We use B, V, and K-band images of the Antennae in a detailed analysis of its stellar populations. Significant large and small-scale spatial variations in the stellar populations are found by examining images pixel by pixel. Flux ratio, or linear color, diagrams allow us to find the stellar populations present in each pixel using a novel technique. This is done through a comparison of the colors of pixels to those of well-studied populations such as elliptical/S0 galaxies, Magellanic Cloud clusters, and Sc galactic nuclei, in addition to stellar population models. This technique allows us to study not only star clusters, but also the diffuse light present in the Antennae. From these flux-ratio diagrams, a luminosity-weighted de-reddened age image of the Antennae is constructed. Our analysis technique complements and extends results based on Hubble Space Telescope images.

We focus here on the gas dynamics of cluster formation and the early stellar dynamical evolution of young clusters. We point out that the condition that a cloud can fragment into a large number of pieces places rather particular constraints on its initial state; we also review the processes that shape the stellar IMF in cluster formation simulations. We show how N-body calculations and observations can be combined to discover the properties of clusters at the point at which they first become stellar dynamical (as opposed to gas dynamical systems). Finally, we touch on the question of how proto-cluster clouds are assembled and reopen the issue of whether dark matter may play a role in globular cluster formation.

We will review the present knowledge of molecular cloud properties and its relation to star formation. We will discuss the evidence for cluster formation associated with giant molecular clouds, and will concentrate on recent results in our Galaxy and the Magellanic Clouds.

I will present a revised CO view of the Large Magellanic Cloud based on the new NANTEN survey, which is ∼ 2–3 times more sensitive than the first one.

Mass functions (MFs) derived from photometric observations of young star clusters of our Galaxy, the Magellanic Clouds (MCs), M31 and M33 have been used to investigate the question of universality of the initial mass function and presence of mass segregation in these systems. Observational determination of the MF slope of young star clusters have an inherent uncertainty of at least ∼ 1.0 dex in the Milky Way and of ∼ 0.4 dex in the MCs. There is no obvious dependence of the MF slope on either galactocentric distance or age of the young star clusters or on the spatial concentration of the stars formed or on the galactic characteristics including metallicity. Effects of mass segregation have been observed in a good number of young stellar groups of our Galaxy and MCs. As their ages are much smaller than their dynamical evolution times, star formation processes seem to be responsible for the observed mass segregation in them.

We present a preliminary analysis of a deep colour-magnitude diagram of NGC 1831, a rich cluster in the LMC. The data were obtained with HST/WFPC2 in the F555W and F814W filters, reaching m555 ⋍ 25 mag. Efficient use of the two-dimensional distribution of data was made by comparing the observed colour-magnitude diagram with model ones. We discuss the process used in accounting for field star contamination and sampling incompleteness in the data. We also describe the algorithm used in building artificial diagrams from model isochrones and the statistical method used in comparing model and data colour-magnitude diagrams. From these comparisons we infer a metallicity Z = 0.008 and a low value, fbin ⋍ 0.25, for the global binary fraction of NGC 1831. We also rule out ages τ ≲ 500 Myr and mass function slope values α ≳ 2.

We present the first infrared luminosity function in the 30 Doradus star cluster obtained with the HST NIC1 camera (0.043″ pixel−1) in the F160W (H-band) filter. Despite diffraction limited resolution (0.15″), crowding and blending is so severe that the cluster centre R136 cannot be studied. Instead a neighbouring NIC1 field (about 15″ away from the centre), for which an empirical PSF could be obtained, was analysed. We obtained photometry for some 750 stars in the range H = 14 to 22 mag. The luminosity function continues rising up to H = 2.1 mag (1 M⊙ at an age of 2 Myr) after which incompleteness corrections become dramatic (exceeding 50%). Contrary to recent optical studies based on a V vs. V—I diagram (Sirianni et al. 2000), we do not infer a flattening or turnover of the implied IMF slope near 2 M⊙.

After summarizing the importance of clusters and variable stars in the context of the distance scale, we focus on two new, and independent, programs to determine the distance to the Large Magellanic Cloud cluster NGC 1866.

Globular clusters in most large galaxies are a mixture of metal-poor and metal-rich (bimodal), but the halo stars are almost entirely metal-rich. This and other lines of evidence argue that the metal-poor globular clusters formed within widely distributed 108 − 109M⊙ gas clouds (supergiant GMCs) during an early burst in which most of the gas was ejected or unused till later rounds of star formation.

New simulations of the growth of pre-galactic potential wells in the early universe now indicate that the initial power-law form of the globular cluster mass distribution (dN/dM ∼ M-1.8) is a miniature replica of the mass distribution of the SGMCs themselves, which grow hierarchically in the CDM potential wells of large protogalaxies.

Mass determinations are difficult to obtain and still frequently characterised by deceptively large uncertainties. We review below the various mass estimators used for star clusters of all ages and luminosities. We highlight a few recent results related to (i) very massive old star clusters, (ii) the differences and similarities between star clusters and cores of dwarf elliptical galaxies, and (iii) the possible strong biases on mass determination induced by tidal effects.

In the first part of this article, we review observations of the mass and luminosity functions of young and old star cluster systems. We also review some of the physical processes that may determine the characteristic mass of globular clusters and the form of their mass function. In the second part of this article, we summarize our models for the disruption of clusters and the corresponding evolution of the mass function. Much of our focus here is on understanding why the mass function of globular clusters has no more than a weak dependence on radius within their host galaxies.

We combine the results of a large HST study of rich LMC clusters with state of the art N-body simulations. We summarise recent highlights from our project: the estimation of age spreads in young LMC clusters, the measurement of the binary fraction of NGC1818, and the determination of deep cluster luminosity functions (LF). We also present preliminary results from an investigation of the core radius versus age relation evident among LMC clusters.

Globular clusters are embedded in galactic potential. Various processes such as dynamical friction, steady tidal field, and shocks by disk and bulge contribute to the dissolution of globular clusters. Most of them are closely coupled with internal dynamical processes. The tidal shock is very effective during the initial phase of the cluster evolution while steady tidal field becomes more important in the later phase. Recent studies revealed that the rotation of the cluster could significantly accelerate the evolution and evaporation. We emphasize that the lifetime of the clusters could be much shorter than the previous estimates of ∼ 20trh for more realistic models.

The dynamical friction timescale for globular clusters to sink to the center of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if the halos have isothermal profiles and the globular clusters formed with the same radial density profile as the underlying stellar population. We examine the summed radial distribution of the entire globular cluster systems and the bright globular cluster candidates in 65 Virgo and Fornax Cluster dEs for evidence of dynamical friction processes. We find that the bright dE nuclei could have been formed from the merger of orbitally decayed massive clusters, but the faint nuclei are several magnitudes fainter than expected. These faint nuclei are found primarily in MV > −14 dEs which have high globular cluster specific frequencies and extended globular cluster systems. In these galaxies, the formation of new star clusters, high central dark matter densities, extended dark matter halos, or tidal interactions may act to prevent dynamical friction from collapsing the entire globular cluster population into a bright nucleus.

The uncertainties in the modelling of some important inputs of stellar evolution must be taken into account for a correct interpretation, both of the HR diagrams of individual stars, and of the integrated colors, of stellar clusters. After a short discussion of Globular Cluster ages, we focus on the problem of convection and discuss the parametrization of convection efficiency and overshooting.

Convective efficiency affects the reliability of the giants colors, especially for high metallicity: age - metallicity relations found from integrated colors of clusters must be regarded with caution.

Non-instantaneous mixing, both in the formal convective region and in the overshooting region must be taken into account: this is necessary to compute nucleosynthesis in ‘hot bottom burning’ envelopes, but it may also affect the color distribution in clump (core helium burning) stars, as shown here for the case of the LMC cluster NGC 1866.

We investigate the origin of the discrepancy between the age estimates based on the Color-Magnitude-Diagram and the integrated spectra of 47 Tuc. Using a newly defined Hγ age indicator, with unprecedented power to disentangle the degeneracy between age and metallicity, we confirm the very large spectroscopic age derived for this cluster. We explore a number of key parameters with potential to decrease the Balmer lines and show the necessity of turnoff stars with lower temperatures. We find that alpha-enhanced isochrones with atomic diffusion leads to a younger spectroscopic age in better agreement with the CMD age.

In this paper I combine the results of a set of population synthesis models with simple Montecarlo simulations of stochastic effects in the number of stars occupying sparsely populated stellar evolutionary phases, to show that the scatter observed in the magnitudes and colors of LMC and NGC 7252 star clusters can be understood in the framework of current stellar evolution theory, without the need to introduce ad-hoc corrections (e.g. artificially increasing the number of AGB stars).

I briefly introduce a database of models that describe the evolution of star clusters in several broad-band photometric systems. Models are based on the latest Padova stellar evolutionary tracks - now including the α-enhanced case and improved AGB models - and a revised library of synthetic spectra from model atmospheres. As of today, we have revised isochrones in Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Thuan-Gunn, and Washington systems. Several other filter sets are included in a preliminary way, like those used by the EIS and SDSS projects. The database contains also integrated magnitudes of single-burst stellar populations and Monte-Carlo simulations that show the stochastic dispersion of the colours as a function of cluster mass, age, and metallicity. The models are useful for several kinds of studies, including estimates of masses and ages of extragalactic star clusters observed by means of broad-band photometry.

Globular clusters formed in galactic mergers (e.g., The Antennae) can now be studied at different stages of their evolution. In young merger remnants (e.g., NGC 7252) these “second-generation” globulars appear by the hundreds as young halo clusters of roughly solar metallicity. While at first bluer and much more luminous than old metal-poor globulars, they become redder after 1–1.5 Gyr and can then be observed as still overluminous red clusters of intermediate age in perturbed-looking E and S0 galaxies (e.g., NGC 1316, 1700, 3610). There is evidence from the color distributions, projected radial distributions, and perhaps also luminosity functions that these clusters eventually assume the properties of red metal-rich globulars observed in many giant ellipticals. Studies of globular clusters in ongoing mergers and young remnants suggest that second-generation globulars form from giant molecular clouds shocked by the rapid pressure increase in the merger-induced starburst. This pressure-induced formation lends credence to Cen's (2001) argument that the general pressure increase during cosmological reionization at z ≈ 7–15 triggered the near-simultaneous formation of the universal population of first-generation metal-poor globulars observed in galaxies of all types.

In a 1st step I present results from our new set of evolutionary synthesis models for Simple (= single burst) Stellar Populations (SSPs) of various metallicities, and in a 2nd step I combine these results with the information we have about the redshift evolution of spiral galaxies’ ISM abundances. The aim is to provide a grid of color and luminosity distributions of any secondary GC population formed at some time in the past in those mergers for comparison with observations.