Properties Of Globular Clusters Research Articles

VLT/UVES spectroscopy of individual stars in three globular clusters in the Fornax dwarf spheroidal galaxy

We present a high resolution (R ~ 43 000) abundance analysis of a total of nine stars in three of the five globular clusters associated with the nearby Fornax dwarf spheroidal galaxy. These three clusters (1, 2 and 3) trace the oldest, most metal-poor stellar populations in Fornax. We determine abundances of O, Mg, Ca, Ti, Cr, Mn, Fe, Ni, Zn, Y, Ba, Nd and Eu in most of these stars, and for some stars also Mn and La. We demonstrate that classical indirect methods (isochrone fitting and integrated spectra) of metallicity determination lead to values of [Fe/H] which are 0.3 to 0.5 dex too high, and that this is primarily due to the underlying reference calibration typically used by these studies. We show that Cluster 1, with [Fe /H] = -2.5, now holds the record for the lowest metallicity globular cluster. We also measure an over-abundance of Eu in Cluster 3 stars that has only been previously detected in a subgroup of stars in M 15. We find that the Fornax globular cluster properties are a global match to what is found in their Galactic counterparts; including deep mixing abundance patterns in two stars. We conclude that at the epoch of formation of globular clusters both the Milky Way and the Fornax dwarf spheroidal galaxy shared the same initial conditions, presumably pre-enriched by the same processes, with identical nucleosynthesis patterns.

Gravity, dimension, equilibrium, and thermodynamics

It is actually possible to interpret gravitation as a property of space in a purely classical way. We note that an extended self-gravitating system equilibrium depends directly on the number of dimensions of the space in which it evolves. Given these precisions, we review the principal thermodynamical knowledge in the context of classical gravity with arbitrary dimension of space. Stability analyses for bounded 3D systems, namely the Antonov instability paradigm, are then associated to some amazing properties of globular clusters and galaxies. To cite this article: J. Perez, C. R. Physique 7 (2006).

Galactic Globular Clusters with Luminous X‐Ray Binaries

Luminous X-ray binaries (LMXBs; >1034 ergs s-1) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to have evolved from wider binaries through dynamical interactions with other stars. We attempt to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L1.5/r, is useful for understanding the presence of luminous LMXBs in the Galactic globular cluster system. We find that while LMXB clusters with large values of L1.5/r preferentially host LMXBs, the systems must also have half-mass relaxation times below th,relax ~ 109 yr. This relaxation time effect probably occurs because several relaxation times are required to modify binary separations, a timescale that must be shorter than cluster ages. The likelihood of finding an LMXB cluster is enhanced if the cluster is metal-rich and if it is close to the bulge region. The dependence on metallicity is most likely either due to differing initial mass functions at the high-mass end or because bulge systems evolve more rapidly from tidal interactions with the bulge. This approach can be used to investigate globular cluster systems in external galaxies, where core properties are unresolved.

Globular cluster formation from gravitational tidal effects of merging and interacting galaxies

We investigate the spatial, kinematic and chemical properties of globular cluster systems formed in merging and interacting galaxies using N-body–smoothed particle hydrodynamics (SPH) simulations. Although we cannot resolve individual clusters in our simulation, we assume that they form in collapsing molecular clouds when the local external gas pressure exceeds 105kB (where kB is the Boltzmann constant). Several simulations are carried out for a range of initial conditions and galaxy mass ratios. The input model spirals are given a halo globular cluster system similar to those observed for the Milky Way and M31. Gravitational tidal effects during galaxy merging and interaction lead to a dramatic increase in gas pressure, which exceeds our threshold and hence triggers new globular cluster formation. We investigate the properties of the globular cluster system in the remnant galaxy, such as the number density, the specific frequency, kinematic properties and the metallicity distribution. Different orbital conditions and mass ratios give rise to a range in globular cluster properties, particularly for the interaction models. Our key results are the following: the newly formed metal-rich clusters are concentrated at the centre of the merger remnant elliptical, whereas the metal-poor ones are distributed to the outer parts because of strong angular momentum transfer. The dissipative merging of present-day spirals, including chemical evolution, results in metal-rich clusters with a mean metallicity that is super-solar, i.e. much higher than is observed in elliptical galaxies. If elliptical galaxies form by dissipative major mergers, then they must do so at very early epochs when their discs contained low-metallicity gas. Our simulations show that the specific frequency can be increased in a dissipative major merger. However, when this occurs it results in a ratio of metal-poor to metal-rich clusters that is less than one, contrary to the ratio observed in many elliptical galaxies.

The Globular Cluster Systems of Ellipticals and Spirals

In this overview of the key properties of globular cluster (GC) systems I show that the GCs in elliptical and spiral host galaxies have more in common than previously thought. After contrasting these properties I briefly comment on GC formation.

Keck imaging of the globular cluster systems in the early-type galaxies NGC 1052 and 7332

The presence of two globular cluster subpopulations in early-type galaxies is now the norm rather than the exception. Here we present two more examples for which the host galaxy appears to have undergone a recent merger. Using multi-colour Keck imaging of NGC 1052 and 7332 we find evidence for a bimodal globular cluster colour distribution in both galaxies, with roughly equal numbers of blue and red globular clusters. The blue ones have similar colours to those in the Milky Way halo and are thus probably very old and metal-poor. If the red globular cluster subpopulations are at least of solar metallicity, then stellar population models indicate young ages. We discuss the origin of globular clusters within the framework of formation models. We conclude that recent merger events in these two galaxies have had little effect on their overall globular cluster systems. We also derive globular cluster density profiles, global specific frequencies and, in the case of NGC 1052, radial colour gradients and azimuthal distribution. In general these globular cluster properties are normal for early-type galaxies.

The 'second parameter': a memory from the globular cluster formation epoch

We study the correlations between the distribution of stars on the horizontal branch (HB), the HB morphology, and some other properties of globular clusters (GCs) in a large sample of GCs. We strengthen previous results that a general correlation exists only between HB morphology and metallicity. Correlations with other properties, e.g. central density and core radius, exist only for GCs within a narrow metallicity range. We conjecture that the lack of correlations with present properties of GCs (besides metallicity) is because the variation of the HB morphologies between GCs having similar metallicities is caused by a process, or processes, the effect of which was determined at the formation time of GCs. This process (or processes) is historically termed the ‘second parameter’, metallicity being the ‘first parameter’. We then argue that the ‘planet second parameter’ model, where the presence of planets and to a lesser degree brown dwarfs and low-mass main-sequence stars is the ‘second parameter’, fits this conjecture. This is because the processes that determine the presence of planets and their properties occur during the formation epoch of the star and its circumstellar disc.

Globular Clusters in the [CLC]d[/CLC]E,N Galaxy NGC 3115 DW1: New Insights from Spectroscopy and [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Photometry

The properties of globular clusters in dwarf galaxies are key to understanding the formation of globu- lar cluster systems and in particular in verifying scenarios in which globular cluster systems of larger galaxies formed (at least partly) from the accretion of dwarf galaxies. Here, we revisit the globular cluster system of the dE,N galaxy NGC 3115 DW1¨a companion of the nearby S0 galaxy NGC 3115¨adding Keck/LRIS spectroscopy and Hubble Space T elescope (HST ) Wide Field Planetary Camera 2 imaging to previous ground-based photometry. Spectra for seven globular clusters reveal normal abundance ratios with respect to the Milky Way and M31 clusters, as well as a relatively high mean metallicity ((Fe/ H) B (1.0 ^ 0.1 dex). Crude kinematics indicate a high velocity dispersion within 10 kpc that could be caused either by dark matterdominated outer regions or by the stripping of outer globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass out to 3 and 10 kpc lies between 1 ) 1010 and 1 ) 1011 and 2 ) 1010 and 4 ) 1011 respectively, depending on the mass estimator used M _ M _ , and the assumptions on cluster orbits and systemic velocity. The HST imaging allows measurement of sizes for two clusters, returning core radii around 2.0 pc, similar to the sizes observed in other galaxies. Spectroscopy allows an estimate of the degree of contamination by foreground stars or background gal- axies for the previous ground-based photometry but does not require a revision of previous results: NGC 3115 DW1 hosts around clusters, which corresponds to a speci—c frequency of N GC 60 ^ 20 on the high end for massive dEs. Given its absolute magnitude mag) and S N 4.9 ^ 1.9, (M V \( 17.7 the properties of its cluster system, NGC 3115 DW1 appears to be a transition between a luminous dE and low-luminosity E galaxy.

Evidence for the Hierarchical Formation of the Galactic Spheroid

The possibility that the Galactic spheroid was assembled from isolated, chemically distinct proto-Galactic fragments is investigated using a Monte Carlo technique designed to simulate the chemical evolution of the Galaxy in hierarchical formation scenarios which involve no gas dissipation. By comparing the observed and simulated metallicity distributions of Galactic globular clusters and halo field stars, we estimate the level of fragmentation in the collapsing proto-Galaxy. Although the formation process is highly stochastic, the simulations frequently show good agreement with the observed metallicity distributions, provided the luminosity function of proto-Galactic fragments had the form dN ∝ Lα dL, where α ~ -2. While this steep slope is strongly at odds with the presently observed luminosity function of the Local Group, it is in close agreement with the predictions of semianalytic and numerical models of hierarchical galaxy formation. We discuss a number of possible explanations for this discrepancy. These simulations suggest that the Galactic halo and its globular cluster system were assembled via the accretion and disruption of ~103 metal-poor, proto-Galactic fragments by the dominant building block: a protobulge whose own metal-rich globular clusters system has been preferentially eroded by dynamical processes. This formation scenario may provide a simple explanation for the different shapes of the Galactic globular cluster and halo star metallicity distributions. Based on the similar properties of globular clusters belonging to spiral and giant elliptical galaxies, we argue that the same process (e.g., hierarchical growth involving little gas dissipation) is responsible for the formation of both giant elliptical galaxies and the bulge-halo components of spiral galaxies.

Globular Cluster Formation Triggered by the Initial Starburst in Galaxy Formation.

We propose and investigate a new formation mechanism for globular clusters in which they form within molecular clouds that are formed in the shocked regions created by galactic winds driven by successive supernova explosions shortly after the initial burst of massive star formation in the galactic centers. The globular clusters have a radial distribution that is more extended than that of the stars because the clusters form as pressure-confined condensations in a shell that is moving outward radially at high velocity. In addition, the model is consistent with existing observations of other global properties of globular clusters, as far as comparisons can be made.

The fundamental straight line of galactic globular clusters

A detailed analysis of the Fundamental Plane properties of globular clusters is performed. If a sample of ordinary King Model clusters is considered, it is found that, in the space (S-space) defined by the parameters (log r 0, log σ 0, μ V (0)), their configuration is similar to a straight line. It is shown that, with rather simple assumptions, a simultaneous explanation of all the observed correlations between S-space parameters can be provided. It is suggested that, at earlier times, globular clusters populated a line in the three-dimensional S-space, i.e. their original dynamical structure was fully determined by a single physical parameter.

On the evolution of the Galactic globular cluster system

In this paper we address the issue of the origin of some observational properties of the galactic globular cluster system. After a preliminary study of some general properties of the main evolutionary processes, we investigate the evolution of systems of globular clusters located in a model of the Milky Way starting from different initial conditions. We study the role of the evolutionary processes in changing the spatial distribution and mass function of the cluster system, in establishing and/or preserving some of the observed correlations and trends between internal properties of globular clusters and between internal properties and location inside the host galaxy and we provide an estimate for the rates of core collapse and disruption of globular clusters. The initial mass function and spatial distribution of the cluster system evolve quite significantly in one Hubble time and the evolution is toward a final state similar to the observed one. If the mass function is initially taken to be a log-normal distribution similar to the one currently observed in our galaxy, its shape is not significantly altered during the entire simulation even though a significant number of clusters are disrupted before one Hubble time, which suggests that the present mass function might represent a sort of 'quasi-equilibrium' distribution.

Evolution of the Galactic Globular Cluster System

Recent surveys of the observational properties of galactic globular clusters have shown the existence of interesting correlations and trends between structural parameters and between structural parameters and location inside the Galaxy (Chernoff & Djorgovski 1989, Djorgovski & Meylan 1994). The origin of most of these correlations is not clear yet and it is not clear to what extent they reflect the primordial conditions or the result of evolution. We have carried out a set of simulations following the evolution of the properties of a globular cluster system (mass function, spatial distribution, correlations between structural parameters) starting from given initial conditions. The evolution of each individual cluster has been followed by the same method applied by Chernoff et al. (1986) and Chernoff & Shapiro (1987). The effects of internal relaxation, disk shocking and dynamical friction have been considered. The main goal of the analysis is that of establishing the role of initial conditions and evolutionary processes in determining the present observational properties.

Correlations of Globular Cluster Properties as Constraints for Dynamical Evolution Models

Correlations between globular cluster (GC) properties are reviewed, including some new work on cluster tidal radii and densities. Most of the observed correlations can be interpreted within the framework of our current understanding of their dynamical evolution. These correlations provide empirical constraints for models of GC formation and evolution.

What determines the stellar mass functions in globular clusters?

We analyze the dependence of stellar mass function slopes for a sample of 17 globular clusters on a variety of cluster parameters. The principal novelty of our approach is the use of appropriate multivariate statistical methods to disentangle the complex situation which is present in this problem: the slopes depend simultaneously on more than one variable, and many cluster parameters are mutually correlated. We find that the mass function slopes in the range M/M(solar) = 0.5-0.8 are largely determined by the position in the Galaxy and to a lesser extent by the cluster metallicity. Clusters closer to the Galactic center or plane have shallower mass function slopes. At a given distance to the Galactic center, clusters closer to the Galactic plane have shallower mass function slopes. At a given R(GC) and/or Z(GP), more metal-rich clusters have shallower mass function slopes. Thus, the monovariate correlations with the position or metallicity are both correct, but only partial, and in terms of slopes, biased descriptions of the situation. We present trivariate least-squares solutions where the mass function slopes can be predicted within the measurement accuracy. This relation can serve as a powerful observational constraint for theories of globular cluster formation and evolution, and it is one of the tightest correlations between globular cluster properties now known.

Globular cluster formation - The fossil record

Properties of globular clusters which have remained unchanged since their formation are used to infer the internal pressures, cooling times, and dynamical times of the protocluster clouds immediately prior to the onset of star formation. For all globular clusters examined, it is found that the cooling times are much less than the dynamical times, implying that the protoclusters must have been maintained in thermal equilibrium by external heat sources, with fluxes consistent with those found in previous work, and giving the observed rho-T relation. Self-gravitating clouds cannot be stably heated, so that the Jeans mass forms an upper limit to the cluster masses. The observed dependence of protocluster pressure upon galactocentric position implies that the protocluster clouds were in hydrostatic equilibrium after their formation. The pressure dependence is well fitted by that expected for a quasi-statically evolving background hot gas, shock heated to its virial temperature. The observations and inferences are combined with previous theoretical work to construct a picture of globular cluster formation.

A theory for the origin of globular clusters

view Abstract Citations (514) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A theory for the origin of globular clusters. Fall, S. M. ; Rees, M. J. Abstract The authors present a theory for the origin of globular clusters during the collapse of a protogalaxy. A thermal instability promotes the development of a two-phase structure in the gas when the cooling time is comparable to the free-fall time. The hot component, which remains near the virial temperature, compresses the cold component into discrete clouds with temperatures near 104K and mean densities in the range 1 - 10 M_sun;pc-3. It is shown that the initial amplitudes of the perturbations required to produce such clouds are of order 10%. When the abundance of heavy elements is less than 10-2Z_sun;, further cooling is inefficient and the minimum mass for gravitational instability is of order 106M_sun;. The authors compare these predictions with the observed properties of globular clusters and find satisfactory agreement, especially if there is some mass loss. Publication: The Astrophysical Journal Pub Date: November 1985 DOI: 10.1086/163585 Bibcode: 1985ApJ...298...18F Keywords: Galactic Evolution; Globular Clusters; Interstellar Gas; Molecular Clouds; Perturbation Theory; Red Shift; Virial Theorem; Astrophysics full text sources ADS | data products SIMBAD (3)

The globular cluster system of the galaxy. IV - The halo and disk subsystems

view Abstract Citations (1117) References (88) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Globular Cluster System of the Galaxy. IV. The Halo and Disk Subsystems Zinn, R. Abstract The spatial distributions, kinematics, and metallicities of 121 globular clusters are examined for evidence of distinct subpopulations in the cluster system. Clusters more metal-poor than (Fe/H) = -0.8 constitute the familiar halo population, while clusters more metal-rich than (Fe/H) = -0.8 have the properties of a disk system. There is a shallow but significant metallicity gradient with R in the halo population. The central concentrations and ellipticities of the halo and disk clusters are similar, while the latter are more evenly distributed in luminosity than the former. Excellent agreement is found between the properties of globular clusters and those of RR Lyrae variables and subdwarfs when these are also divided by metallicity at (Fe/H) = -0.8, which suggests that clusters are good tracers of the halo and old disk stellar populations. The scale height of the disk clusters appears to be about 500 pc, and it is argued that disk clusters are relics of a transient, thick-disk phase in the evolution of the Galaxy. Publication: The Astrophysical Journal Pub Date: June 1985 DOI: 10.1086/163249 Bibcode: 1985ApJ...293..424Z Keywords: Galactic Structure; Globular Clusters; Metallicity; Milky Way Galaxy; Stellar Composition; Angular Velocity; Ellipticity; Histograms; Radial Velocity; Stellar Luminosity; Subdwarf Stars; Variable Stars; Astrophysics full text sources ADS | data products SIMBAD (119) Related Materials (4) Erratum: 1986ApJ...304..579Z Part 1: 1980ApJS...42...19Z Part 2: 1980ApJ...241..602Z Part 3: 1984ApJS...55...45Z

Short-term X-ray variability of the globular cluster source 4U 1820 - 30 (NGC 6624)

Analytical techniques for improved identification of the temporal and spectral variability properties of globular cluster and galactic bulge X-ray sources are described in terms of their application to a large set of observations of the source 4U 1820 - 30 in the globular cluster NGC 6624. The autocorrelation function, cross-correlations, time skewness function, erratic periodicities, and pulse trains are examined. The results are discussed in terms of current models with particular emphasis on recent accretion disk models. It is concluded that the analyzed observations provide the first evidence for shot-noise variability in a globular cluster X-ray source.

Test computations on the dynamical evolution of star clusters

Test calculations have been carried out on the evolution of star clusters using the fluiddynamical method devised by Larson (1970a). Large systems of stars have been considered with specific concern with globular clusters. With reference to the analogous ‘standard’ model by Larson, the influence of varying in turn the various free parameters (cluster mass, star mass, tidal radius, mass concentration of the initial model) has been studied for the results. Furthermore, the partial release of some simplifying assumptions with regard to the relaxation time and distribution of the ‘target’ stars has been considered. The change of the structural properties is discussed, and the variation of the evolutionary time scale is outlined. An indicative agreement of the results obtained here with structural properties of globular clusters as deduced from previous theoretical models is pointed out.