共查询到20条相似文献，搜索用时 922 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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UBV stellar photometry of bright stars in GC M5 – II. Physical parameters of horizontal branch stars
P.V. Baev H. Markov N. Spassova 《Monthly notices of the Royal Astronomical Society》2001,328(3):944-950
The physical parameters of the stars in the central region of the globular cluster M5 (NGC 5904) were determined from UBV photometry using Kurucz's synthetic flux distributions and some empirical relations. It is found that the bluest horizontal branch (HB) stars have higher luminosities than predicted by canonical zero-age horizontal branch models. Parameters of the mass distribution on the HB stars are determined. It is shown that the gap in the blue HB previously reported by Markov et al. in Paper I is probably a statistical fluctuation. 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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Mirek Giersz Douglas C. Heggie † 《Monthly notices of the Royal Astronomical Society》2009,395(2):1173-1183
We describe Monte Carlo models for the dynamical evolution of the nearby globular cluster NGC 6397. The code includes treatments of two-body relaxation, most kinds of three- and four-body interactions involving primordial binaries and those formed dynamically, the Galactic tide and the internal evolution of both single and binary stars. We arrive at a set of initial parameters for the cluster which, after 12 Gyr of evolution, gives a model with a fairly satisfactory match to the surface brightness profile, the velocity dispersion profile and the luminosity function in two fields. We describe in particular those aspects of the evolution which distinguish this cluster from M4, which has a roughly similar mass and Galactocentric distance, but a qualitatively different surface brightness profile. Within the limitations of our modelling, we conclude that the most plausible explanation for the difference is fluctuations: both clusters are post-collapse objects, but sometimes have resolvable cores and sometimes not. 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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It is expected that specific globular clusters (GCs) can contain up to a hundred of millisecond pulsars. These pulsars can accelerate leptons at the shock waves originated in collisions of the pulsar winds and/or inside the pulsar magnetospheres. Energetic leptons diffuse gradually through the GC Comptonizing stellar and microwave background radiation. We calculate the GeV–TeV γ-ray spectra for different models of injection of leptons and parameters of the GCs assuming reasonable, of the order of 1 per cent, efficiency of energy conversion from the pulsar winds into the relativistic leptons. It is concluded that leptons accelerated in the GC cores should produce well localized γ-ray sources which are concentric with these GCs. The results are shown for four specific GCs (47 Tuc, Ter 5, M13 and M15), in which significant population of millisecond pulsars have been already discovered. We argue that the best candidates, which might be potentially detected by the present Cherenkov telescopes and the planned satellite telescopes (AGILE, GLAST), are 47 Tuc on the Southern hemisphere, and M13 on the Northern hemisphere. We conclude that detection (or non-detection) of GeV–TeV γ-ray emission from GCs by these instruments put important constraints on the models of acceleration of leptons by millisecond pulsars. 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
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A. D. Mackey A. M. N. Ferguson M. J. Irwin N. F. Martin A. P. Huxor N. R. Tanvir S. C. Chapman R. A. Ibata G. F. Lewis A. W. McConnachie 《Monthly notices of the Royal Astronomical Society》2010,401(1):533-546
We report on deep imaging of a remote M31 globular cluster, MGC1, obtained with Gemini/GMOS. Our colour–magnitude diagram for this object extends ∼5 mag below the tip of the red-giant branch and exhibits features consistent with an ancient metal-poor stellar population, including a long, well-populated horizontal branch. The red-giant branch locus suggests MGC1 has a metal abundance [M/H]≈−2.3 . We measure the distance to MGC1 and find that it lies ∼160 kpc in front of M31 with a distance modulus μ= 23.95 ± 0.06 . Combined with its large projected separation of R p = 117 kpc from M31, this implies a deprojected radius of R gc = 200 ± 20 kpc , rendering it the most isolated known globular cluster in the Local Group by some considerable margin. We construct a radial brightness profile for MGC1 and show that it is both centrally compact and rather luminous, with MV =−9.2 . Remarkably, the cluster profile shows no evidence for a tidal limit and we are able to trace it to a radius of at least 450 pc, and possibly as far as ∼900 pc. The profile exhibits a power-law fall-off with exponent γ=−2.5 , breaking to γ=−3.5 in its outermost parts. This core-halo structure is broadly consistent with expectations derived from numerical models, and suggests that MGC1 has spent many gigayears in isolation. 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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