共查询到19条相似文献,搜索用时 296 毫秒
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球状星团与星系的形成和演化过程密切相关,研究球状星团是研究星系形成和演化的重要途径之一。综述了球状星团在观测上的统计结论,观测对理论模型的限制,利用数值模拟和半解析模型研究球状星团形成和演化等阶段的进展。理论研究主要包括球状星团的形成、密度轮廓、初始质量函数,球状星团在质量、大小、空间上的演化,星系并合对球状星团形成和演化的影响等。通过介绍当前研究的主要模型、相关的研究结果以及该领域尚未解决的一些难题,阐明球状星团与星系的关系,深化对球状星团和星系形成和演化的认识。 相似文献
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扼要地介绍了色球和日冕加热问题的研究历史。随着空间太阳观测技术的进步,人们认识到色球和日冕加热机制主要与MHD过程有关。因此,在本文中着重介绍四种MHD色球和日冕加热机制:(1)阿尔芬波;(2)MHD湍动;(3)场向电流;(4)磁重联。由于这四种加热机制的有效性都需要通过高分辨率观测来判定,所以空间太阳观测对于研究色球和日冕加热问题具有重大意义。 相似文献
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扼要地介绍了色球和日冕加热问题的研究历史。随着空间太阳观测技术的进步,人们认识到色球和日冕加热机制主要与MHD过程有关。因此,在本文中着重介绍四种MHD色球和日冕加热机制:(1)阿尔芬波;(2)MHD波动;(3)场向电流;(4)磁重联。由于这四种加热机制的有效性都需要通过高分辨率观测来判定,所以空间太阳 相似文献
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自从人们获得河外射电源的第一个结构图像以来,30多年时间已经过去了。在这段时间的后半期,人们对许多源中的喷流状结构作了大量研究。目前,我们正在分析所获得的有关喷流的第一代结果。这包括高分辨率观测,数值模拟和理论研究等诸方面的成就。在本文 详细地讨论河外射电源中的喷流。 相似文献
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从理论和观测两个方面来介绍和讨论出现在太阳爆发过程中的磁重联电流片及其物理本质和动力学特征。首先介绍在理论研究和理论模型中,磁重联电流片是如何在爆发磁结构当中形成并发展的,对观测研究有什么指导意义。然后介绍观测工作是从哪几个方面对理论模型预测的电流片进行证认和研究的。第三,将介绍观测研究给出了哪些过去所没有能够预期的结果,这些结果对深入研究耀斑一CME电流片以及其中的磁重联过程的理论工作有什么重要的、挑战性的意义。第四,讨论最新的与此有关的理论研究和数值实验。最后,对未来的研究方向和重要课题进行综述和展望。 相似文献
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日冕物质抛射的理想MHD模型研究 总被引:3,自引:0,他引:3
概括了日冕物质抛射的一些观测结果和它们与其它太阳活动现象的相关性。简要回顾了较早期日冕物质抛射的理论研究,着重介绍了最近研究得较多的理论机制,即能量储存机制,以及其中的磁通量绳突变模型与其它理论模型的MHD数值和解析研究以及相应的重要应用. 相似文献
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HI observations of high-velocity clouds(HVCs) indicate that they are interacting with their ambientmedium. The question on the dynamical and thermal stabilization of a cold dense neutral cloud in a hot, thin, and magnetized ambient halo plasma is investigated by plasma-neutral gas simulations.The simulations show the formation of a comet-likehead-tail structurecombined with a magnetic barrier whichexerts a stabilizing pressure on the cloud and hindershot plasma from diffusing into the cloud. 相似文献
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Sandford SA 《Meteoritics & planetary science》1996,31(4):449-476
Tremendous progress has been made in the field of interstellar dust in recent years through the use of telescopic observations, theoretical studies, laboratory studies of analogs, and the study of actual interstellar samples found in meteorites. It is increasingly clear that the interstellar medium (ISM) contains an enormous diversity of materials created by a wide range of chemical and physical processes. This understanding is a far cry from the picture of interstellar materials held as recently as two decades ago, a picture which incorporated only a few generic types of grains and few molecules. In this paper, I attempt to review some of our current knowledge of the more abundant materials thought to exist in the ISM. The review concentrates on matter in interstellar dense molecular clouds since it is the materials in these environments from which new stars and planetary systems are formed. However, some discussion is reserved for materials in circumstellar environments and in the diffuse ISM. The paper also focuses largely on solid materials as opposed to gases since solids contain a major fraction of the heavier elements in clouds and because solids are most likely to survive incorporation into new planetary systems in identifiable form. The paper concludes with a discussion of some of the implications resulting from the recent growth of our knowledge about interstellar materials and also considers a number of areas in which future work might be expected to yield important results. 相似文献
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Observations of the submillimeter polarized dust emission is an important tool to study the role of the magnetic fields in the evolutions of molecular clouds and in the star formation processes. The Submillimeter Array (SMA) is the first imaging submillimeter interferometer. The installation of quarter wave plates in front of the 345 GHz receivers has allowed to carry out polarimetric observations. We present high angular resolution 345 GHz SMA observations of polarized dust emission towards the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of formation of low-mass stars in magnetized molecular clouds at scales of a few hundred AU; gravity has overcome magnetic support and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system and the initial misalignment of the magnetic and spin axes may have been important in the formation of the binary system. 相似文献
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M.A. de Avillez 《Astrophysics and Space Science》2000,272(1-3):23-30
Three-dimensional simulations of the disk-halo interaction show the formation of a thick HI and HII gas disk with different
scale heights. The thick HI disk prevents the disk gas from expanding freely upwards, unless some highly energetic event such
as chimneys occurs, whereas the thick HII disk acts as a disk-halo interaction region from where the hot ionized gas flows
freely into the halo. The upflowing gas reaches the maximum height at z ∼ 9.3 ± 1 kpc becoming thermally unstable due to radiative losses, and condenses into HI clouds. Because the major fraction
of the gas is gravitationally bound to the Galaxy, the cold gas returns to the disk. The descending clouds will have at some
height high velocities. In a period of 200 Myr of fountain evolution, some 10 percent of the total number of clouds are HVCs.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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D.R. Gonçalves A.C.S. Friaça V. Jatenco-Pereira 《Monthly notices of the Royal Astronomical Society》2001,328(2):409-418
The formation of quasar broad-line region (BLR) clouds via thermal instability in the presence of Alfvén heating has been discussed by Gonçalves, Jatenco-Pereira & Opher. In particular, these studies showed the relevance of Alfvén heating in establishing the stability of BLR clouds in the intercloud medium. The present paper shows the results of time-dependent calculations (we use a time-dependent hydrodynamic code) following the evolution of BLR clouds, since their formation from the 107 -K intercloud medium. We also calculate the UV and optical line emission associated with the clouds in order to compare with observations. Our results are compared with those of UV and optical monitoring of well-studied AGN, which suggest that the BLR is most probably composed of at least two different regions, each one giving rise to a kind of line variability, since low- and high-ionization lines present different patterns of variability. We discuss the alternative scenario in which lines of different ionization could be formed at the same place but heated/excited by distinct mechanisms, considering the Alfvén heating as the non-radiative mechanism. 相似文献
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The Arecibo H i Strip Survey probed the halos of approximately 300 cataloged galaxies and the environments of approximately 14 groups with sensitivity to neutral hydrogen masses >/=107 M middle dot in circle. The survey detected no objects with properties resembling the high-velocity clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically MHi=107.5 M middle dot in circle objects distributed throughout groups and galaxy halos at distances of approximately 1 Mpc, the survey should have made approximately 70 HVC detections in groups and approximately 250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of =200 kpc from the galaxies or group barycenters. If the clouds are in virial equilibrium, their average dark matter fraction must be 98% or higher. 相似文献
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J. Flix J. E. Taylor M. Martínez F. Prada J. Silk J. Cortina 《Astrophysics and Space Science》2005,297(1-4):299-308
In a cold dark matter (CDM) framework of structure formation, the dark matter haloes around galaxies assemble through successive
mergers with smaller haloes. This merging process is not completely efficient, and hundreds of surviving halo cores, or subhaloes, are expected to remain in orbit within the halo of a galaxy like the Milky Way. While the dozen visible satellites of the
Milky Way may trace some of these subhaloes, the majority are currently undetected. A large number of high-velocity clouds
(HVCs) of neutral hydrogen are observed around the Milky Way, and it is plausible that some of the HVCs may trace subhaloes undetected in the optical. Confirming
the existence of concentrations of dark matter associated with even a few of the HVCs would represent a dramatic step forward
in our attempts to understand the nature of dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle
physics currently suggest neutralinos as a natural well-motivated candidate for the non-baryonic dark matter of the universe.
If this is indeed the case, then it may be possible to detect dark matter indirectly as it annihilates into neutrinos, photons
or positrons. In particular, the centres of subhaloes might show up as point sources in gamma-ray observations. In this work,
we consider the possibility that some of the unidentified EGRET γ-ray sources trace annihilating neutralino dark matter in
the dark substructure of the Local Group. We compare the observed positions and fluxes of both the unidentified EGRET sources
and the HVCs with the positions and fluxes predicted by a model of halo substructure, to determine up to what extent any of
these three populations could be associated. 相似文献