. 617–630, October–December, 1999. 相似文献
5.
Compressible homogeneous spheres with constant adiabatic index γ were studied for their dynamical stability by Chandrasekhar
and he found that for each value of u (≡ mass to size ratio), there is a value of γ = γc, such that for γ < γc, the configuration is dynamically unstable. On examining the properties of the Chandrasekhar's spheres (homogeneous spheres
with constant γ) it is found that these spheres are non-isentropic, and the speed of sound within these spheres is finite.
The authors find that (i) for the causality condition to be fulfilled throughout the configuration, the value of γ ≤ [2/(surface
redshift)], (ii) for a given value of u, the binding coefficient, αr = (Mr -M)/M, vanishes for some value of γ = γb and for all the values of γ < γb the configurations are unbound, and (iii) for u≤ (1/3), one can find configurations which are bound, dynamically stable, and the speed of sound is less than that of light
throughout the configuration, whereas, for u >(1/3), the physically viable models of homogeneous density distribution are not possible. If the configuration is considered
to be isentropic, then both γ and the speed of sound become infinite throughout the configuration.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
N. B. Zheleznov 《Solar System Research》2010,44(2):136-143
The probability of an asteroid colliding with a planet can be estimated by the Monte Carlo method, in particular, through
the statistical simulation of the possible initial conditions for the motion of an asteroid based on the probability density
distribution set by the respective covariance matrix to be further projected with the orbital model onto the supposed time
point of the collision. Hence, the collision probability is calculated as the ratio between the number of projected (virtual)
asteroids striking the planet and their total number. The main problem is that different elements of the initial conditions
(orbit or state vector) are correlated and, therefore, cannot be simulated independently. These correlations are reflected
in the nondiagonal covariance matrix of the solution. The matrix is diagonalized by an orthogonal transformation. In the uncertainty
domain constructed from the diagonal matrix elements, the initial values for each of the six orbital elements are simulated
independently from the other elements, but with the accounting for their normal distribution. The program for calculating
the normal distribution is based on the central limit theorem. Each sample of the initial values for the six orbital elements
is transferred to the initial reference frame using an inverse transformation. Then, numerical integration is used to track
the asteroid’s motion along the respective orbit to predict a possible impact event. Asteroids 99942 Apophis and 2007 WD5
are used as examples to show that disregarding the correlations when diagonalizing the covariance matrix to set the initial
conditions may seriously distort the collision probability estimates. The paper gives the probabilities of the collisions
of Apophis with the Earth and asteroid 2007 WD5 with Mars calculated by the author from observation sets showing nonzero collision
probabilities. The author’s estimates are compared to those calculated by NASA. 相似文献
7.
C. Beveridge 《Solar physics》2006,236(1):41-57
Magnetic topology is a powerful tool for constraining certain physical properties of a given magnetic configuration, including
the strengths and locations of current sheets, relative helicity and the magnetic free energy available for reconnection.
A critical feature of magnetic topology is the separator, a field line bordering several different regions of connectivity.
With existing methods, these field lines are at best computationally expensive and at worst impossible to find. A new method
is presented for finding the Minimal Separator Set, all of the separators that necessarily exist in a configuration, and to
use this information in combination with the optical analogy and a simulated annealing method to ‘cool’ an initial guess for
each separator into a good approximation. 相似文献
8.
R. P. Kane 《Solar physics》2007,243(2):205-217
For many purposes (e.g., satellite drag, operation of power grids on Earth, and satellite communication systems), predictions of the strength of
a solar cycle are needed. Predictions are made by using different methods, depending upon the characteristics of sunspot cycles.
However, the method most successful seems to be the precursor method by Ohl and his group, in which the geomagnetic activity
in the declining phase of a sunspot cycle is found to be well correlated with the sunspot maximum of the next cycle. In the
present communication, the method is illustrated by plotting the 12-month running means aa(min ) of the geomagnetic disturbance index aa near sunspot minimum versus the 12-month running means of the sunspot number Rz near sunspot maximum [aa(min ) versus Rz(max )], using data for sunspot cycles 9 – 18 to predict the Rz(max ) of cycle 19, using data for cycles 9 – 19 to predict Rz(max ) of cycle 20, and so on, and finally using data for cycles 9 – 23 to predict Rz(max ) of cycle 24, which is expected to occur in 2011 – 2012. The correlations were good (∼+0.90) and our preliminary predicted
Rz(max ) for cycle 24 is 142±24, though this can be regarded as an upper limit, since there are indications that solar minimum
may occur as late as March 2008. (Some workers have reported that the aa values before 1957 would have an error of 3 nT; if true, the revised estimate would be 124±26.) This result of the precursor
method is compared with several other predictions of cycle 24, which are in a very wide range (50 – 200), so that whatever
may be the final observed value, some method or other will be discredited, as happened in the case of cycle 23. 相似文献
9.
Patrick Michel Giovanni H. Valsecchi 《Celestial Mechanics and Dynamical Astronomy》1996,65(4):355-371
We discuss the efficiency of the so-called mixed-variable symplectic integrators for N-body problems. By performing numerical experiments, we first show that the evolution of the mean error in action-like variables is strongly dependent on the initial configuration of the system. Then we study the effect of changing the stepsize when dealing with problems including close encounters between a particle and a planet. Considering a previous study of the slow encounter between comet P/Oterma and Jupiter, we show that the overall orbital patterns can be reproduced, but this depends on the chosen value of the maximum integration stepsize. Moreover the Jacobi constant in a restricted three-body problem is not conserved anymore when the stepsize is changed frequently: over a 105 year time span, to keep a relative error in this integral of motion of the same order as that given by a Bulirsch-Stoer integrator requires a very small integration stepsize and much more computing time. However, an integration of a sample including 104 particles close to Neptune shows that the distributions of the variation of the elements over one orbital period of the particles obtained by the Bulirsch-Stoer integrator and the symplectic integrator up to a certain integration stepsize are rather similar. Therefore, mixed-variable symplectic integrators are efficient either for N-body problems which do not include close encounters or for statistical investigations on a big sample of particles. 相似文献
10.
In the method of variation of parameters we express the Cartesian coordinates or the Euler angles as functions of the time
and six constants. If, under disturbance, we endow the “constants” with time dependence, the perturbed orbital or angular
velocity will consist of a partial time derivative and a convective term that includes time derivatives of the “constants”.
The Lagrange constraint, often imposed for convenience, nullifies the convective term and thereby guarantees that the functional
dependence of the velocity on the time and “constants” stays unaltered under disturbance. “Constants” satisfying this constraint
are called osculating elements. Otherwise, they are simply termed orbital or rotational elements. When the equations for the
elements are required to be canonical, it is normally the Delaunay variables that are chosen to be the orbital elements, and
it is the Andoyer variables that are typically chosen to play the role of rotational elements. (Since some of the Andoyer
elements are time-dependent even in the unperturbed setting, the role of “constants” is actually played by their initial values.)
The Delaunay and Andoyer sets of variables share a subtle peculiarity: under certain circumstances the standard equations
render the elements nonosculating. In the theory of orbits, the planetary equations yield nonosculating elements when perturbations
depend on velocities. To keep the elements osculating, the equations must be amended with extra terms that are not parts of the disturbing function [Efroimsky, M., Goldreich, P.: J. Math. Phys. 44, 5958–5977 (2003); Astron. Astrophys. 415, 1187–1199 (2004); Efroimsky, M.: Celest. Mech. Dyn. Astron. 91, 75–108 (2005); Ann. New York Acad. Sci. 1065, 346–374 (2006)]. It complicates both the Lagrange- and Delaunay-type planetary equations and makes the Delaunay equations
noncanonical. In attitude dynamics, whenever a perturbation depends upon the angular velocity (like a switch to a noninertial
frame), a mere amendment of the Hamiltonian makes the equations yield nonosculating Andoyer elements. To make them osculating,
extra terms should be added to the equations (but then the equations will no longer be canonical). Calculations in nonosculating
variables are mathematically valid, but their physical interpretation is not easy. Nonosculating orbital elements parameterise
instantaneous conics not tangent to the orbit. (A nonosculating i may differ much from the real inclination of the orbit, given by the osculating i.) Nonosculating Andoyer elements correctly describe perturbed attitude, but their interconnection with the angular velocity
is a nontrivial issue. The Kinoshita–Souchay theory tacitly employs nonosculating Andoyer elements. For this reason, even
though the elements are introduced in a precessing frame, they nevertheless return the inertial velocity, not the velocity
relative to the precessing frame. To amend the Kinoshita–Souchay theory, we derive the precessing-frame-related directional
angles of the angular velocity relative to the precessing frame. The loss of osculation should not necessarily be considered
a flaw of the Kinoshita–Souchay theory, because in some situations it is the inertial, not the relative, angular velocity
that is measurable [Schreiber, K. U. et al.: J. Geophys. Res. 109, B06405 (2004); Petrov, L.: Astron. Astrophys. 467, 359–369 (2007)]. Under these circumstances, the Kinoshita–Souchay formulae for the angular velocity should be employed (as
long as they are rightly identified as the formulae for the inertial angular velocity). 相似文献
11.
The rates of electron capture on heavier elements under the extreme conditions predicted for dwarf star supernovae have been computed, incorporating modifications that seem to be indicated by present experimental results. An estimate of the maximum possible value of such rates is also given. The distribution of nuclei in nuclear statistical equilibrium has been calculated for the range of expected supernovae conditions, including the effects of the temperature dependence of nuclear partition functions. These nuclide abundance distributions are then used to compute nuclear equilibrium thermodynamic properties. The effects of the electron capture on such equilibrium matter are discussed. The results of supernova numerical hydrodynamics incorporating the computed equilibrium properties and the influence of electron capture are presented. In the context of the ‘carbon detonation’ supernova model, the dwarf central density required to assure core collapse to a neutron star configuration is found to be slightly higher than that obtained by Bruenn (1972) with the electron capture rates of Hansen (1966). 相似文献
12.
D. D. Ryutov 《Astrophysics and Space Science》2011,336(1):21-26
It is shown that some aspects of the accretion disc physics can be experimentally simulated with the use of an array of properly
directed plasma jets created by intense laser beams. For the laser energy of 1 to 3 kJ, one can create a quasi-planar disc
with the Reynolds number exceeding 104 and magnetic Reynolds number in the range of 10–100. The way of seeding the disc with the magnetic field by using a cusp
magnetic configuration is described. 相似文献
13.
Masayoshi Sekiguchi 《Celestial Mechanics and Dynamical Astronomy》2004,90(3-4):355-360
The bifurcation of central configuration in the Newtonian N-body problem for any odd number N ≥ 7 is shown. We study a special
case where 2n particles of mass m on the vertices of two different coplanar and concentric regular n-gons (rosette configuration)
and an additional particle of mass m0 at the center are governed by the gravitational law he 2n+1 body problem. This system is of two degrees of freedom and permits
only one mass parameter μ =m
0/m. This parameter μ controls the bifurcation. If n≥ 3, namely any odd N ≥ 7, then the number of central configurations is three
when μ ≥ μ
c
, and one when μ ≥ μ
c
. By combining the results of the preceding studies and our main theorem, explicit examples of bifurcating central configuration
are obtained for N ≤ 13, for any odd N ∈ [15,943], and for any N ≥ 945. 相似文献
14.
S. C. Wilks H. Chen E. Liang P. Patel D. Price B. Remington R. Shepherd M. Tabak W. L. Kruer 《Astrophysics and Space Science》2005,298(1-2):347-355
We discuss the necessary requirements to create dense electron-positron plasmas in the laboratory and the possibility of using
them to investigate certain aspects of various astrophysical phenomena, such as gamma ray burst engines. Earth-based electron-positron
plasmas are created during the interaction of ultra-intense laser pulses impinging on a solid density target. The fact that
positrons can be generated during this interaction has already been demonstrated by Cowan et al. (2000). However, several
questions concerning the number, energy, and dynamics of these positrons have yet to be answered. Through insight gathered
from PIC simulations, we postulate that the e+e− plasma leaves the creation region in dense jets, with relativistic energies. In order to estimate the number density of the
positrons created, we begin by first experimentally measuring the hot electron temperatures and densities of such interactions
using a compact electron spectrometer. Once the electron distribution is known, the positron creation rate, Γ, can be estimated.
This same experimental diagnostic can also, with minor modification, measure the energy distribution of positrons. Initial
estimates are that, with proper target and laser configurations, we could potentially create one of the densest arraignments
of positrons ever assembled on earth. This experimental configuration would only last for a few femtoseconds, but would eventually
evolve into astrophysically relevant pure electron-positron jets, possibly relevant to e+e− outflow from black holes. 相似文献
15.
In addition to the detection of an asteroid moon or a binary asteroid, the knowledge of the satellite’s true orbit is of high importance to derive fundamental physical parameters of the binary system such as its mass and to shed light on its possible formation history and dynamical evolution (prograde/retrograde orbit, large/small eccentricity or inclination, etc.). A new methodology for preliminary orbit determination of binary asteroids – and visual binaries in general – is proposed. It is based on Thiele–Innes method combined with a ‘trial and error’ Monte-Carlo technique. This method provides the full set of solutions (bundle of orbits, with the 7 orbital elements) even for a reduced number of observations. The mass is a direct by-product of this orbit determination, from which one can next infer the bulk-density and porosity. In addition to the bundle of orbits, the method provides the marginal probability densities of the foreseen parameters. Such error analysis – since it avoids linear approximation – can be of importance for the prediction of the satellite’s position in the plane-of-sky during future stellar occultations or subsequent observations, but also for the analysis of the orbit’s secular evolution. After briefly describing the method, we present the algorithm and its application to some practical cases, with particular emphasis on asteroids binaries and applications on orbital evolution. 相似文献
16.
Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band. 相似文献
17.
In this paper we seek the origin of the axial component of the magnetic field in filaments by adapting theory to observations.
A previous paper (Mackay, Gaizauskas, and van Ballegooijen, 2000) showed that surface flows acting on potential magnetic fields
for 27 days – the maximum time between the emergence of magnetic flux and the formation of large filaments between the resulting
activity complexes – cannot explain the chirality or inverse polarity nature of the observed filaments. We show that the inclusion
of initial helicity, for which there is observational evidence, in the flux transport model results in sufficiently strong
dextral fields of inverse polarity to account for the existence and length of an observed filament within the allotted time.
The simulations even produce a large length of dextral chirality when just small amounts of helicity are included in the initial
configuration. The modeling suggests that the axial field component in filaments can result from a combination of surface
(flux transport) and sub-surface (helicity) effects acting together. Here surface effects convert the large-scale helicity
emerging in active regions into a smaller-scale magnetic-field component parallel to the polarity inversion line so as to
form a magnetic configuration suitable for a filament. 相似文献
18.
The photometric elements of the Algol type binary TT Hydrae derived by the authors from theirUBV observations during 1973–77 have been combined with the spectroscopic elements given by Sanford (1937) and Sahade and Cesco
(1946) to obtain the absolute dimensions of the system. It is found that the spectroscopic orbital elements given by Sanford
represent the evolutionary status of the secondary component better than those of Sahade and Cesco. The primary appears to
be an Al v main sequence star of mass
and radius ∼2.3R
⊙. The secondary fills its Roche lobe; it can be represented by a K0iii star of mass
and radius ∼6.0R
⊙. Better spectroscopic data are needed for confirmation of these results. 相似文献
19.
We consider the problem of 4 bodies of equal masses in R
3 for the Newtonian r−1 potential. We address the question of the absolute minima of the action integral among (anti)symmetric loops of class H
1 whose period is fixed. It is the simplest case for which the results of [4] (corrected in [5]) do not apply: the minima cannot
be the relative equilibria whose configuration is an absolute minimum of the potential among the configurations having a given
moment of inertia with respect to their center of mass. This is because the regular tetrahedron cannot have a relative equilibrium
motion in R
3 (see [2]). We show that the absolute minima of the action are not homographic motions. We also show that if we force the
configuration to admit a certain type of symmetry of order 4, the absolute minimum is a collisionless orbit whose configuration
‘hesitates’ between the central configuration of the square and the one of the tetrahedron. We call these orbits ‘hip-hop’.
A similar result holds in case of a symmetry of order 3 where the central configuration of the equilateral triangle with a
body at the center of mass replaces the square.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
Spotless days (i.e., days when no sunspots are observed on the Sun) occur during the interval between the declining phase of the old sunspot
cycle and the rising phase of the new sunspot cycle, being greatest in number and of longest continuous length near a new
cycle minimum. In this paper, we introduce the concept of the longest spotless segment (LSS) and examine its statistical relation
to selected characteristic points in the sunspot time series (STS), such as the occurrences of first spotless day and sunspot
maximum. The analysis has revealed statistically significant relations that appear to be of predictive value. For example,
for Cycle 24 the last spotless day during its rising phase should be about August 2012 (± 9.1 months), the daily maximum sunspot
number should be about 227 (± 50; occurring about January 2014±9.5 months), and the maximum Gaussian smoothed sunspot number
should be about 87 (± 25; occurring about July 2014). Using the Gaussian-filtered values, slightly earlier dates of August
2011 and March 2013 are indicated for the last spotless day and sunspot maximum for Cycle 24, respectively. 相似文献