共查询到20条相似文献，搜索用时 15 毫秒
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Simon P. Goodwin 《Monthly notices of the Royal Astronomical Society》1998,294(1):47-60
N -body simulations are made with a variety of initial conditions, in particular clumpy and flattened distributions, to attempt to constrain the possible initial conditions of globular clusters, using the observations that young LMC globular clusters appear relaxed after only 20 to 40 Myr. It is found that violent relaxation is able to erase most of the initial substructure in only ≈ 6 crossing times. However, initially very clumpy distributions (≲ 100 clumps) form clusters that are too concentrated to resemble real globular clusters. Such clusters also often have large clumps in long-lasting (≳ 30 crossing times) orbits which do not appear in observed cluster profiles. It is also found that even modest amounts of initial flattening produce clusters that are too elliptical to resemble real globular clusters. In such a scenario, cloud–cloud collisions and similar energetic processes would be unlikely to produce sufficiently spherical globular clusters. It is suggested that globular clusters form from roughly spherical initial conditions with star formation occurring either smoothly or in many small clumps. 相似文献
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J. W. Menzies 《Astrophysics and Space Science》1995,230(1-2):187-194
Southern hemisphere astronomers are well placed to observe globular clusters in the Galaxy, and significant work has been done on them in South Africa. This brief review covers research on colour-magnitude diagrams and spectroscopic analyses of cluster stars as well as on miscellaneous stars representing late evolutionary stages. 相似文献
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A. Bragaglia E. Carretta R.G. Gratton S. Lucatello A. Milone G. Piotto V. D'Orazi S. Cassisi C. Sneden R.L. Bedin 《Astronomische Nachrichten》2011,332(3):258-259
We observed two main sequence stars in the globular cluster NGC 2808, using X‐shooter. We selected one of the targets on the blue main sequence (bMS) and one on the red main sequence (rMS) and measured abundances for several light elements that are expected to be different in stars of first and second generations in globular clusters. The differences between the bMS and the rMS stars amply exceed the errors and is in agreement with a difference in products of hydrogen burning at high temperature. More data are required to put the findings on more solid basis and to try to distinguish between the different models proposed to explain the formation of globular clusters (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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The crossing of the Galactic disc by a globular cluster (GC) could produce star formation due to gravitational focusing or compression of disc material. We report on simulations of the effect on disc material which reveal that the crossing can sometimes cause local gravitational focusing of disc material. We also present the salient points of a little-known paper by Levy, which shows that strong compression can result from the shock wave generated by GC disc crossing. The main thrust of our paper is a search for remnants of disc crossings by GCs. Using the gravitational potential of the Galaxy to locate the position of the most recent crossings of a subset of fifty-four GCs reveals that systematic errors and uncertainties in initial conditions limit the scope for unequivocal identification. From the subset of fifty-four, six possible search sites with the best constraints are retained for further scrutiny. Three of the six potentially promising search areas in the disc are from GCs NGC 3201, 6397 and 6838, for which we cannot rule out some observed star associations observed nearby as being remnants. The other three of the six areas are too large to provide meaningful identification of remnants. Also, a possible remnant (open cluster NGC 6231) is shown not to be due to GC impact, contrary to a previous report. In a more wide-ranging screening of 155 GCs, we identify which GCs are compatible with being responsible for the formation of any of the Galaxy's five most prominent star superclusters. 相似文献
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Dean E. McLaughlin Pauline Barmby William E. Harris Duncan A. Forbes Gretchen L. H. Harris 《Monthly notices of the Royal Astronomical Society》2008,384(2):563-590
We present internal surface brightness profiles, based on Hubble Space Telescope /ACS imaging in the F 606 W bandpass, for 131 globular cluster (GC) candidates with luminosities L ≃ 104 –3 × 106 L⊙ in the giant elliptical galaxy NGC 5128. Several structural models are fitted to the profile of each cluster and combined with mass-to-light ratios ( M / L values) from population-synthesis models, to derive a catalogue of fundamental structural and dynamical parameters parallel in form to the catalogues recently produced by McLaughlin & van der Marel and by Barmby et al. for GCs and massive young star clusters in Local Group galaxies. As part of this, we provide corrected and extended parameter estimates for another 18 clusters in NGC 5128, which we observed previously. We show that, like GCs in the Milky Way and some of its satellites, the majority of globulars in NGC 5128 are well fitted by isotropic Wilson models, which have intrinsically more distended envelope structures than the standard King lowered isothermal spheres. We use our models to predict internal velocity dispersions for every cluster in our sample. These predictions agree well in general with the observed dispersions in a small number of clusters for which spectroscopic data are available. In a subsequent paper, we use these results to investigate scaling relations for GCs in NGC 5128. 相似文献
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C. Bonatto E. Bica S. Ortolani B. Barbuy 《Monthly notices of the Royal Astronomical Society》2009,397(2):1032-1040
With Two-Micron All-Sky Survey (2MASS) photometry and proper motions, Bonatto et al. suggested that FSR 1767 is a globular cluster (GC), while with J and K NTT/SOFI photometry Froebrich, Meusinger & Scholz concluded that it is not a star cluster. In this study, we combine previous and new evidence that are consistent with a GC. For instance, we show that the horizontal branch (HB) and red giant branch (RGB) stars, besides sharing a common proper motion, have radial density profiles that consistently follow the King's law independently. Reddening maps around FSR 1767 are built using the bulge RGB as reference and also Schlegel's extinction values to study local absorptions. Both approaches provide similar maps and show that FSR 1767 is not located in a dust window, which otherwise might have produced the stellar overdensity. Besides, neighbouring regions of similar reddening as FSR 1767 do not present the blue HB stars that are a conspicuous feature in the colour–magnitude diagram of FSR 1767. We report the presence of a compact group of stars located in the central parts of FSR 1767. It appears to be a detached post-collapse core, similar to those of other nearby low-luminosity GCs projected towards the bulge. We note that while the NTT/SOFI photometry of the star cluster FSR 1716 matches perfectly that from 2MASS, it shows a considerable offset for FSR 1767. We discuss the possible reasons why both photometries differ. We confirm our previous structural and photometric fundamental parameters for FSR 1767, which are consistent with a GC. 相似文献
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J. Gerssen 《Astronomische Nachrichten》2004,325(2):84-87
The high central stellar densities in globular clusters provide a unique environment to study the fundamental dynamical process of two‐body relaxation. This process is the main driver of the dynamical evolution in the center of a globular cluster and has a profound effect on the structure of the cluster and on its stellar environment. We have obtained stellar absorption line spectra with STIS to measure the radial velocities of individual stars in the crowded center of the globular cluster M15. These data increase the number of stars with known radial velocities within the central arcsec by a factor of about three and significantly improve the constraints on the mass distribution in M15. The data provide the most detailed look of the central structure of any globular cluster and show that there is a compact dark central mass component. Similar studies using ground based facilities can be efficiently performed by employing Integral Field Units. We have started a project to better constrain the central mass density in the globular cluster M3 using the GMOS‐IFU on Gemini North. The data will also allow us to better understand the central rotation which is neither explained nor predicted by any globular cluster model. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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M. Gieles 《Monthly notices of the Royal Astronomical Society》2009,394(4):2113-2126
Several recent studies have shown that the star cluster initial mass function (CIMF) can be well approximated by a power law, with indications for a steepening or truncation at high masses. This contribution considers the evolution of such a mass function due to cluster disruption, with emphasis on the part of the mass function that is observable in the first ∼1 Gyr. A Schechter type function is used for the CIMF, with a power-law index of −2 at low masses and an exponential truncation at M * . Cluster disruption due to the tidal field of the host galaxy and encounters with giant molecular clouds flattens the low-mass end of the mass function, but there is always a part of the 'evolved Schechter function' that can be approximated by a power law with index −2. The mass range for which this holds depends on age, τ, and shifts to higher masses roughly as τ0.6 . Mean cluster masses derived from luminosity-limited samples increase with age very similarly due to the evolutionary fading of clusters. Empirical mass functions are, therefore, approximately power laws with index −2, or slightly steeper, at all ages. The results are illustrated by an application to the star cluster population of the interacting galaxy M51, which can be well described by a model with M * = (1.9 ± 0.5) × 105 M⊙ and a short (mass-dependent) disruption time destroying M * clusters in roughly a Gyr. 相似文献
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选取前文中所列出的29个累积光谱型为F型的球状星团中的3个作为样本，深入研究了初始观测资料的不确定性和选用不同的银河系引力势模型，对样本星团轨道参数的影响。首先采用Monte Carlo方法产生3个样本球状星团的模拟初始观测数据，而后，以这些模拟数据为初始条件，在3种不同的银河系引力势模型下进行轨道计算，得到此3个样本的模拟轨道参数。模拟计算的结果表明：根据模拟初始数据生成的样本轨道参数分布形态大致可分为高斯分布、准高斯分布和非高斯分布等3类；初始观测数据的不确定性对样本轨道参数分布的影响，与样本星团的选择和轨道参数的类型有关；选用不同的银河系引力势模型，对3个样本星团的各个轨道参数的分布和形态结构也会产生不同程度的影响。该工作的结果，可供深入研究球状星团的整体运动和动力学性质等问题参考。 相似文献
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We first present a self-consistent dynamical model in which ω Cen is formed from an ancient nucleated dwarf galaxy merging with the first generation of the Galactic thin disc in a retrograde manner with respect to the Galactic rotation. Our numerical simulations demonstrate that during merging between the Galaxy and the ω Cen host dwarf with M B ≃−14 mag and its nucleus mass of 107 M⊙ , the outer stellar envelope of the dwarf is nearly completely stripped, whereas the central nucleus can survive from the tidal stripping because of its compactness. The developed naked nucleus has a very bound retrograde orbit around the young Galactic disc, as observed for ω Cen, with apocentre and pericentre distances of ∼8 and ∼1 kpc, respectively. The Galactic tidal force can induce radial inflow of gas to the centre of the dwarf and consequently triggers moderately strong nuclear starbursts in a repetitive manner. This result implies that efficient nuclear chemical enrichment resulting from the later starbursts can be closely associated with the origin of the observed relatively young and metal-rich stars in ω Cen. Dynamical heating by the ω Cen host can transform the young thin disc into the thick disc during merging. 相似文献