共查询到20条相似文献,搜索用时 109 毫秒
1.
R. Samadi M.J. Goupil Y. Lebreton Å Nordlund F. Baudin 《Astrophysics and Space Science》2003,284(1):221-224
The excitation rate P of solar p-modes is computed with a model of stochastic excitation which involves constraints on the averaged properties of the solar turbulence. These constraints are obtained from a 3D simulation. Resulting values for P are found 4.5 times larger than when the calculation assumes properties of turbulent convection which are derived from an 1D solar model based on Gough (1977)'s formulation of the mixing-length theory (GMLT). This difference is mainly due to the assumed values for the mean anisotropy of the velocity field in each case.Calculations based on 3D constraints bring the P maximum closer to the observational one.We also compute P for several models of intermediate mass stars (1 M 2 M).Differences in the values of P
max between models computed with the classical mixing-length theory and GMLT models are found large enough for main sequence stars to suggest that measurements of P in this mass range will be able to discriminate between different models of turbulent convection. 相似文献
2.
A model of 5M
hydrogen-helium star with no metals has been considered. The properties of the adiabatic oscillations were obtained, and the vibrational instability of the model has been investigated. The model is a double-shell burning star. Calculations were done for the fundamental mode as well as the first and the second harmonics. The model has been found vibrationally unstable against radial adiabatic pulsations. 相似文献
3.
Raj Bali 《Astrophysics and Space Science》1985,116(1):81-88
The object of this paper is to investigate the behavior of a magnetic field in a viscous fluid cosmological model. It has been assumed that the expansion () is proportional to the eigenvalue
1
of the shear tensor
i
j
and the coefficient of shearing viscosity is proportional to the scalar of expansion. The paper also discusses the behavior of the model when the magnetic field tends to zero and comments on some other physical properties. 相似文献
4.
The structure and stability of rapidly uniformly rotating supermassive stars is investigated using the full post-Newtonian equations of hydrodynamics. The standard model of a supermassive star, a polytrope of index three, is adopted. All rotation terms up to and including those of order 4, where is the angular velocity, are retained. The effects of rotation and post-Newtonian gravitation on the classical configuration are explicitly evaluated and shown to be very small. The dynamical stability of the model is treated by using the binding energy approach. The most massive objects are found to be dynamically unstable when =1/c
2.p
c
/
c
2.2 × 10–3, wherep
c
and
c
are the central pressure and density, respectively. Hence, the higher-order terms considered in this analysis do not appreciably alter the previously known stability limits.The maximum mass that can be stabilized by uniform rotation in the hydrogen-burning phase is found to be 2.9×106
M
, whereM
is the solar mass. The corresponding nuclear-generated luminosity of 6×1044 erg/sec–1 is too small for the model to be applicable to the quasi-stellar objects. The maximum kinetic energy of a uniformly rotating supermassive star is found to be 3×10–5
Mc
2, whereM is the mass of the star. Masses in excess of 1010
M
are required if an adequate store of kinetic energy is to be made available to a pulsar like QSO. However such large masses have rotation periods in excess of 100 yr and thus could not account for any short term periodic variability. It is concluded then that the uniformly rotating supermassive star does not provide a suitable base for a model of a QSO. 相似文献
5.
We present an attempt for an extension of the modified Boltzmann model, which was introduced by Callebautet al. (1982) as an improvement of the polytropic models, to the case of chemically-heterogeneous stars in equilibrium, containing H and He, by proposing a density profile of the formp=C
1
T
N
exp (–H
m(–*)/kT) +C
2
T
N
exp (–He
m(–*)/kT. Analytical properties are derived and numerical as well as analytical arguments are presented for the conclusion that this hypothesis for a density profile imposes an almost constant chemical profile to the model as a whole, thereby making it in this form unsuited for the study of heterogeneous stars. A comparison is made with the former Boltzmann model in the homogeneous limit. 相似文献
6.
Enrique Guzmán 《Astrophysics and Space Science》1991,179(2):331-334
In this paper we have obtained the general vacuum solution for Bianchi type-II in the Brans-Dicke theory for total anistropyR
1R
2R
3. It is known that by use of our method, we can find the general solution for Bianchi type-II vacuum case in the general relativity theory, first given by Taub (1951). Some physical properties of this model are also discussed. 相似文献
7.
H. J. Fahr 《Solar physics》1973,30(1):193-206
The effect of a new energy source due to energies transferred from supra-thermal secondary ions on the temperature profile of the solar wind has been considered. For this purpose a solution of a tri-fluid model of the solar wind including solar electrons, protons, and -particles, and starting with the boundary conditions of Hartle and Barnes at 0.5 AU is given. On the base of the assumption that suprathermal He+-ions which have four times the temperature of suprathermal protons are predominantly coupled to solar -particles by Alfvén waves, it is shown that the temperature T
of solar -particles should be appreciably higher than those T
p of solar protons beyond the orbit of the Earth. For 1 AU a temperature excess T
over T
p according to that which has been found in some solar wind ion spectrograms can only be explained for a small part of the orbit of the earth which is inside the cone of enhanced helium densities. Around 1 AU the temperatures T
and T
p are found to decrease much slighter with solar distance than given in the two-fluid model of Hartle and Barnes. Beyond 1.7 and 2.2 AU the temperatures T
and T
p even start increasing with solar distance and come up to about 105 at about 10 AU. These predictions should lend some support to future temperature measurements with deep-space probes reaching Solar distances of some AU.Forschungsberichte des Astronomischen Institutes, Bonn, 72-10. 相似文献
8.
A study of pulsational properties with evolution has been done for a 15.6M
star withX
e
=0.90 andY
e
=0.08. Pulsational properties in the hydrogen-burning stages have been compared with those in helium-burning stages. A comparison with observed characteristics of Cepheids, classical Cepheids and supergiant variables has been made during the course of its evolution. In addition, models of 5,9 and 15M
withX
e
=0.708,Y
e
=0.272 have also been studied for pulsational properties during the helium burning stage. It is also seen that pulsational instability is sensitive to changes in initial chemical composition and opacity parameters,n ands. A low helium abundance could be a reason for the stability of the models, even when lying in the instability strip of the H-R diagram. 相似文献
9.
Yu. A. Fadeyev 《Astrophysics and Space Science》1984,100(1-2):329-339
The radial nonlinear pulsations of a model withM=0.8M
,M
bol=–6 mag andT
eff=5500 K have been studied. The pulsations are shown to exist in the form of the standing wave only in the innermost layers withR<0.2R
ph. In the outer layers, the standing wave transforms into running waves, the frequency of which decreases with an increasingR. the pulsation period at the photosphere is found to be twice as long as the pulsation period atR<0.2R
ph. The difference between the pulsation periods causes alternation of deep and shallow minima in the temporal dependences of the kinetic energy and radii and can be used to explain the nature of RV Tau variables. It is shown that at the distanceR>3R
ph, the time-independent mass flux caused by shocks takes place. The rate of mass loss is found to beM10–4
M
yr–1. 相似文献
10.
Ronald E. Taam 《Astrophysics and Space Science》1981,77(1):257-265
The properties of the hydrogen burning shell in the envelope of an accreting neutron star have been studied for a range of mass accretion rates, neutron star radii, and metal abundances of the accreted matter. It is found that the hydrogen burning shells lie at densities ranging from 105–6×106 gm cm–3. For mass accretion rates in excess ofM
c2 hydrogen and helium burn together. ForM
c1MM
c2, the hydrogen burning shell is stabilized by the limited CNO cycle. Implications of these results to the X-ray burst phenomena are briefly discussed. 相似文献
11.
The magnetic field in solar active regions forms a highly structured pattern without an apparent length scale. We study this pattern in detail for a plage and its surroundings observed with the Swedish Solar Observatory on La Palma. The magnetogram has a resolution of about 1/3, after image optimisation. We analysed the geometric properties of isolated patches of magnetic flux. Patches with a linear size up to 3 appear to be statistically self-similar, with a fractal dimension ofD
f
= 1.54 ± 0.05 for the relation between area and linear size. This value agrees very well with the dimensionD
f
= 1.56 which is found in percolation theory for clusters of tracers placed randomly on a lattice with a tracer density below a critical threshold. The distribution of observed cluster areas also agrees with that of clusters on such a random lattice. The correspondence between properties of observations and of clusters on randomly filled lattices suggests that- well after emergence - the magnetic flux on the Sun is randomly distributed at least up to sizes of about 3 and possibly larger. 相似文献
12.
A critical examination of the components of the recent impulsive loop flare model of Takakura is made. It is found that his analysis of the stability of the electron distribution resulting from anomalous heat conduction is in error and electron plasma waves would not be excited. Rather, in the regions where the electron/proton temperature ratioT
e/T
i 10, electrostatic ion-cyclotron waves would be excited and in the regions whereT
e 10, ion-acoustic waves would be excited. Ratios ofT
e/T
i 10 occur only in the late time development behind the conduction fronts. Since the anomalous resistivity due to electrostatic ion-cyclotron waves is fortuitously about 70% of the one used by Takakura, the general development will follow closely the one calculated by him. Because the anomalous resistivity due to ion-acoustic waves is about 95 times the one used by Takakura, the development in the parts of the loop whereT
e/T
i 10 for late times would be altered considerably.Also Guest Worker at NOAA Space Environment Laboratory, Boulder, Colorado, U.S.A. 相似文献
13.
J. E. Lancaster 《Celestial Mechanics and Dynamical Astronomy》1970,2(4):481-493
Analytical techniques are employed to demonstrate certain invariant properties of families of moon-to-earth trajectories. The analytical expressions which demonstrate these properties have been derived from an earlier analytical solution of the restricted three-body problem which was developed by the method of matched asymptotic expansions. These expressions are given explicitly to orderµ
1/2 where is the dimensionless mass of the moon. It is also shown that the inclusion of higher order corrections does not affect the nature of the invariant properties but only increases the accuracy of the analytic expressions.The results are compared with the work of Hoelker, Braud, and Herring who first discovered invariant properties of earth-to-moon trajectories by exact numerical integration of the equations of motion. (Similar properties for moon-to-earth trajectories follow from the principle of reflection). In each instance the analytical expressions result in properties which are equivalent, to orderµ
1/2, with those found by numerical integration. Some quantitative comparisons are presented which show the analytical expressions to be quite accurate for calculating particular geometrical characteristics.
Nomenclature
Orbital Elements near the Moon energy - angular momentum - semi-major axis - eccentricity - inclination - argument of node - argument of pericynthion Orbital Elements near the Earth h e energy - l e angular momentum - i inclination - argument of node - argument of perigee - t f time of flight Other symbols parameters used in matehing - U a function of the energy near the earth - a function of the angular momentum near the earth - r p perigee radius - perincynthion radius - radius at node near moon - true anomaly of node near moon - initial angle between node near moon and earth-moon line - a function ofU, , andi - earth phase angle - dimensionless mass of the moon - U 0, U1 U=U 0+U 1 - i 0, i1/2, i1 i=i 0+µ 1/2 i 1/2+µ i 1 - 0, 1/2, 1 = 0+µ 1/2 i 1/2+µ i 1 - p longitude of vertex line - n latitude of vertex line - R o ,S o ,N o functions ofU 0 and - a function ofU 0, and 相似文献14.
Calculations are made for the center-limb variations of the K2 and K3 components of the solar Ca ii K line using an optically thick model of the chromosphere. The center-limb variations are shown to require an increase of Doppler width with height in the chromosphere and to depend critically upon the location of the point where
D has increased by a factor e. Good agreement with observations is found when, and only when, the increase in
D occurs nearly simultaneously with the increase in chromospheric temperature. 相似文献
15.
E. C. Bowers 《Astrophysics and Space Science》1973,21(2):399-423
Starting from the Vlasov equation the steady state and stability properties of the electron sheet in the Cowley neutral sheet model of the geomagnetic tail are considered. Electrostatic ion plasma oscillations propagating from dusk to dawn are found to be unstable provided the thermal spread normal to the current is sufficiently large. Assuming the population of the neutral sheet to be supplied by the polar wind it is shown how a localisation of the cross tail electric field could lead to the instability first appearing around midnight. It is conjectured that the localisation of the cross tail electric field could continually feed the instability, so leading to enough turbulence to give enhanced reconnection of the magnetic field.List of symbols
f
distribution function
-
B
magnetic field strength far from the neutral sheet
-
a
sheet half thickness
-
total potential drop across the tail which is localised to the dusk end of the tail in Cowley's model
-
potential for the steady state electric field normal to the electron current sheet. This potential exists in that region of the tail that excludes the localised region of cross tail electric field
-
average velocity across the tail of electrons in the current sheet
-
v
average velocity of the electrons normal to the current sheet
-
p
canonical momentum of a particle
-
energy of a particle
-
KT
electron energy normal to the sheet (1/2m
e
v
2
)
-
KT
i
ion energy (1/2m
i
V
2
)
-
electron gyrofrequency far from the neutral sheet
- i
ion gyrofrequency far from the neutral sheet
-
Ay
steady state vector potential for the magnetic field
-
A
–Ay/aB
0 (normalised vector potential)
When perturbing the steady state, dashes have been used to denote the time dependent first order quantities. Where no confusion could arise the dashes are dropped, e.g.Ey=Ey since there is no zero orderEy in the region considered in the stability analysis. 相似文献
16.
An idea is developed that the vacuum in the gravitational field acquires properties of an elastic medium described by a definite tension
ik
. The vacuum is stated to also participate in the formation of the space-time metric, together with the usual matter. So, the matter, vacuum and metric form a complex unity determined by the solution of the field equations. The vacuum may prove to play an essential role in the extremely strong fields existing in superdense celestial bodies. The tensor
ik
is not to be identified with the pseudo-tensor of the energy-momentum of the gravitational field the idea of which is preserved.The problem of vacuum is investigated in the case of the central symmetry static field. A number of properties of the tensor
ik
is found using the symmetry of the field and comparison with the post-Newton limit. The external and internal problems, as well as the procedure of joining the solutions on the surface of a celestial body, have been formulated. The stellar surface is determined in the usual way:P(r) = 0 whereP is the matter pressure. The theory includes three dimensionless parametersa=p/,b=p
/ (,p, p
are the density of the vacuum energy and of its pressures in the radial and transverse directions) and determining the vacuum elastic properties. Generally speaking, they depend on the valueP/c2 in the stellar centre where is the mass density. From general physical considerations it is shown that 0 1 + lim
P (l/q). The field equations are solved for the simple version of the theoryb=–a. There are solutions corresponding to superdense celestial bodies with masses considerably exceeding that of the Sun. 相似文献
17.
D. L. Lambert 《Solar physics》1968,3(1):118-124
A simplified representation of the temperature distribution in the solar photosphere is proposed: (
0) =
0 -
1 log
0. An expression is derived for the emergent continuous spectrum from the simple model. The limitations and applications of the simple model are discussed. 相似文献
18.
W. -H. Ip 《Astrophysics and Space Science》1974,31(1):57-79
The problem of planetary accretion in a jet stream is studied using the model developed by Alfvén and Arrhenius. We find that there are basically three types of planetary accretion: namely, fast process
c
<
i
, slow process
c
~
i
and delayed process
c
>
i
where
c
is the characteristic time of the occurrence of catastrophic accretion and
i
the time-scale of mass injection to the planetary system (3×108 yr). These different time scales of accretion are found to be closely related to the primordial thermal profiles and equatorial inclinations of the planets. Finally, Venus' retrograde rotational spin is shown to be a possible result of accretion process in a jet stream. 相似文献
19.
R. V. Isakov A. A. Ruzmaikin D. D. Sokoloff M. V. Faminskaya 《Astrophysics and Space Science》1981,80(1):145-155
The asymptotic properties of a turbulent disk dynamo at large dimensionless numbersR
andR
characterizing the helicity and the differential rotation are analysed. Three types of generations in the dependence of the relations betweenR
andR
are found: 2-dynamo and two types of -dynamo. For each of these types the rates of growth are obtained and the forms of solution are pointed out. Boundaries of the disk dynamo approximation are given. 相似文献
20.
Chuan-Yi Tu 《Solar physics》1971,109(1):149-186
A new solar wind model has been developed by including in the model the Alfvénic fluctuation power spectrum equation proposed by Tu et al. (1984). The basic assumptions of the model are as follows: (1) for heliocentric distances r > 10 R
, the radial variation of the power spectrum of Alfvénic fluctuations is controlled by the spectrum equation (1), (2) for heliocentric distances r < 10 R
, the radial variation of the fluctuation amplitude is determined by the Alfvén wave WKB solution, (3) no energy cascades from the low-frequency boundary of the Alfvénic fluctuation power spectrum into the fluctuation frequency range, and the energy which cascades from the high-energy boundary of the spectrum into the higher frequency range is transported to heat of the solar wind flow. Some solutions of this model which, on one hand, describe the major properties of the Alfvénic fluctuations and the high-speed flow observed by Helios in the space range between 0.3–1 AU and, on the other hand, are consistent with the observational constraints at the coronal base have been obtained under the following conditions: (1) the spectrum index of the fluctuations is near to -1 for almost the whole frequency range at 10 R
, (2) the particle flux density at 1 AU is not greater than 3 × 108 cm–2 s–1, (3) the solution is for spherically-symmetric flow geometry or the solution passes through the outermost of the three critical points of the rapidly diverging flow geometry with f
max = 7. Some solutions passing through the innermost critical point of the rapidly diverging flow geometry with f
max = 7 have been found, however, with too low pressure at the coronal base to compare with the observational constraints. Heat addition or other kind of momentum addition for r < 10 R
is required to modify this model to yield better agreement with observations. A cascade energy flux function which leads to Kolmogorov power law in the high-frequency range of Alfvénic fluctuations is presented in Appendix A. More detailed discussions about the characteristics, the boundary conditions and the solution of the spectrum equation (1) are given in Appendix B.Projects supported by the Science Fund of the Chinese Academy of Sciences. 相似文献