Warped discs and the directional stability of jets in active galactic nuclei

The radial distribution of globular clusters in galaxies is always less peaked to the centre than that of the halo stars. Extending previous work to a sample of Hubble Space Telescope globular cluster systems in ellipticals, we evaluate the number of clusters potentially lost to the galactic centre as the integrals of the difference between the observed globular cluster system distribution and the underlying halo light profile. In the sample of galaxies examined it is found that the initial populations of globular clusters may have been ∼30 per cent to 50 per cent richer than now. If these 'missing' globular clusters have decayed and have been partly destroyed in the very central galactic zones, they have carried there a significant quantity of mass that, plausibly, contributed to the formation and feeding of a massive object therein. It is relevant to note that the observed correlation between the core radius of the globular cluster system and the parent galaxy luminosity can be interpreted as a result of evolution.     

Zur Metallhäufigkeit in offenen Sternhaufen

The noteworthy deviations and differences given in the individual values of the metal abundance of open clusters were studied. It could be shown that the metal abundance of open clusters, which are situated in or near the galactic plane (|z| ≦ 0.3 Kpc) are not only correlated wiht the galactocentric distance (RGC ) but also with the galactic longitude l of the clusters. This correlation is not given in the open clusters of high |z|-distances. There, the metal abundance is connected as well with the galactocentric radius (RGC ) as with the distances of the objects form the galactic plane.     

DDO integrated photometry of globular clusters and initial chemical evolution of the galaxy

Integrated photometry of 91 galactic globular clusters was carried out with theUBV and DDO systems. A method to determine reddening is presented. Metal abundances were obtained by means of a calibration ofC(42–45)₀ vs [Fe/H]. Thirteen other clusters whose metallicity values were made compatible with those observed here, were added to the sample totalizing a homogenous set of [Fe/H] estimates for 104 globular clusters. This sample is used to analyze the spatial distribution of metallicities. An estimate of the intrinsic metal abundance frequency distribution of the globular cluster system is provided. The chemical properties of the globular cluster system are compared with one-zone-chemical-enrichment models which take into account gas loss from star formation regions. It is concluded that these models cannot describe the complete chemical enrichment of the globular cluster system.     

Evolution of globular cluster systems in three galaxies of the Fornax cluster

We studied and compared the radial profiles of globular clusters and of the stellar bulge component in three galaxies of the Fornax cluster observed with the WFPC2 of the Hubble Space Telescope ( HST ). The stars are more concentrated toward the galactic centres than globular clusters, in agreement with what has already been observed in many other galaxies: if the observed difference is the result of evolution of the globular cluster systems starting from initial profiles similar to those of the halo–bulge stellar components, a relevant fraction of their initial mass (74, 47 and 52 per cent for NGC 1379, 1399 and 1404, respectively) should have disappeared in the inner regions. This mass has probably contributed to the nuclear field population, local dynamics and high-energy phenomena in the primeval life of the galaxy. An indication in favour of the evolutionary interpretation of the difference between the globular cluster system and stellar bulge radial profiles is given by the positive correlation we found between the value of the mass lost from the globular cluster system and the central galactic black hole mass in the set of seven galaxies for which these data are available.     

The rotation of the globular cluster system

In this paper we study the possibility that all globular clusters rotate in the direct sense relatively to the galactic disk motion. We show with a simple model that this is possible if the projection of the cluster orbits on the galactic plane are elongated. We discuss the validity of this hypothesis.     

Migration of supermassive black holes in galactic nuclei

The migration of central black holes in galactic nuclei through their encounters with galactic globular clusters is studied. The black hole moves in the field of the galactic bulge with a fixed potential. The dependences of the black-hole drift amplitude on orbital parameters of the globular cluster, its mass, and bulge parameters have been found. The drift amplitude of the central black hole can reach several parsecs in our Galaxy and several tens of parsecs in early-type (Sa) and late-type (Sc) spiral galaxies.     

Ages of globular clusters. II

Evolutionary model sequences for (X, Z)=(0.7,4×10^–3) are constructed by using the same input physics and programming code as those of Saioet al. (1977). From these results the ages of globular clusters are estimated under the assumption of constant helium abundance (Y=0.3). The results suggest that there is a correlation between age and metal abundance for the globular clusters and that metal enrichment in the Galaxy slowly proceeded in several billion years from the value of the extreme Population II stars to that of the Population I stars. Comparison with some models of chemical evolution of galaxies is briefly made.     

2D Fokker–Planck models of rotating clusters

Globular clusters rotate significantly, and with the increasing amount of detailed morphological and kinematical data obtained in recent years on galactic globular clusters many interesting features show up. We show how our theoretical evolutionary models of rotating clusters can be used to obtain fits, which at least properly model the overall rotation and its implied kinematics in full 2D detail (dispersions, rotation velocities). Our simplified equal mass axisymmetric rotating model provides detailed two-dimensional kinematical and morphological data for star clusters. The degree of rotation is not dominant in energy, but also non-negligible for the phase-space distribution function, shape and kinematics of clusters. Therefore, the models are well applicable for galactic globular clusters. Since previously published papers on that matter by us made it difficult to do detailed comparisons with observations, we provide a much more comprehensive and easy-to-use set of data here, which uses as entries dynamical age and flattening of observed cluster and then offers a limited range of applicable models in full detail. The method, data structure and some exemplary comparison with observations are presented. Future work will improve modelling and data base to take a central black hole, a mass spectrum and stellar evolution into account.     

Galaxies and Star Clusters – From the Computer to the Real World

G01 New evidence for a connection between massive black holes and ULX G02 Long‐Term Evolution of Massive Black Hole Binaries G03 NBODY Meets Stellar Population Synthesis G04 N‐body modelling of real globular star clusters G05 Fokker‐Planck rotating models of globular clusters with black hole G06 Observational Manifestation of chaos in spiral galaxies: quantitative analysis and qualitative explanation G07 GRAPE Clusters: Beyond the Million‐Body Problem G08 Orbital decay of star clusters and Massive Black Holes in cuspy galactic nuclei G09 An Edge‐on Disk Galaxy Catalog G10 Complexes of open clusters in the Solar neighborhood G11 Search for and investigation of new stellar clusters using the data from huge stellar catalogues G12 Computing 2D images of 3D galactic disk models G13 Outer Pseudoring in the Galaxy G14 Where are tidal‐dwarf galaxies? G15 Ultra compact dwarf galaxies in nearby clusters G16 Impact of an Accretion Disk on the Structure of a stellar cluster in active galactic nuclei G17 Order and Chaos in the edge‐on profiles of disk galaxies G18 On the stability of OB‐star configurations in the Orion Nebula cluster G19 Older stars captured in young star clusters by cloud collapse G20 General features of the population of open clusters within 1 kpc from the Sun G21 Unstable modes in thin stellar disks G22 From Newton to Einstein – Dynamics of N‐body systems G23 On the relation between the maximum stellar mass and the star cluster mass     

Dynamical friction on globular clusters in core-triaxial galaxies: is it a cause of massive black hole accretion?

The present work extends and deepens previous examinations of the evolution of globular cluster orbits in elliptical galaxies, by means of numerical integrations of a wide set of orbits in five self-consistent triaxial galactic models characterized by a central core and different axial ratios. These models are valid and complete in the representation of regular orbits in elliptical galaxies. Dynamical friction is definitely shown to be an efficient cause of evolution for the globular cluster systems in elliptical galaxies of any mass or axial ratio. Moreover, our statistically significant sample of computed orbits confirms that the globular cluster orbital decay times are, at least for clusters moving on box orbits, much shorter than the age of the galaxies. Consequently, the mass carried into the innermost galactic region in the form of decayed globular clusters may have contributed significantly to feeding and accreting a compact object therein.     

The mass of the Andromeda galaxy

This paper argues that the Milky Way galaxy is probably the largest member of the Local Group. The evidence comes from estimates of the total mass of the Andromeda galaxy (M31) derived from the three-dimensional positions and radial velocities of its satellite galaxies, as well as the projected positions and radial velocities of its distant globular clusters and planetary nebulae. The available data set comprises 10 satellite galaxies, 17 distant globular clusters and nine halo planetary nebulae with radial velocities. We find that the halo of Andromeda has a mass of together with a scalelength of 90 kpc and a predominantly isotropic velocity distribution. For comparison, our earlier estimate for the Milky Way halo is Although the error bars are admittedly large, this suggests that the total mass of M31 is probably less than that of the Milky Way . We verify the robustness of our results to changes in the modelling assumptions and to errors caused by the small size and incompleteness of the data set.
Our surprising claim can be checked in several ways in the near future. The numbers of satellite galaxies, planetary nebulae and globular clusters with radial velocities can be increased by ground-based spectroscopy, while the proper motions of the companion galaxies and the unresolved cores of the globular clusters can be measured using the astrometric satellites Space Interferometry Mission ( SIM ) and Global Astrometric Interferometer for Astrophysics ( GAIA ). Using 100 globular clusters at projected radii 20 R 50 kpc with both radial velocities and proper motions, it will be possible to estimate the mass within 50 kpc to an accuracy of 20 per cent. Measuring the proper motions of the companion galaxies with SIM and GAIA will reduce the uncertainty in the total mass caused by the small size of the data set to 22 per cent.     

The period–luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations ( JHK _s) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period–luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster. The period–luminosity relation can be reproduced using the pulsation equation     assuming that all the stars have the same mass. Cluster RR Lyr variables were found to lie on an extrapolation of the period–luminosity relation. These results provide important constraints on the parameters of the variable stars.
Using Two Micron All-Sky Survey (2MASS) data, we show that the type II Cepheids in the Large Magellanic Cloud (LMC) fit our period–luminosity relation within the expected scatter at the shorter periods. However, at long periods (   P > 40  d, i.e. in the RV Tau star range) the LMC field variables are brighter by about one magnitude than those of similar periods in galactic globular clusters. The long-period cluster stars also differ from both these LMC stars and galactic field RV Tau stars in a colour–colour diagram. The reasons for these differences are discussed.     

The Mass Function of Young Star Clusters in the "Antennae" Galaxies

We determine the mass function of young star clusters in the merging galaxies known as the "Antennae" (NGC 4038/9) from deep images taken with the Wide Field Planetary Camera 2 on the refurbished Hubble Space Telescope. This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age, and hence the mass, of each cluster. We find that the mass function of the young star clusters (with ages less, similar160 Myr) is well represented by a power law of the form psi&parl0;M&parr0;~M-2 over the range 104 less, similarM less, similar106 M middle dot in circle. This result may have important implications for our understanding of the origin of globular clusters during the early phases of galactic evolution.     

Chemical evolution of the galactic halo

The chemical enrichment in the galactic halo is studied, on the basis of the numerical model developed in Paper I, with paricular attention to the overabundances of O and light elements with respect to Fe shown by metal poor stars. Some representative nucleosynthesis pictures for stars of both Population I and Population II are considered and their yields are compared with observations of relative abundances in the Sun and in the halo, to identify the possible reasons of the observed compositional differences. It is found that solar elemental ratios can be reproduced if intermediate mass stars are allowed to give some contribution to the production of Fe by type-I supernovae, while the ratios of abundances observed in the halo are more similar to the relative yields produced by massive stars. These features are shared by all the nucleosynthesis schemes which have been considered. Using the best model of Paper I, we show that the steep star formation induced by the collapse has a decisive effect in maintaining the overabundances of light elements during the whole evolution of the halo. The relevance of this conclusion is discussed also in the light of a possible interpretation of the differences between the two abundance scales for globular clusters.     

On a Formation Scenario of Star Clusters

Most formation scenarios of globular clusters assume a molecular cloud as the progenitor of the stellar system. However, it is still unclear, how this cloud is transformed into a star cluster, i.e. how the destructive processes related to gas removal or low star formation effiency can be avoided. Here a scheme of supernova (SN) induced cluster formation is studied. According to this scenario an expanding SN shell accumulates the mass of the cloud. This is accompanied by fragmentation resulting in star formation in the shell. Provided the stellar shell expands sufficiently slow, its self-gravity stops the expansion and the shell recollapses, by this forming a stellar system. I present N-body simulations of collapsing shells which move in a galactic potential on circular and elliptic orbits. It is shown that typical shells (10⁵ M_⊙, 30 pc) evolve to twin clusters over a large range of galactocentric distances. Outside this range single stellar systems are formed, whereas at small galactocentric distances the shells are tidally disrupted. In that case many small fragments formed during the collapse survive as single bound entities. About 1/3 of the twin cluster systems formed on circular orbits merge within 400 Myr. On elliptic orbits the merger rate reduces to less than 4%. Thus, there could be a significant number of twin clusters even in our Galaxy, which, however, might be undetected as twins due to a large phase shift on their common orbit. This revised version was published online in September 2006 with corrections to the Cover Date.     

A new interpretation of the metallicity histogram of globular clusters

A new interpretation is given to the low metallicity peak of the bimodal metallicity histogram of galactic globular clusters. It is proposed that these globular clusters are primordial,i.e., formed out of big-bang matter. Their nonvanishing metallicity is attributed to pollution by supermassive stars like R 136a. The first stellar generation is formed out of the ‘dirty’ primordial matter.     

Bursts of gravitational radiation from active galactic nuclei and globular clusters

The characteristics of gravitational bursts from active galactic nuclei, and globular clusters are obtained for three astrophysical situations:(i) scattering of stars by massive black holes residued at the centers of galaxies and globular clusters; (ii) the close encounters of stars in the nuclear regions of these objects; (iii) scattering of stars by black holes of stellar mass containing in the stellar population of galactic nuclei and clusters. The most effective source of gravitational bursts appears to be a scattering of stars by the massive central black holes which produces the bursts with dimensionless amplitudeh10^–19–10^–21 and frequencies from 10^–1 to 10^–5 Hz. The characteristics obtained correspond to the possiblities of a future gravitational-wave experiment with use of laser Doppler tracking of interplanetary spacecrafts.     

银河系球状星团轨道参数分布的Monte Carlo模拟

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。     

The chemical compositions and ages of globular clusters

Recent technological advances have led to a dramatic improvement in the quality of photometric and spectroscopic data obtainable on stars in globular clusters. Evidence from CCD-based colour magnitude diagrams points to clear differences in age between some clusters. High dispersion spectra show that abundance variations cannot explain the observed differences. In particular, it seems that NGC 288 must be 2–3 Gyr older than NGC 362. The same spectra show that although there is a spread in some molecular band strengths in NGC 362, the total C+N+O abundance remains constant, indicating that the material has undergone varying amounts of nuclear processing. No variations are seen in the abundances of iron group elements. Lower dispersion spectra for a large sample of faint stars in 47 Tucanae, obtained with a multi-object optical fibre system, show that unevolved main sequence stars in that cluster share the same CNO variations as the bright giants. The conclusion from all these data is that the intra-cluster CNO variations are neither truly primordial nor due to evolutionary mixing. It may be that there was a sufficiently extended period of star formation for material from first generation stars to be used in later generations, or that some pollution has occurred due to mass loss. Finally, it is noted that if ‘prehistoric’ clusters exist with ages of around 50 Gyr, as hypothesised in some cosmological models, these should probably still be rather obvious and readily recognised. Paper presented at the 6th Asian-Pacific Regional Meeting on Astronomy of the International Astronomical Union, India, 1993.     

Multiple populations in globular clusters Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, but rather are made up of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second-parameter problem, it also opens new perspectives on the relation between globular clusters and the halo of our Galaxy, and by extension on all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focussing on the most recent studies. Several points remain to become properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.