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以球状星团NGC (New General Catalogue) 104、NGC 5139、NGC 6121为实验样区, 选取了视差等10个恒星参数, 通过引入地学中的空间分析理论和相应的分析框架为定量描述球状星团成员星的空间分布特征提出了一种基于地学的研究范式. 通过计算全局和局部莫兰(Moran)指数得到球状星团成员星各恒星参数的空间分布特征. 研究结果表明: 球状星团NGC 104、NGC 5139、NGC 6121成员星的各恒星参数在总体上呈现出空间正相关特性, 表现出空间集聚特征, 但不同恒星参数之间存在差异; 局部空间分布也呈现聚集特征, 而不同的成员星呈现出不同的空间分布特性和趋势. 总体而言, 用地学空间相关分析系统地定量化描述球状星团成员星空间分布特征, 能够为球状星团的研究提供新的思路. 相似文献
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本文详细地介绍了利用球状星团颜色一星等图确定球状星团年龄的方法、难点和一般结果。这种方法与估计宇宙年龄的其他两种方法彼此完全独立,但估出的宇宙年龄大体相符。球状星团与疏散星团年龄相比要老得多,表明它们是在银河系形成初期形成的恒星系统。球状星团的金属丰度分布,以及空间分布、运动性质与金属丰度的相关性,提供了球状星团形成过程的信息。邻近星系中球状星团的观测更丰富了人们对球状星团的了解。 相似文献
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根据球状星团动力学演化理论,本文探讨了球状星团致密度的演化与球状星用质量和位置的关系。结果表明,银心距和球状星团质量都与致密度的演化紧密相关。一般来说,银心距很大时致密度演化极少,球状星团质量愈大致密度演化愈缓慢。对现有球状星团致密度的分布作了统计研究,结果表明,在银心距较小的区域,致密度的分布与理论分析结果一致。在大银心距处发现,质量不同的球状星团其致密度分布有明显不同,它可能反映了球状星团形成阶段其致密度与质量紧密相关。 相似文献
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球状星团历来是业余天文观测者和专业天文学家特别喜爱观测研究的对象之一所谓星团,是由十几颗以上的恒星组成的、受各成员星之间引力束缚在一起的恒星群,其成员星的空间密度显著高于周围的恒星场。根据星团包含的恒星数、星团的形状和在银河系中位置分布的不同,星团又分为疏散星团和球状星团。疏散星团一般由十几颖、数百颗到上千颗恒星组成,结构松散、形状也不规则,它们 相似文献
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动力学过程和恒星演化及二者的互相影响都会对球状星团的演化产生重要影响.由于金属丰度会影响恒星的演化轨迹,与之相伴随的恒星质量损失率的变化也会对球状星团的动力学过程造成影响.通过一系列N体模拟研究金属丰度对球状星团的质量损失率、半径等的影响,并分析其原因,同时研究了大质量恒星以及星团初始数密度分布的影响.模拟中采用的球状星团模型初始成员星数目N=50000,运行于类银河系的引力势中并考虑成员星的演化.结果显示,由于低金属丰度恒星拥有较快的演化时标,所以贫金属球状星团在早期会拥有较高的质量损失,但与此同时它们的核塌缩时间会比后者显著推迟,因此在核塌缩之后其质量损失会被富金属星团反超.另外由于大质量恒星演化导致的质量损失较大,所以大质量星的存在会使金属丰度更加显著地影响球状星团早期的扩张以及随后的核塌缩过程,同时星团的初始数密度分布也对该效应有着不可忽视的影响. 相似文献
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We present N -body simulations (including an initial mass function) of globular clusters in the Galaxy in order to study effects of the tidal field systematically on the properties of the outer parts of globular clusters. Using nbody6 , which correctly takes into account the two-body relaxation, we investigate the development of tidal tails of globular clusters in the Galactic tidal field. For simplicity, we have employed only the spherical components (bulge and halo) of the Galaxy, and ignored the effects of stellar evolution which could have been important in the very early phase of the cluster evolution. The total number of stars in our simulations is about 20 000, which is much smaller than the realistic number of stars. All simulations had been done for several orbital periods in order to understand the development of the tidal tails. In our scaled-down models, the relaxation time is sufficiently short to show the mass segregation effect, but we did not go far enough to see the core collapse, and the fraction of stars lost from the cluster at the end of the simulations is only ∼10 per cent. The radial distribution of extra-tidal stars can be described by a power law with a slope around −3 in surface density. The directions of tidal tails are determined by the orbits and locations of the clusters. We find that the length of tidal tails increases towards the apogalacticon and decreases towards the perigalacticon. This is an anti-correlation with the strength of the tidal field, caused by the fact that the time-scale for the stars to respond to the potential is similar to the orbital time-scale of the cluster. The escape of stars in the tidal tails towards the pericentre could be another reason for the decrease of the length of tidal tails. We find that the rotational angular velocity of tidally induced clusters shows quite different behaviour from that of initially rotating clusters. 相似文献
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《New Astronomy》2002,7(7):395-433
The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population—all of the stars being old and metal-poor—similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to ∼0.3 M⊙. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way. 相似文献
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We have analyzed the formation, structure, and dynamical evolution of the population of stars that escaped from open clusters by numerical simulations using S. Aarseth’s modified NBODY6 code. In the Galactic tidal field, the population of stars that escaped from a cluster is shown to be elongated along the orbit of the cluster symmetrically about its core in the form of stellar tails of increasing sizes. We analyze the parameters of stellar tails as a function of such initial simulation conditions as the number of stars, the cluster density, the eccentricity of the Galactic cluster orbit in the plane of the Galactic disk, and the z velocity component. As a result, we constructed a grid of model stellar tails of open clusters. The grid includes such time-dependent parameters of the stellar tails as the length, the cross section, the number of stars, the velocity distribution, etc. Our simulations allow us to clarify the origin of moving clusters and stellar streams and to assess the role of star clusters in forming the stellar velocity field in the solar neighborhood. 相似文献
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Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal
(dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting
the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common
feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven
star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies.
The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally
∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the
remaining gas over the gravitational potential of the dSph galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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In our previous papers, we showed that at the final phases of the dynamical evolution of an open cluster, an extended population of stars elongated along its Galactic orbit, the stellar tail of the cluster, is formed. The tail stars that escaped from the cluster at different times move in a common orbit with low relative velocities. Experiencing a weak interaction with Galactic field stars, these objects, the relics of open clusters, can exist for a fairly long time. In this paper, we investigate the structures of such stellar tails in the nearest open clusters: Hyades, Pleiades, Praesepe, Alpha Persei, Coma, IC 2391, and IC 2602. To this end, we performed several numerical simulations of the dynamical evolution of these clusters in the tidal field of the Galaxy. Our computations of the dynamical evolution were based on known cluster age estimates and real Galactic orbits. The initial conditions were chosen in such a way that the parameters of the simulated clusters corresponded to their observed parameters. As a result, we obtained models of the stellar tails for the nearest open clusters and estimated such parameters of the tails as their sizes, densities, locations relative to the solar neighborhood, and others. 相似文献
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Jorge Peñarrubia Matthew G. Walker Gerard Gilmore 《Monthly notices of the Royal Astronomical Society》2009,399(3):1275-1292
We use N -body simulations to study the tidal evolution of globular clusters (GCs) in dwarf spheroidal (dSph) galaxies. Our models adopt a cosmologically motivated scenario in which the dSph is approximated by a static Navarro, Frenk & White halo with a triaxial shape. We apply our models to five GCs spanning three orders of magnitude in stellar density and two in mass, chosen to represent the properties exhibited by the five GCs of the Fornax dSph. We show that only the object representing Fornax's least dense GC (F1) can be fully disrupted by Fornax's internal tidal field – the four denser clusters survive even if their orbits decay to the centre of Fornax. For a large set of orbits and projection angles, we examine the spatial and velocity distribution of stellar debris deposited during the complete disruption of an F1-like GC. Our simulations show that such debris appears as shells, isolated clumps and elongated overdensities at low surface brightness (≥26 mag arcsec−2 ), reminiscent of substructure observed in several Milky Way dSphs. Such features arise from the triaxiality of the galaxy potential and do not dissolve in time. The kinematics of the debris depends strongly on the progenitor's orbit. Debris associated with box and resonant orbits does not display stream motions and may appear 'colder'/'hotter' than the dSph's field population if the viewing angle is perpendicular/parallel to the progenitor's orbital plane. In contrast, debris associated with loop orbits shows a rotational velocity that may be detectable out to a few kpc from the galaxy centre. Chemical tagging that can distinguish GC debris from field stars may reveal whether the merger of GCs contributed to the formation of multiple stellar components observed in dSphs. 相似文献
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L. Monaco M. Bellazzini F. R. Ferraro E. Pancino 《Monthly notices of the Royal Astronomical Society》2005,356(4):1396-1402
We present an analysis of the density profile in the central region of the Sagittarius dwarf spheroidal galaxy. A strong density enhancement of Sgr stars is observed. The position of the peak of the detected cusp is indistinguishable from the centre of M54. The photometric properties of the cusp are fully compatible with those observed in the nuclei of dwarf elliptical galaxies, indicating that the Sgr dSph would appear as a nucleated galaxy independently of the presence of M54 at its centre. 相似文献
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M. Bellazzini F. R. Ferraro R. Buonanno 《Monthly notices of the Royal Astronomical Society》1999,307(3):619-636
A detailed study of the star formation history of the Sagittarius dwarf spheroidal galaxy is performed through the analysis of data from the Sagittarius Dwarf Galaxy Survey (SDGS). Accurate statistical decontamination of the SDGS colour–magnitude diagrams (CMDs) allows us to obtain many useful constraints on the age and metal content of the Sgr stellar populations in three different regions of the galaxy.
A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]∼−2.0 to [Fe/H]∼−0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the star formation history of the Sgr dSph is derived. According to this scheme, star formation began at a very early time from a low metal content interstellar medium and lasted for several Gyr, coupled with progressive chemical enrichment. The star formation rate (SFR) had a peak from 8 to 10 Gyr ago, when the mean metallicity was in the range −1.3≤[Fe/H]≤−0.7. After that maximum, the SFR rapidly decreased and a very low rate of star formation took place until ∼1–0.5 Gyr ago. 相似文献
A coarse metallicity distribution of Sgr stars is derived, ranging from [Fe/H]∼−2.0 to [Fe/H]∼−0.7, the upper limit being somewhat higher in the central region of the galaxy. A qualitative global fit to all the observed CMD features is attempted, and a general scheme for the star formation history of the Sgr dSph is derived. According to this scheme, star formation began at a very early time from a low metal content interstellar medium and lasted for several Gyr, coupled with progressive chemical enrichment. The star formation rate (SFR) had a peak from 8 to 10 Gyr ago, when the mean metallicity was in the range −1.3≤[Fe/H]≤−0.7. After that maximum, the SFR rapidly decreased and a very low rate of star formation took place until ∼1–0.5 Gyr ago. 相似文献
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简述了精确测定相对自行的方法,特别介绍了在用2~3个历元的底片和CCD观测结果推导恒星自行的过程中如何消除光学视场畸变、星等差和色差的具体办法;并介绍了用星系把相对自行推算绝对自行的方法。还介绍了用自行资料研究银河系结构和演化的一些前沿课题,其中包括星团研究、与银河系兼并的矮星系的发现、暗物质的检测、外星行星的探测和银河系中心黑洞的质量估算等。最后评价了自行在研究银河系中的重要性,论述了我国研制4m光学/近红外望远镜的重要意义。 相似文献
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Eric Lagadec Albert A. Zijlstra G. C. Sloan Peter R. Wood Mikako Matsuura Jeronimo Bernard-Salas J. A. D. L. Blommaert M.-R. L. Cioni M. W. Feast M. A. T. Groenewegen Sacha Hony J. W. Menzies J. Th. van Loon P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2009,396(1):598-608
We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C2 H2 and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range 1.0–3.3 × 10−8 M⊙ yr−1 . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample. 相似文献