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1.
Thermosolutal-convective instability of a stellar atmosphere is considered in the presence of suspended particles. The criteria for monotonic instability are derived which are found to hold good in the presence of uniform rotation and uniform magnetic field, separately, on the thermosolutal-convective instability in the presence of suspended particles. 相似文献
2.
The thermosolutal-convective instability of a composite stellar atmosphere is considered in the presence of variable horizontal magnetic field and collisional effects. The criteria for monotonic instability are obtained which generalize the criterion derived for thermal-convective instability in the absence of above effects. 相似文献
3.
P. Devadas Rao N. Ramamani Saleh Mohammed Alladin 《Journal of Astrophysics and Astronomy》1987,8(1):17-31
N-Body simulations have been performed to study the tidal effects of a primary stellar system on a secondary stellar system
of density close to the Roche density. Two hyperbolic, one parabolic and one elliptic encounters have been simulated. The
changes in energy, angular momentum, mass distribution, and shape of the secondary system have been determined in each case.
The inner region containing about 40 per cent of the mass was found to be practically unchanged and the mass exterior to the
tidal radius was found to escape. The intermediate region showed tidal distension. The thickness of this region decreased
as we went from hyperbolic encounters to the elliptic encounter keeping the distance of closest approach constant. The numerical
results for the fractional change in energy have been compared with the predictions of the available analytic formulae and
the usefulness and limitations of the formulae have been discussed. 相似文献
4.
Thermosolutal-convective instability of a composite plasma in a stellar atmosphere is considered. The effect of a variable horizontal magnetic field and the simultaneous effect of a uniform rotation and a variable horizontal magnetic field have been considered on the thermosolutal-convective instability. We have derived the sufficient conditions for the existence of monotonic instability. It is found that the criteria for monotonic instability hold good in the presence of a variable horizontal magnetic field as well as in the presence of a uniform rotation and a variable horizontal magnetic field. 相似文献
5.
Thermosolutal-convective instability of a composite plasma in a stellar atmosphere is considered to include the effects, separately, due to finite Larmor radius (FLR) and Hall currents in the presence of a uniform horizontal magnetic field. The sufficient conditions for the existence of monotonic instability are derived and are found to hold good both in the presence, separately, of FLR and Hall current effects. 相似文献
6.
Thermosolutal-convective instability of a stellar atmosphere is considered. The criteria for monotonic instability are derived. The effects of a variable horizontal magnetic field and the simultaneous presence of a uniform rotation and a uniform horizontal magnetic field have been considered on the thermosolutal-convective instability. The criteria derived for monotonic instability are found to hold good in the presence of a variable horizontal magnetic field as well as in the presence of a uniform rotation and a uniform horizontal magnetic field. 相似文献
7.
Thermosolutal-convective instability of a stellar atmosphere in the presence of a stable solute gradient is considered to include the effects, separately, due to finite Larmor radius (FLR) and Hall currents in the presence of a uniform horizontal magnetic field. The criteria derived for monotonic instability are found to hold true in the presence of FLR and Hall effects. 相似文献
8.
The formation of Na I lines in X-ray illuminated atmospheres is investigated by abandoning the assumption of local thermodynamic equilibrium (LTE). Calculations are performed on the basis of a 21-level Na I model atom for the LTE model atmospheres of irradiated F-G stars obtained with allowance for a reflection effect in the first approximation. The state of extreme “overrecombination” is shown to exist for the populations of all Na I levels in the case of external illumination. Absorption features in the profiles of “cool” and “normal” Na I lines have been found to be enhanced compared to the LTE approximation. Effects of the angle of incidence and intensity of the external radiation on the formation of level populations and line profiles when abandoning LTE are analyzed. The existence of overrecombination for Na I is explained by the small X-ray heating function and the large optical cooling function. Na I level populations are shown to depend weakly on the presence of “overionization” for Na II in the atmospheres of irradiated stars. 相似文献
9.
We study the various approximations used to investigate the eigenmode spectrum for systems with highly elongated stellar orbits. The approximation in which the elongated orbits are represented by thin rotating spokes, with the rotation imitating the precession of real orbits, is the simplest and most natural one. However, we show that using this pictorial approximation does not allow the picture of stability to be properly presented. We show that for stellar systems with a plane disk geometry, this approach does not allow unstable spectral modes to be obtained even in the leading order in small parameter, which characterizes the spread of nearly radial orbits in angular momentum. For spherical systems, where the situation is more favorable, the spectrum can be determined but only in the leading order in this parameter. A rigorous approach based on the solution of more complex integral equations given here should be used to properly investigate the stability of stellar systems. 相似文献
10.
Observations on the pulsation pattern in the time profile of short duration solar radio bursts at decametre wavelengths are
presented. The pulsations are found to be present predominantly in the saturation phase of the burst. A tentative physical
model based on the non-linear development of the waves interacting in a turbulent medium is invoked to explain the origin
of the pulsations. 相似文献
11.
Two dimensional spectral classification of stars in the neighborhood of the stellar cluster NGC 2175
Spectral classifications of 1224 O-M stars in different luminosity classes in the neighborhood of NGC 2175 are presented.
The classifications are based on low dispersion objective prism spectra. B-V and U-B data for 106 of the 1224 stars are used
to obtain MK classifications using the photometric Q method.
__________
Translated from Astrofizika, Vol. 51, No. 2, pp. 339–341 (May 2008). 相似文献
12.
We study the nonstationary recombination of hydrogen in the atmosphere of SN 1987A by taking into account ion-molecular processes. The hydrogen excitation due to nonstationary recombination is shown to be enough to explain the observed hydrogen lines in a time interval until day 30 in the absence of additional excitation mechanisms. Thus, the problem of a deficit in the hydrogen excitation that has recently been found in modeling the hydrogen spectrum of SN 1987A at an early photospheric stage by assuming statistical ionization equilibrium is resolved. The mass of the radioactive 56Ni with a spherically symmetric distribution in the outer layers is shown to be close to 10?6 M ⊙. Our model predicts the appearance of a blue peak in the Hα profile between days 20 and 30. This peak bears a close similarity to the observed peak known as the Bochum event. The presence of this peak in the model is attributable to nonstationary recombination and to a substantial contribution of hydrogen neutralization involving H? and H2. 相似文献
13.
Yu. A. Fadeyev 《Astronomy Letters》2003,29(11):775-782
Based on a self-consistent solution of the equations of gas dynamics, kinetics of hydrogen atomic level populations, and radiative transfer, we analyze the structure of a shock wave that propagates in a partially ionized hydrogen gas. We consider the radiative transfer at the frequencies of spectral lines by taking into account the effects of a moving medium in the observer's frame of reference. The flux in Balmer lines is shown to be formed behind the shock discontinuity at the initial hydrogen recombination stage. The Doppler shift of the emission-line profile is approximately one and a half times smaller than the gas flow velocity in the Balmer emission region, because the radiation field of the shock wave is anisotropic. At Mach numbers M1?10 and unperturbed gas densities σ1=10?10 g cm?3, the Doppler shift is approximately one third of the shock velocity U1. The FWHM of the emission-line profile δ ? is related to the shock velocity by δ ? ≈ k ? U1, where k ? = 1, 0.6, and 0.65 for the Hα, Hβ, and Hγ lines, respectively. 相似文献
14.
The saturation conditions for bending modes in inhomogeneous thin stellar disks that follow from an analysis of the dispersion relation are compared with those derived from N-body simulations. In the central regions of inhomogeneous disks, the reserve of disk strength against the growth of bending instability is smaller than that for a homogeneous layer. The spheroidal component (a dark halo, a bulge) is shown to have a stabilizing effect. The latter turns out to depend not only on the total mass of the spherical component, but also on the degree of mass concentration toward the center. We conclude that the presence of a compact (not necessarily massive) bulge in spiral galaxies may prove to be enough to suppress the bending perturbations that increase the disk thickness. This conclusion is corroborated by our N-body simulations in which we simulated the evolution of near-equilibrium, but unstable finite-thickness disks in the presence of spheroidal components. The final disk thickness at the same total mass of the spherical component (dark halo + bulge) was found to be much smaller than that in the simulations where a concentrated bulge is present. 相似文献
15.
The characteristics of the most unstable fundamental mode and the first harmonic excited in the convection zone of a variety
of solar envelope models are shown to be in reasonable agreement with the observed features of granulation and supergranulation.
On leave of absence from Government Digvijai College, Rajnandgaon 491441 相似文献
16.
We consider disk and spherical subsystems of stars with nearly radial orbits under conditions when the well-known radial orbit instability is not possible. This requires that the precession of stellar orbits be retrograde, i.e., in the direction opposite to the orbital rotation of stars. We show that an instability that is an analogue of the loss-cone instability known in plasma physics can then develop in the presence of a “loss cone” in the angular momentum distribution of stars, which ensures a deficit or even absence of stars with low angular momenta. Examples of systems with a loss cone are the centers of galaxies or star clusters with massive black holes. The instability can produce a flux of stars onto the galactic center, i.e., it can serve as a mechanism of fueling the nuclear activity of galaxies. Mathematically, the problem is reduced to analyzing simple characteristic equations that describe small perturbations in a disk and a sphere of radially highly elongated stellar orbits. In turn, these characteristics equations are derived through a number of successive simplifications of the general linearized Vlasov equations (i.e., the system that includes the collisionless Boltzmann kinetic equation and the Poisson equation) in action—angle variables. 相似文献
17.
K. C. Sharma 《Astrophysics and Space Science》1986,125(2):299-304
The problem of thermal-convective instability of a hydromagnetic composite plasma in a stellar atmosphere has been studied with Hall effects. The criterion for monotonic instability has been found to be unchanged by the presence of Hall effects. 相似文献
18.
19.
K. C. Sharma 《Astrophysics and Space Science》1987,133(1):177-182
Thermal-convective instability of a hydromagnetic-composite plasma in a stellar atmosphere has been studied in the presence of finite Larmor radius. It is found that the criterion for monotonic instability holds good in the presence of finite Larmor radius. 相似文献
20.
Using a direct numerical procedure we approximate a ’realistic’ potential for an elliptical galaxy by that of a perturbed
harmonic oscillator. The quality of fitting is checked using two criteria. First by computing the value of the fitting parameterf, and second by comparing the behaviour of orbits for the two potentials. Both criteria suggest that the fitting is good when
the total energyh is smaller than the energy of escape in the unharmonic potential. 相似文献