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1.
A viscous fluid cosmological model in presence of magnetic field and zero-mass scalar fields is developed. The non-negativity condition of viscous fluid pressure prescribes a certain minimum value oft vis-a-vis of the scale factorQ(t) and at this epoch the model is found to be singularity free. 相似文献
2.
Exact Bianchi-type VIII and IX models in the presence of zero-mass scalar fields are presented, when the source of the gravitational field is a perfect fluid withP=. Some physical and geometrical properties of the models are also discussed. 相似文献
3.
In connection with the problem of the initial singularity in the framework of the Brans-Dicke scalar-tensor isotropic cosmology, the dynamics of the early stages of the expansion is studied on the basis of the general analytical solutions for flat models withP=n (0n1). The sourceless scalar field, which plays a role of effective source of the geometry, entirely changes the character of the initial expansion, whereas in the absense of it the dynamics of the models almost does not differ from GR-case. If the connecting parameter <–6 then the sourceless scalar field removes the initial singularity for any equations of state, and provides the regular transition form the compression to the expansion through the non-stationary state with
. If is positive, the singularity of vacuum nature necessarily exists; the sourceless scalar-field being prevalent over the material sources (at least for P1/3) near the singularity. For P>1/3 parallel with the vacuum singularity the singular initial state with dominant role of gravitating matter is possible. In the course of expansion, the influence of gravitating matter and curvature-which as nonessential at the early stage in comparison with sourceless -field — becomes dominant, and all models approach the Machian ones which differs slightly from corresponding GR-models. If is negative (<–6) the sourceless -field slows down the initial expansion and increases essentially the real age of the Universe; it can also influence the primordial nuclear synthesis by diminishing in particular the content of He4. It is shown that a knowledge of the sign gives us the information about the nature of the initial Universe state.
Zusammenfassung In diesem Artikel betrachtet man die Einwirkung des quellosen skalaren Feld auf die Dynamik der früheren Stadien der Ausdehnung der kosmologischen Modellen. Man zeigt auch die Möglichkeit der Existenz eines unsingulären ursprünglichen Zustand in der Brans-Dickeschen skalar-tensorischen Kosmologie.
Résumé On considère dans cet article l'influence du champ scalaire sans source sur la dynamique des étapes premières de l'expansion des modèles cosmologiques. On montre, en outre, la possibilité de l'existence de l'état initiale nonsingulier dans la cosmologie scalaire-tensorique de Brans-Dicke.相似文献
4.
The past evolution of the rotation of Venus has been studied by a numerical integration method using the hypothesis that only solar tidal torques and core-mantle coupling have been active since formation. It is found quite conceivable that Venus had originally a rotation similar to the other planets and has evolved in 4.5×109 years from a rapid and direct rotation (12-hour spin period and nearly zero obliquity) to the present slow retrograde one.While the solid tidal torque may be quite efficient in despinning the planet, a thermally driven atmospheric tidal torque has the capability to drive the obliquity from 0° towards 180° and to stabilize the spin axis in the latter position. The effect of a liquid core is discussed and it is shown that core-mantle friction hastens the latter part of the evolution and makes even stronger the state of equilibrium at 180°. The model assumes a nearly stable balance between solid and atmospheric tides at the current rotation rate interpreting the present 243 day spin period as being very close to the limiting value.A large family of solutions allowing for the evolution, in a few billions years, of a rapid prograde rotation to the present state have been found. Noticeably different histories of evolution are observed when the initial conditions and the values of the physical parameters are slightly modified, but generally the principal trend is maintained.The proposed evolutionary explanation of the current rotation of Venus has led us to place constraints on the solid bodyQ and on the magnitude of the atmospheric tidal torque. While the constraints seem rather severe in the absence of core-mantle friction (aQ15 at the annual frequency is required, and a dominant diurnal thermal response in the atmosphere is needed), for a large range of values of the core's viscosity, the liquid core effect allows us to relax somewhat these constraints: a solid bodyQ of the order 40 can then be allowed. ThisQ value implies that a semi-diurnal ground pressure oscillation of 2 mb is needed in the atmosphere in order for a stable balance to occur between the solid and atmospheric tides at the current rotation rate. No model of atmospheric tides on Venus has been attempted in this study, however the value of 2 mb agrees well with that predicted by the model given in Dobrovolskis (1978). 相似文献
5.
V. A. Ruban 《Astrophysics and Space Science》1977,46(2):L23-L28
It is shown that, for the scalar-tensor cosmology (STC) by Jordan-Brans-Dicke (JBD), in general anisotropic solution the oscillatory mixmaster regime near the singularity will be destroyed by the scalar source-free field and replaced by monotonousV
3-collapse into the point or into the line and plane (only in caseG0) even in the presence of the primordial source-free electromagnetic (EM) field. 相似文献
6.
Osamu Kaburaki 《Astrophysics and Space Science》1981,74(2):333-356
The electromagnetic field produced by a magnetic dipole moment, , which is rotating obliquely surrounded by a corotating plasma sphere, is investigated. This corotating-plasma approximation has the same order of accuracy as the force-free one but has somewhat different physical implications. In the former the effect of non-electromagnetic forces such as the inertial force are included, though in somewhat artificial manner, as a departure from the strict MHD condition and this fact seems to guarantee the existence of physical solutions.Analogous to the relativistic force-free equation, a set of two differential equations (the corotation equation) are derived for the scalar functions associated with the electric and magnetic fields. A self-consistent solution of these equations is given and it is shown that this solution has no singularity, in spite of apparent divergence in the formal solution, on the light cylinder. It is concluded from this solution that, even in the extreme case of the largest possible corotation-radius (i.e.b=r
L
, wherer
L
is the light radius), the existence of a corotating plasma does not alter the field structure drastically from the vacuum case. It is also suggested through this treatment that inclusion of the inertial term in generalized Ohm's law might be essential in considering the centrifugal-wind problem. 相似文献
7.
Luis O. Pimentel 《Astrophysics and Space Science》1985,112(1):175-183
Under the assumption of a power-law between the expansion factor of the Universe, and the scalar field (a
n=c=const.) tensor theory with cosmological constant are reduced to quadrature. Several exact solutions are obtained, among them inflationary universes that have barotropic equation of state. 相似文献
8.
The 1968–2000 data on the mean magnetic field (MMF, longitudinal component) of the Sun are analysed to study long-time trends of the Sun's magnetic field and to check MMF calibration. It is found that, within the error limits, the mean intensity of photospheric magnetic field (the MMF strength, |H|), did not change over the last 33 years. It clearly shows, however, the presence of an 11-year periodicity caused by the solar activity cycle. Time variations of |H| correlate well with those of the radial component, |B
r|, of the interplanetary magnetic field (IMF). This correlation (r=0.69) appears to be significantly higher than that between |B
r| and the results of a potential source-surface extrapolation, to the Earth's orbit, of synoptic magnetic charts of the photosphere (using the so-called `saturation' factor –1 for magnetograph measurements performed in the line Fei 525.0 nm; Wang and Sheeley, 1995). It seems therefore that the true source surface of IMF is the `quiet' photosphere – background fields and coronal holes, like those for MMF. The average `effective' magnetic strength of the photospheric field is determined to be about 1.9 G. It is also shown that there is an approximate linear relation between |B
r| and MMF intensity |H| (in gauss)|B
r|(H
0)min×(1+C|H|)where =1.5×10–5 normalizes the photospheric field strength to 1 AU distance from the Sun, (H
0)min=1.2 G is some minimal `effective' intensity of photospheric background fields and C=1.3 G–1 an empirical constant. It is noted that good correlation between time variations of |H| and |B
r| makes suspicious a correction of the photospheric magnetic fields with the use of saturation factor –1. 相似文献
9.
E. M. Drobyshevski 《Earth, Moon, and Planets》1988,43(2):87-99
The totality of the results obtained in the VEGA, GIOTTO and SUISEI projects does not lend itself to interpretation within the old sublimation models of comet activity considering solar radiation as the sole source of energy. Among them are the systematic excess of the velocity and temperature of the gas escaping from the nucleus over the theoretical values, the outflow being concentrated in several ( ~ 12–15) narrow ( ~ 300 m in size at the nucleus) hypersonic jets carrying very large amounts of remarkably fine CHON dust and located along continuous lines on the nuclear surface, intense release from the nucleus or very close to it (2 × 103km) of CO (Q
Co/Q
H2
O = 0.05–0.2) with a smaller amount of CO2 (Q
CO
2/ Q
H2
O 0.015), large near-nucleus abundances of C, C+ (Q
C/Q
CO 0.29), etc.The new observations, together with some earlier data still poorly understood (e.g. the appearance in the coma of large amounts of C3) can be accounted for by assuming the cometary ices to contain, apart from hydrocarbons, nitrogen-containing compounds, etc. also of free oxygen ( ~ 15 wt.%). Under these conditions, burning should occur in the products of sublimation under deficiency of oxidizer accompanied by the production of soot, smoke, etc. The burning should propagate under the surface crust and localize primarily at a few sites.The presence of oxygen in cometary ices follows from a new eruption theory assuming the minor bodies of the Solar System to have formed in explosions of the massive ice envelopes saturated by electrolysis products on distant moonlike bodies of the type of Ganymede and Callisto. 相似文献
10.
Relativistic cosmological field equations are obtained for a non-static stationary Bertotti-Robinson-type space-time for interacting perfect fluid and electromagnetic field. The cosmological solution to the field equations are obtained and the nature of the electromagnetic field as well the perfect fluid are studied. The electromagnetic field generated here corresponds to a special generic case and the perfect fluid distribution degenerates into a barotropic perfect fluid with equation of statep+=0, >0. It is shown here that the interacting barotropic fluid can generate gravitation only when the cosmological constant being a function ofx in a dynamic field. 相似文献
11.
The decay of a longitudinal plasmon into two neutrinos is studied in the presence of a strong magnetic field. Contrary to the transverse case, for longitudinal plasmons the existence of a new mode, entirely dependent on the magnetic field, greatly enhances the energy loss at high densities. Denoting byQ
HandQ
0the neutrino energy losses with and without magnetic field respectively, the situation is as follows: atH1011 G andT109K,Q
0105
Q
Hfor <1011g cm–3, WhileQ
H1010
Q
0for >1011g cm–3. A second physically interesting feature is the anisotropic character of the energy loss which is highly peaked along the field lines, giving rise to a shorter cooling time in that direction than in any other. 相似文献
12.
With the aim of investigating the resonance system of NO, equivalent width calculations have been made for the 213.575 nm, Q
1(31.5) line of the 1–0 band of -system of NO for the photospheric HSRA model, and for the line 214.012 nm, Q
1(25.5) line of the same band system for Zwaan's (1974) sunspot model.Calculations show that -band system would not show up in the photospheric spectrum whereas a sunspot model yields an equivalent width of 72 mÅ suggesting that sunspots may provide relatively more favourable conditions for the detections of the resonance systems of some abundant molecules in the inaccessible ultraviolet region. 相似文献
13.
W. M. Ostermann 《Astrophysics and Space Science》1990,164(1):87-93
All knownV data from the literature for the Scuti star HR 1170 have been reanalysed by using single-frequency Fourier and multiple-frequency least squares analysis. The calculated periods areP
1=0
.
d
09942 andP
2=0
.
d
08392. The second frequency has not been found before and gives better residuals and almost constant amplitudes for the individual data sets. The period ratio andQ values indicate that this star is not pulsating in pure radial modes. 相似文献
14.
G. A. Gurzadyan 《Astrophysics and Space Science》1979,62(1):67-77
It is shown that the far-ultraviolet radiation (shorter than 2000 Å) discovered by ANS observations in the few T Tauri-type stars does not have any relation to the two-photon emission of hydrogen, as suggested by some authors. This is obtained from the observational data of the numerical values of the ratioQ
*(2q)/Q
*() for these stars, whereQ
*(2q) is the complete number of the observed 2q-photons andQ
*() is the number of observedH-photons. The observational values ofQ
*(2q)/Q
*() for four T Tauri-type stars turned out to be in the region of 20–90, while the theoretical value of this relation is 6. Hence, the observed fluxes in the region <2000 Å are 3–15 times larger than the theoretically expected values.The emission discovered in the region <2000 Å is of non-thermal origin, and can be identified with high probability with thetransition radiation. The latter originates as a result of the electromagnetic interaction of so-called fast electrons (E1.5 MeV) with dust particles in the gas-dust clouds surrounding these stars. The theoretical spectral curves of the transition radiation, for a few values of the plasma frequency 0 for the dust particles, are calculated taking into account also the self-absorption effect of the radiation in the cloud and the absorption in the interstellar medium. Qualitatively, these curves (Figures 2, 3 and 4) are in good accord with the observed spectral distribution curves for the T Tauri-type stars (Figure 1). In particular, in both cases a minimum of radiation flux occurs near to 2200 Å, and a maximum near 1800 Å.The starting point of our analysis has been the concept of the identity of the processes, non-thermal and non-stationary in character, taking place at the time of the flare phenomenon of UV Cet-type stars in one case, and at the generation of continuous emission and the excitation of the emission lines in T Tauri-type stars on the other. In the latter case, the T Tauri-type stars can be regarded aspermanently flaring stars, with a very high frequency of flare events. 相似文献
15.
Tolman's V solution has found wide application to the redshift problems in astrophysics but it has a drawback that there is singularity at the centre. We have developed a two-density model with a parabolic density distribution in the core ande
r
2n
in the envelope. The most distinctive feature of the model is that there is a continuity of all four variables (E, P, , and ) at the boundary. In other analytic core-envelope models the continuity of only three varaibles is possible. This two-density structure has been used to model neutron stars and their mass, size, and red shifts have been obtained. 相似文献
16.
T. J. Bogdan 《Solar physics》1986,103(2):311-315
Previous efforts to construct solar coronal fields using surface magnetograph data have generally employed a least squares minimization technique in order to determine the spherical harmonic expansion coefficients of the magnetic scalar potential. Provided there is no source surface high up in the corona, we show that knowledge of the line-of-sight component of the surface magnetic field, B
i
= B
r
sin + B
cos , is sufficient to uniquely determine the potential coronal magnetic field by an explicit construction of the magnetic scalar potential for an arbitrary B
l
(, ).The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
17.
M. J. Miketinac 《Astrophysics and Space Science》1974,28(1):193-203
Frequencies of non-radial oscillation of polytropic models of stars, belonging to spherical harmonics of ordersl=1, 2 and 3, are evaluated, in a second approximation, by a variational method. Equilibrium configurations in the presence of toroidal magnetic fields are obtained numerically without any restriction on the field strength. The value of the ratio of the specific heats, , is assumed to be equal to 5/3 and only two polytropic indeces,n=1.5 and 3.0, are considered. It is found that a polytropic star stays stable for magnetic fields considerably stronger than expected from the results obtained by the weak field perturbation methods. 相似文献
18.
The influence of initial helium content on the evolutionary characteristics of super-metalrich stars (Z=0.10) has been investigated. The evolution of models in the range –0.05L/L
<1.0 has been followed up to the relative luminosity maximum in the subgiant branch, and under the assumptionY=0.20 orY=0.40.Comparison with previous results suggests the existence of theoretical constraints that could be adopted as metal indicators for the observed H–R diagrams of old open clusters. 相似文献
19.
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