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891.
Cool giant and supergiant stars generally present low velocity winds with high mass-loss rates. Several models have been proposed
to explain the acceleration process of these winds. Although dust is known to be present in these objects, the radiation pressure
on these particles is uneffective in reproducing the observed physical parameters of the wind. The most promising acceleration
mechanism cited in the literature is the transference of momentum and energy from Alfvén waves to the gas. Usually, these
models consider the wind to be isothermal. We present a stellar wind model in which the Alfvén waves are used as the main
acceleration mechanism, and determine the temperature profile by solving the energy equation taking into account both the
radiative losses and the wave heating. We also determine, self-consistently, the magnetic field geometry as the result of
the competition between the magnetic field and the thermal pressure gradient. As the main result, we show that the magnetic
geometry presents a super-radial index in the region where the gas pressure is increasing. However, this super-radial index
is greater than that observed for the solar corona. 相似文献
892.
张宽政 《长沙航空职业技术学院学报》2006,6(4):78-81
井田制内含耕地资源所有权和经营权分离,租佃制则是耕地资源所有权与经营权分离的普遍化,二者都以耕地资源私有制和资源有偿利用为基础。给我们的启示是:公有资源按照所有权与经营权相分离的模式必产生公有制经济形式多样。 相似文献
893.
通过分析高频磁场作用下熔体连续处理时夹杂的去除效率表明,熔体连续处理条件下的夹杂去除效率比熔体静止时的略低,而夹杂与熔体之间的密度差对电磁分离时的去除效率影响不大。提高去除效率的有效途径是增大磁感应强度或降低分离器管径,但减小管径的同时需要提高磁场频率,以保持α/δ的值在2附近才可以取得最佳的分离效果。 相似文献
894.
895.
José Luis Ballester 《Space Science Reviews》2006,122(1-4):129-135
Prominence seismology is a rapidly developing topic which seeks to infer the internal structure and properties of solar prominences
from the study of its oscillations. Two-dimensional high-resolution observations suggest that filaments can be considered
as made by small scale fibrils, having a cool region, stacked one after another in the vertical and horizontal directions.
An extense observational background about oscillations in filaments has been gathered during the last 20 years and these observations
point out that fibrils or groups of fibrils can oscillate independently. From the theoretical point of view, small amplitude
oscillations in single and multifibril configurations have been studied as a first step to explain observational features. 相似文献
896.
897.
J. Craig Wheeler 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2004,34(12):2744-2749
A summary is given of the presentations at the COSPAR workshop on γ-ray bursts with some personal commentary on the contributions, the SN/GRB connection, and on the role of magnetic fields in γ-ray bursts and their afterglows. Of special interest were the accumulated arguments for strong collimation and associated reduction in the total required energy for γ-ray bursts. Significant discussion was also devoted to the issues associated with iron and metal lines in X-ray spectra. It is important to note that some of the afterglows seem to require ambient densities 1 g cm−3, rather incompatible with a massive star environment. Of associated difficulty is the fact that few, if any, afterglows seem consistent with the r−2 wind expected for a massive star model. There are reasons to think that if γ-ray bursts are associated with supernovae they are of Type Ic. This suggests that any wind present might be rich in carbon and oxygen, not hydrogen or helium. If γ-ray bursts are narrowly collimated, then the burst is only probing a small portion of any wind, perhaps just that time-dependent and isotropic structure directly along the rotation axis. The characteristics of “hypernovae” may be the result of orientation effects in a mildly inhomogeneous set of progenitors, rather than requiring an excessive total energy or luminosity. The recent event GRB 021004 provided a rich photometric and spectroscopic record and perhaps the most direct evidence yet for the association of a specific γ-ray burst with a massive star progenitor. If the magnetic field plays a significant role in launching a relativistic γ-ray burst jet from within a collapsing star, then the magnetic field may also play a role in the propagation, collimation, and stability of that jet within and beyond the star. The magneto-rotational instability (MRI) can operate under conditions of moderate rotation. This means that the MRI will be at work generating strong fields exponentially rapidly even as the disk of material begins to form and makes a transition from a non-Keplerian to quasi-Keplerian flow in the collapsar and related models. 相似文献
898.
A. A. Van Ballegooijen 《Space Science Reviews》1994,68(1-4):299-307
The interaction of a stellar magnetosphere with a thin accretion disk is considered. Specifically, I consider a model in which (1) the accretion disk is magnetically linked to the star over a large range of radii and (2) the magnetic diffusivity of the disk is sufficiently small that there is little slippage of field lines within the disk on the rotation time scale. In this case the magnetic energy built up as a result of differential rotation between the star and the disk is released in quasi-periodic reconnection events occuring in the magnetosphere (Aly and Kuijpers 1990). The radial transport of magnetic flux in such an accretion disk is considered. It is show that the magnetic flux distribution is stationary on the accretion time scale, provided the time average of the radial component of the field just above the disk vanishes. A simple model of the time-dependent structure of the magnetosphere is presented. It is shown that energy release in the magnetosphere must take place for (differential) rotation angles less than about 3 radians. The magnetic flux distribution in the disk depends on the precise value of the rotation angle. 相似文献
899.
张稼丰 《南京航空航天大学学报》1991,(Z1)
网络拓扑法已被实践证明为一种有效实用的数值计算方法。它以数学分析简单、物理概念明确、单元剖分灵活、适用性强等优点,已被越来越多的场计算专家所采用,成为一种较优秀的数值分析方法。本文在普通网络拓扑法的基础上,以温度场计算为例,通过建立其能量泛函并求其极值,导出一种高精度的网络拓扑法——对偶网络拓扑法。在给定的求解精度条件下,由于两组网络计算时产生误差之间的补偿作用,使用该方法能有效地减少计算存贮空间、缩短计算用时。这对用微机求解复杂场问题是很有意义的。本文最后的实例虽然是针对温度场分析的,但对其他场问题完全适用,故具有普遍性。 相似文献
900.
分别从磁化理论和数理统计两个方面分析了影响耐蚀软磁合金1J116磁性能的因素。对某钢厂不同生产时期的生产质量水平通过理论计算进行了评估;研究了化学元素C不同含量对材料磁性能的影响;通过计算得到了提高磁性能的有效C含量范围。 相似文献