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961.
962.
Ramadan M. Ali 《Astrophysics and Space Science》2007,310(3-4):201-204
Cylindrically symmetric perfect fluid solutions are derived for the Levi-Civita metric. The pressure P is finite. The matter density is greater than the stresses in the material. The solutions are inside cylinders of bounded
radius at which the pressure vanishes. The range of σ, for which the sources have been matched to the Levi-Civeta metric is ∞>σ>0. The solutions are regular and satisfy energy conditions 相似文献
963.
During several campaigns focused on prominences we have obtained coordinated spectral observations from the ground and from
space. The SOHO/SUMER spectrometer allows us to observe, among others, the whole Lyman series of hydrogen, while the Hα line was observed by the MSDP spectrograph at the VTT. For the Lyman lines, non-LTE radiative-transfer computations have
shown the importance of the optical thickness of the prominence – corona transition region (PCTR) and its relation to the
magnetic field orientation for the explanation of the observed line profiles. Moreover, Heinzel, Anzer, and Gunár (2005, Astron. Astrophys.
442, 331) developed a 2D magnetostatic model of prominence fine structures that demonstrates how the shapes of Lyman lines vary,
depending on the orientation of the magnetic field with respect to the line of sight. To support this result observationally,
we focus here on a round-shaped filament observed during three days as it was crossing the limb. The Lyman profiles observed
on the limb are different from day to day. We interpret these differences as being due to the change of orientation of the
prominence axis (and therefore the magnetic field direction) with respect to the line of sight. The Lyman lines are more reversed
if the line of sight is across the prominence axis as compared to the case when it is aligned along its axis. 相似文献
964.
965.
We investigate the figure rotation of dark matter haloes identified in Λ cold dark matter (CDM) simulations. We find that when strict criteria are used to select suitable haloes for study, five of the 222 haloes identified in our z = 0 simulation output undergo coherent figure rotation over a 5 h −1 Gyr period. We discuss the effects of varying the selection criteria and find that pattern speeds for a much larger fraction of the haloes can be measured when the criteria are relaxed. Pattern speeds measured over a 1 h −1 Gyr period follow a lognormal distribution, centred at Ωp = 0.2 h rad Gyr−1 with a maximum value of 0.94 h rad Gyr−1 . Over a 5 h −1 Gyr period, the average pattern speed of a halo is about 0.1 h rad Gyr−1 and the largest pattern speed found is 0.24 h rad Gyr−1 . Less than half of the selected haloes showed alignment between their figure rotation axis and minor axis, the exact fraction being somewhat dependent on how one defines a halo. While the pattern speeds observed are lower than those generally thought capable of causing spiral structure, we note that coherent figure rotation is found over very long periods and argue that further simulations would be required before strong conclusions about spiral structure in all galaxies could be drawn. We find no correlation between halo properties such as total mass and the pattern speed. 相似文献
966.
We consider a conventional stellar α2 ω -dynamo with dynamo generators localized in two spherical shells separated by a passive layer. The signs of the α-effect as well as rotational shear in the dynamo active layers can be chosen to give dynamo waves that propagate in opposite directions (poleward and equatorward) if the layers are considered separately in the framework of the Parker migratory dynamo. In a sequence of numerical experiments we show that the variety of dynamo-generated magnetic configurations in the system under discussion is quite rich. We identify the possibility of almost independent dynamo waves existing in the two layers as well as enslavement of one layer by the other, and of activity waves generated by a joint action of the two layers. We suggest some qualitative explanations of the behaviour and discuss also the limited nature of these explanations. This variety of phenomena suggests previously underexploited freedoms to understand how predictions of dynamo theory may accommodate the observed solar and stellar activity phenomenology. 相似文献
967.
The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as M a = 6 × 10−4 M⊙ , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to M a < 10−4 M⊙ ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all M a ≤ 6 × 10−4 M⊙ ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux). 相似文献
968.
Fang Yang Chen Hu Yan-Mei Chen Jian-Min Wang 《中国天文和天体物理学报》2007,7(3):353-363
We compiled a sample of 98 radio-quiet active galactic nuclei observed by ASCA, Chandra, XMM-Newton, INTEGRAL and Swift with the aim of testing the formation of hot corona and the magnetic shear stress operating in a disk-corona system. We found a strong correlation between the hard X-ray luminosity, bolometric luminosity L Bol and Eddington luminosity LEdd, in the sense that the fraction f of hard X-ray to the bolometric luminosity is inversely proportional to the Eddington ratio. This correlation favors the shear stress tensor being of the form of trφ∝ Pgas, with which the disk-corona structure is stable. 相似文献
969.
David L. Meier 《Astrophysics and Space Science》2005,300(1-3):55-65
In this paper I propose that the inner part of a black hole accretion inflow (< 100 rg) may enter a magnetically dominated, magnetosphere-like phase in which the strong, well-ordered fields play a more important
role than weak, turbulent fields. In the low/hard state this flow is interior to the standard ADAF usually invoked to explain
the observed hot, optically thin emission. Preliminary solutions for these new MDAFs are presented.
Time-dependent X-ray and radio observations give considerable insight into these processes, and a new interpretation of the
X-ray power spectrum (as arising from many disk radii) may be in order. While an evaporative ADAF model explains the noise
power above 0.01 Hz, an inner MDAF is needed to explain the high-frequency cutoff near 1 Hz, the presence of a QPO, and the
production of a jet. The MDAF scenario also is consistent with the phenomonological models presented at this meeting by several
authors. 相似文献
970.