共查询到20条相似文献，搜索用时 31 毫秒
1.
Stanisław P. Kasperczuk 《Celestial Mechanics and Dynamical Astronomy》2000,76(4):215-227
In a recent paper Ballersteros and Ragnisco (1998) have proposed a new method of constructing integrable Hamiltonian systems.
A new class of integrable systems may be devised using the following sequence:
, where A is a Lie algebra
is a Lie–Poisson structure on R
3, C is a Casimir for
is a reduced Poisson bracket and (A, ▵) is a bialgebra. We study the relation between a Lie-Poisson stucture Λ and a reduced Poisson bracket
, which is a key element in using the Lie algebra A to constructing this sequence. New examples of Lie algebras and their
related integrable Hamiltonian systems are given.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Hα luminosities of a sample of galaxies in nearby compactgroups are presented. Our purpose is to study the influence of thegroup
environment on the star formation rates (SFRs) of the galaxies in thegroups, provided that the Hα luminosity is a good tracer of theSFR of disc galaxies. Measuring the global L
/L
B of the groups – including early-type galaxies – we find that the average value of the Hα emission is not significantly different from thatmeasured for field galaxies, and that most of the groups that show thehighest
level of L
/L
B, with respect to a set of synthetic groups built out of field galaxies, show tidal features in at least one of their members.
Finally, we have exploredthe relationship between the ratio L
/L
B and severalrelevant dynamical parameters of the groups (velocity dispersion, crossingtime, radius and mass-to-luminosity
ratio) and have found no clearcorrelation. This suggests that the exact dynamical state of a groupdoes not appear to control
the SFR of the group as a whole.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
R. Louise 《Astrophysics and Space Science》1982,86(2):505-508
The model is based on the assumption that logarithmic spiral arms of most spiral galaxies are due to the totating density wave
obeying the steady wave equation
. It is shown that this equation accounts also for the so-called Titius-Bode's law in the solar system. 相似文献
4.
5.
N.A. Silant'ev 《Astrophysics and Space Science》1997,249(1):95-109
Different techniques of calculation and estimation of turbulentdiffusivities D
of passive fields in infinite incompressiblemedia showing isotropic, homogeneous and stationary turbulence areconsidered. The numerical comparison of the methods is made mostlyfor two representative limiting models of turbulence – withpeak-like spectrum and for vast Kolmogorov's type spectrum. The timedependence of two-point velocity correlators is assumed to beexponential. The backbone of the comparison is steady-state turbulentdiffusivities calculated in nonlinear DIA-approach with thecorrections due to contribution of four-order velocity correlators.As a result of this comparison the most satisfactory approximatemethods are proposed both for steady-state and time-dependentturbulent diffusivities. The results may be used for the most correctchoice of
-dependence in various astrophysical problems. 相似文献
6.
H. Rovithis-Livaniou 《Astrophysics and Space Science》1977,52(2):271-306
The aim of the present paper is to find the eclipse perturbations, in the frequency-domain, of close eclipsing systems exhibiting partial eclipses.After a brief introduction, in Section 2 we shall deal with the evaluation of thea
n
(l)
integrals for partial eclipses and give them in terms ofa
0
0
,a
0
0
(of the associated -functions) and
integrals; while Section 3 gives the eclipse perturbations
arising from the tidal and rotational distortion of the two components. The
are given for uniformly bright discs (h=1) as well as for linear and quadratic limb-darkening (h=2 and 3, respectively).Finally, Section 4 gives a brief discussion of the results and the way in which they can be applied to practical cases. 相似文献
7.
I. Stellmacher 《Celestial Mechanics and Dynamical Astronomy》1999,75(3):185-200
The motion of Hyperion is an almost perfect application of second kind and second genius orbit, according to Poincaré’s classification.
In order to construct such an orbit, we suppose that Titan’s motion is an elliptical one and that the observed frequencies
are such that 4n
H−3n
T+3n
ω=0, where n
H, n
T are the mean motions of Hyperion and Titan, n
ω is the rate of rotation of Hyperion’s pericenter. We admit that the observed motion of Hyperion is a
periodic motion
such as
. Then,
.N
H, N
T, k∈ N
+. With that hypothesis we show that Hyperion’s orbit tends to a particular periodic solution among the periodic solutions
of the Keplerian problem, when Titan’s mass tends to zero. The condition of periodicity allows us to construct this orbit
which represents the real motion with a very good approximation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
9.
Maxim Lyutikov 《Astrophysics and Space Science》1998,264(1-4):411-421
A theory of pulsar radio emission generation, in which the observed waves are produced directly by the maser-type plasma instabilities
on the anomalous cyclotron-Cherenkov resonance
and the Cherenkov-drift resonance
, is capable of explaining the main observational characteristics of pulsar radio emission. The instabilities are due to the
interaction of the fast particles of the primary beam and from the tail of the distribution with the normal modes of a strongly
magnetized one-dimensional electron-positron plasma. The waves emitted at these resonances are vacuum-like electromagnetic
waves that may leave the magnetosphere directly. The cyclotron-Cherenkov instability is responsible for core emission pattern
and the Cherenkov-drift instability produces conal emission. The conditions for the development of the cyclotron-Cherenkov
instability are satisfied for the both typical and millisecond pulsars provided that the streaming energy of the bulk plasma
is not very high γ
p
= 5 ÷ 10. In a typical pulsar the cyclotron-Cherenkov and Cherenkov-drift resonances occur in the outer parts of magnetosphere
at r
res
≈ 109cm. This theory can account for various aspects of pulsar phenomenology including the morphology of the pulses, their polarization
properties and spectral behavior.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
S. A. Stephens 《Astrophysics and Space Science》1981,76(1):87-103
A reliable representation to the invariant cross-section for the production of antiprotons (
) in inclusive reactions has been obtained, which fits the data extremely well from threshold to ISR energies. Using this, the production spectrum of
by cosmic ray interaction with interstellar gas is calculated and is compared with other existing calculations. The equilibrium spectrum
in the Galaxy has been derived from about 100 MeV to a few hundred GeV in kinetic energy using Leaky Box Model for the propagation of cosmic rays, by taking into account all energy loss processes. It is found that
/P ratio calculated here is very much smaller than the observed ratio. In view of the fact that this excess of observed
/P ratio is much larger than the measured upper limits on the fraction of antimatter in the form of antinuclei, the excess
/P ratio has not been attributed to the existence of antimatter. Instead, it is shown that Closed Galaxy Model for the propagation of cosmic rays predicts more antiprotons than observed. Therefore, it is suggested that if cosmic rays contain about 50% new component of local origin, the resulting Closed Galaxy Model explains well the observations. The present calculations also predict too small a flux of
below a few hundred MeV to make this energy region ideally suited to look for antiprotons of primordial origin. 相似文献
11.
The motion of spin particles in the Schwarzschild field is examined in this paper. It is shown that Mathisson-Papapetrou equations under additional conditions
, where
is the Lie derivative of the Killing vector
j
, permit only radial motion, motion in the equatorial plane and in the equilibrium points. All these types of motion are considered more in detail. 相似文献
12.
Werner Lohmann 《Astrophysics and Space Science》1977,51(1):173-176
The very young open star cluster NGC 2362 was investigated by the strip method on charts of two photographs taken with the 1-m Schmidt telescope of the European Southern Observatory. Up to the limiting magnitudeM
v
*
=5
.
m
8 the cluster contains 100 stars and can be described by the Gaussian density law (6). Further results are: Mass
= 246
, central mass density
0 = 43.1
= 246
pc-3 , radiusR2.6 pc, mean velocity of the stars
= 0.64 km s–1.
Mitteilungen Serie A. 相似文献
Mitteilungen Serie A. 相似文献
13.
J. Faulkner 《Astrophysics and Space Science》2003,285(2):339-339
The impact of Fred Hoyle's work on the structure and evolution of red giants, particularly his breakthrough contribution with
Martin Schwartzschild (1955), is described and assessed. Working with his students in the early 1960s, Hoyle presented new
physical ways of understanding some of the approximations used, and results obtained, in that seminal paper. His initial viewpoint
on the critical role of the outer surface boundary condition was replaced by a more subtle, if related one, which emphasized
the peculiar difficulty of storing much mass outside a dense stellar core. That viewpoint that – low-mass red giants are essentially
white dwarfs with a serious mass-storage problem – is still extremely fruitful. Recently, I have extended Hoyle's approach
to explain not only many of the structural properties of red giants themselves, but also to link and unify the structures
of low-mass stars from the main sequence through both the red giant and horizontal branch phases of evolution. Many aspects
of these stars that had remained mysterious for decades have now fallen into place, and some questions have been answered
that were not even posed before. With red giants as the simplest example, this recent work emphasizes that stars, in general,
may have at least two distinct but very important centres: (i) a geometrical centre, and (ii) a separate nuclear centre, residing in a shell outside a zero-luminosity dense core for example. This two-centre perspective leads to an explicit,
analytic, asymptotic theory of low-mass red giant structure. In this theory, there arises a naturally important in situ measure of central compactness: the parameter
. That parameter, like others, is derived self-consistently and explicitly, and can be used to show how close a given model's
properties are to ultimate asymptotic relationships. The results obtained also imply that the problem of understanding why
such stars become red giants is one of anticipating a remarkable yet natural structural bifurcation which occurs in them.
In the resulting theory, both the ratio
and products like
prove to be important, self-consistently derived quantities. Two striking theorems involving such quantities express between
them the very essence of red giant behaviour, proving analytically for the first time that stars with dense cores are necessarily
(i) extremely luminous, and (ii) very large. Perhaps the most astonishingly unexpected single result is that for the very
value Nature provides for the relevant nuclear energy-generating temperature exponent (CNO's η=15), ρsh and
behave in a well-defined, precisely inverse manner. This emphasizes that the internal behaviour of such stars is definitely anti-homologous rather than homologous, thus showing how very unfortunate the term `shell homology' is. Finally, I sketch a viewpoint which (i) links the structural
and evolutionary behaviour of stars from the main-sequence through horizontal branch phases of evolution, and also (ii) has
implications for post-main-sequence developments in more massive stars.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
M. Aslam Chaudhry 《Astrophysics and Space Science》1998,262(3):263-270
Two representations of the extended gamma functions
[(b,x)] are proved. These representations are exploited to find a transformation relation between two Fox's H-functions. These results are used to solve Fox's H-function in terms of Meijer's G-function for certain values of the parameters. A closed form representation of the kernel of the Bessel type integral transform is also proved. 相似文献
15.
A linear excitation of electromagnetic modes at frequencies
, in a plasma through which two electron beams are contra-streaming along the magnetic field is investigated. This may be a source of the observed
emissions at auroral latitudes. 相似文献
16.
D. C. Schwank 《Astrophysics and Space Science》1976,43(2):459-468
Weight functions for the determination of the periods of linear adiabatic non-radial oscillations have been calculated in the same manner as Epstein's classic treatment of purely radial oscillations. Quadrupole (l=2) oscillations for thef and lower orderp andg-modes were considered. One group of static models were polytropes in the range 1.0n4.0 with
; thus included were configurations that were convectively stable, unstable and neutrally stable throughout. Another group consisted ofn=3.0 polytropes with convective shells or convective cores; 1 was set at different values in each region in order to produce stability (
) or instability (
). The weight function provides a pictorial means for assessing the relative importance of each region of a given static model with respect to generating a given non-radial mode. 相似文献
17.
I. H. Urch 《Astrophysics and Space Science》1977,49(2):443-472
The diffusion of charged particles through a weak stochastic electro-magnetic field which is superimposed on a constant background magnetic field
is considered. The stochastic electromagnetic fields are assumed to consist of unpolarized Alfvén waves propagating at arbitrary angles to the direction
. When the Alfvén waves are propagating in directions other than
and the particle gyro-radius,r
g, is sufficiently large (but may be smaller than the correlation length of the stochastic fields) it is shown that the particle flux perpendicular to the direction
is
, wherev is the particle speed andf the particle density. The expression forK
differs from those calculated by previous authors. For small particle gyro-radii the flux S has a different functional form and is identical to that found by Urch (1977) to describe particle diffusion when the Alfvén waves only propagate in the direction
. 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.
20.
E. Evangelidis 《Astrophysics and Space Science》1977,46(2):309-319
The dispersion relation has been derived for density waves propagating at an arbitrary angle. The analysis has shown the existence of a resonance which for a two-arm galaxy can be stable, neutral or unstable as
}2$$
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, respectively. 相似文献