We present a new analysis of the Galactic structure in the field of Lacerta. The investigation uses precise uvbyβ photometry and the recalculated Hipparcos parallaxes to provide an improved homogeneous distance scale to the main stellar groups in the field. The OB association Lac OB1 (Lac OB1b) is reliably identified as a very compact young group at an average distance of 520 ± 80 pc. The characteristics of the older Lac OB1a group and the open cluster NGC 7243 are extensively studied based on photometric diagrams and the inferred distances. A good agreement between the photometry-derived distances and the recalculated Hipparcos parallaxes is found for the Lac OB1 group, but in general discrepancies still exist for this particular field.

Galactic open clusters are excellent laboratories to study stellar populations that are coeval and have a common chemical composition. These clusters allow for the investigation of stellar and chemical abundance evolution, binary star systems, mass and luminosity functions, Galactic metallicity gradient, and disk structure, among other things. The Southern Open Cluster Study (socs) is a database of a select set of 24 open clusters with a broad range of ages and metallicities. It is an extension of the northern WIYN Open Cluster Study (wocs). The main goal of both studies is to obtain comprehensive photometric, spectroscopic, and astrometric information of these key open clusters. Here, we provide an overview of the socs survey and present results of the wide-field photometry on two socs clusters, NGC 3532 and Tombaugh 2.

Star clusters are found in all sorts of environments, and their formation and evolution is inextricably linked to the star-formation process. Their eventual destruction can result from a number of factors at different times, but the process can be investigated as a whole through the study of cluster age distributions. Observations of populous cluster samples reveal a distribution following a power law of index approximately −1. In this work, we use M33 as a test case to examine the age distribution of an archetypal cluster population and show that it is, in fact, the evolving shape of the mass detection limit that defines this trend. That is to say, any magnitude-limited sample will appear to follow a dN/dτ = τ−1 relation, while cutting the sample according to mass gives rise to a composite structure, perhaps implying a dependence of the cluster disruption process on mass. In the context of this framework, we examine different models of cluster disruption from both theoretical and observational perspectives.

The majority of stars in the Galactic field and halo are part of binary or multiple systems. A significant fraction of these systems have orbital separations in excess of thousands of astronomical units, and systems wider than a parsec have been identified in the Galactic halo. These binary systems cannot have formed through the ‘normal’ star-formation process, nor by capture processes in the Galactic field. We propose that these wide systems were formed during the dissolution phase of young star clusters. We test this hypothesis using N-body simulations of evolving star clusters and find wide binary fractions of 1–30%, depending on initial conditions. Moreover, given that most stars form as part of a binary system, our theory predicts that a large fraction of the known wide ‘binaries’ are, in fact, multiple systems.

In the study of open clusters, the distinction between cluster and field members and the assignment of membership probabilities for each individual star in the relevant region are central issues. Here, we present an analysis based on the use of a fully automated method that relies on a hybrid heuristics, based on genetic algorithms and hill-climbing optimisation, as well as different probability distribution functions for the observables. We analysed the use of some variations for the parameterisation of those distribution functions. Finally, a catalogue comprising kinematic parameters and associated membership probability lists for all the open clusters in the Bordeaux PM2000 region was derived.

We derive photometric metallicities for 56 intermediate-age and old star clusters in the Large Magellanic Cloud from the slopes of their red-giant branches in near-infrared color–magnitude diagrams (CMDs). The cluster sample covers the LMC bar and disk to a distance of ~2 kpc from the LMC center. The derived cluster metallicity distribution spans a range of [Fe/H] = +0.0 . . . 1.4 and is similar to the metallicity distribution of field stars. We find that the average cluster metallicities in certain fields of the LMC bar and disk may be different, but there seems to be no compelling evidence for a global cluster metallicity gradient in the LMC.

We summarize our results based on observations with the NIRI camera on the Gemini North telescope of three Hii galaxies (Mrk 36, UM 408 and UM 461), obtained to identify and determine the ages and masses of the elementary components (the star cluster population) of the starburst regions in compact Hii galaxies. Our preliminary results indicate that the masses of the stellar clusters in these galaxies range from ~104 to ~106 M⊙, with associated ages of a few Myr. The most massive star clusters fall in the so-called super star cluster category. The identification of these clusters suggests that the formation and evolution of massive star clusters is the dominant mode of star formation in these galaxies. Their spatial distribution and ages seem to indicate that star formation is simultaneous over these timescales in some of our objects. We also review our recent description of the spatial distribution of physical conditions in the Hii galaxy UM 408 using the GMOS integral-field unit on Gemini South. The spatial distribution of the oxygen abundance does not show any significant variation or gradient across the galaxy on scales of hundreds of parsecs, within our observational uncertainties, confirming that this compact Hii galaxy, like other previously studied dwarf irregular galaxies, is chemically homogeneous.

W UMa systems can be found everywhere in the Galaxy. They can be used as a distance tracer. Therefore, W UMa systems are very important to investigate the structure of the Galaxy. The distance to W UMa systems in globular clusters (GCs) is determined using a period–color–luminosity relation. It is found that the mean distance (ra) of W UMa systems is consistent with their host cluster distances (rGC) deduced from their intrinsic distance moduli if rGC ≤ 10 kpc. There is a significant difference between ra and rGC for rGC ≥ 10 kpc. We discuss the reasons causing this deviation.

We present a study of star clusters in the starburst galaxy M82 using the BVI mosaic images taken with the Hubble Space Telescope's Advanced Camera for Surveys. We have selected about 850 clusters with V < 23 mag based on their morphological information. The brightest cluster is as bright as V ~ 16.5 mag (MV ~ −11.2 mag) and most clusters are fainter than V = 18 mag (MV = −9.7 mag). The V-band luminosity function is represented by a power law with a slope α ~ −2.0 in the range of −9.5 < MV < −7 mag. The star clusters in M82 are mainly distributed in the galaxy's disk. However, about a dozen clusters are found far from the disk and are considered to belong to the halo of M82. The color–magnitude diagrams of star clusters show that most star clusters are highly reddened. We have derived the age of the star clusters using a spectral-energy-distribution fitting method with Bruzual & Charlot simple stellar population models. We discuss the age distribution as well as the photometric properties of the star clusters in regard to the formation history of the M82 cluster population.

We present a characterization method based on spectral cross-correlation to obtain the physical parameters of the controversial stellar aggregate ESO442–SC04. The data used was obtained with GMOS at the Gemini South telescope, and includes spectra of 17 stars in the central region of the object and 6 standard stars. fxcor was used iteratively to obtain self-consistent radial velocities for the standard stars and average radial velocities for the science spectra. Spectral types, effective temperatures, surface gravities and metallicities were determined using fxcor to correlate cluster spectra with the ELODIE spectral library and select the best correlation matches using the Tonry & Davis ratio. Analysis of the results suggests that the stars in ESO442–SC04 are not bound and, therefore, they do not constitute a physical system.

We present blue optical spectra of 92 members of h and χ Per obtained with the WIYN telescope at Kitt Peak National Observatory. From these spectra, several stellar parameters were measured for the B-type stars, including v sin i, Teff, log gpolar, M∗, and R∗. Strömgren photometry was used to measure Teff and log gpolar for the Be stars. We also analyze photometric data of cluster members and discuss the near- to mid-infrared excesses of Be stars.

NGC 6611, Trumpler 14, Trumpler 15, Trumpler 16, and Collinder 232 are very young open clusters located in star-formation regions in the Eagle Nebula and Carina in the Milky Way, and NGC 346 in the Small Magellanic Cloud. With different instrumentation and techniques, it has been possible to detect and classify new Herbig Ae/Be and classical Be stars and to provide new tests/comparisons of the Be stars' appearance models. Special (He-strong) stars in these star-formation regions are also discussed.

Star clusters are privileged laboratories for studying the evolution of massive stars (OB stars). One particularly interesting question concerns the phases during which the classical Be stars occur, which—unlike HAe/Be stars—are not pre-main-sequence objects, nor supergiants. Rather, they are extremely rapidly rotating B-type stars with a circumstellar decretion disk formed by episodic ejections of matter from the central star. To study the impact of mass, metallicity, and age on the Be phase, we observed Small Magellanic Cloud (SMC) open clusters with two different techniques: (i) with the ESO–WFI in slitless mode, which allowed us to find the brighter Be and other emission-line stars in 84 SMC open clusters, and (ii) with the VLT–FLAMES multifiber spectrograph to determine accurately the evolutionary phases of Be stars in the Be-star-rich SMC open cluster NGC 330. Based on a comparison to the Milky Way, a model of Be stellar evolution, appearance as a function of metallicity and mass, and spectral type is developed, involving the fractional critical rotation rate as a key parameter.

We have obtained high-resolution VLT/FLAMES spectra of a number of giants in the LMC cluster Hodge 11 and the surrounding field. We present results of a detailed model-atmosphere abundance analysis for a variety of elements, including Fe, Ca, Ti, Si, Sc, Ni and Cr.

The properties of the binary components of the eclipsing variables found in the 47 Tuc core region as part of a search for extrasolar planets have been re-examined following reanalysis of the image photometry and calibration. This is part of an ongoing study to find fundamental properties of stars of different metallicity in a variety of environments through eclipsing-binary analyses.

We present a new technique to analyze color–magnitude diagrams, such as those of stellar clusters, based on the cross-entropy global-optimization procedure. The method uses theoretical isochrones available in the literature and minimizes an objective function that describes the quality of a fit to the color–color and color–magnitude diagrams. The objective function is based on the two-dimensional distances of the points in the color–magnitude and color–color diagrams to the theoretical curves and is modified by weights that take into account the stellar distance to the observed cluster center, observed magnitude uncertainties and the King profile of the cluster, among others. The parameters determined simultaneously are distance, reddening, age and metalicity. The method uses a Monte Carlo approach to obtain uncertainties on the determined parameters for the cluster. We present results for 10 well-studied open clusters and show that the results are consistent with previous studies.

The WEBDA database of open clusters (hereafter OCs) in the Galaxy contains 970 OCs, of which 911 have age determinations, 920 have distance measurements, and 911 have color-excess data. Base on the statistical analysis of global properties of open clusters, we investigate disk properties such as the height above the Galactic plane. We find that old open clusters (age ≥ 1 Gyr) are preferentially located far from the Galactic plane with 〈|z|〉~394.5 pc. They lie in the outer part of the Galactic disk. The young open clusters are distributed in the Galactic plane almost symmetrically with respect to the Sun, with a scale height perpendicular to the Galactic plane of 50.5 pc. The age distribution of open clusters can be fit approximately with a two-component exponential decay function: one component has an age scale factor of 225.2 Myr, and the other consists of longer-lived clusters with an age scale of 1.8 Gyr, which are smaller than those derived by Janes & Phelps (1994) of 200 Myr and 4 Gyr for the young and old OCs, respectively. As a consequence of completeness effects, the observed radial distribution of OCs with respect to Galactocentric distance does not follow the expected exponential profile. Instead, it falls off both for regions external to the solar circle and more sharply towards the Galactic Center, which is probably due to giant molecular cloud disruption in the center. We simulate the effects of completeness, assuming that the observed distribution of the number of OCs with a given number of stars above the background is representative of the intrinsic distribution of OCs throughout the Galaxy. Two simulation models are considered, in which the intrinsic number of the observable stars are distributed (i) assuming the actual positions of the OCs in the sample, and (ii) random selection of OC positions. As a result, we derive completeness-corrected radial distributions which agree with an exponential disk throughout the observed Galactocentric distance in the range of 5–15 kpc, with scale lengths in the range of 1.6–2.8 kpc.

The metallicities of 15 Small Magellanic Cloud (SMC) clusters are determined from the Calcium ii-triplet lines in spectra obtained using the FORS2 multi-object spectrograph on the VLT (Paranal, Chile). The metallicity distribution, metallicity gradient and age–metallicity relationship in the SMC are determined and analyzed using new and literature data for these and other, previously studied SMC objects. Only the main results regarding these topics are presented and discussed.

Open cluster remnants (OCRs) are fundamental objects to investigate open cluster dissolution processes (e.g., Bica et al. 2001; Carraro 2002; Pavani et al. 2003; Carraro et al. 2007; Pavani & Bica 2007). They are defined as poorly populated concentrations of stars, with enough members to show evolutionary sequences in colour–magnitude diagrams (CMDs) as a result of the dynamical evolution of an initially more massive physical system. An OCR is intrinsically poorly populated, which makes its differentiation from field-star fluctuations difficult. Among the possible approaches to establish the nature of OCRs, we adopted CMD analysis combined with a robust statistical tool applied to 2mass data. In addition, photometry is the main information source available for possible OCRs (POCRs). We developed a statistical diagnostic tool to analyse the CMDs of POCRs and verify them as physical systems, explore membership probabilityies taking into account field contamination and derive age, distance and reddening values in a self-consistent way. We present the results of our analysis of 88 POCRs that are part of a larger sample that is widely distributed across the sky, with a significant density contrast of bright stars compared to the Galactic field. The 88 objects are projected onto low-density Galactic fields, at relatively high latitudes (|b| > 15°). Studies of larger POCR samples will provide a better understanding of OCR properties and constraints for theoretical models, including new insights into the evolution of open clusters and their dissolution rates. The results of this ongoing survey will provide a general picture of these fossil stellar systems and their connection to Galactic-disk evolution.

Over the last few years, different observations have suggested the existence of intermediate-mass (~103 M⊙) black holes in the centers of globular clusters. However, the issue is still a matter of debate, as current observations have alternative explanations. We previously developed a hydrodynamical model for the interstellar medium in these systems to explain the luminosity of the central X-ray source found in NGC 6388, assuming a black hole accreting from the insterstellar medium. Here, we explore the predictions of our model regarding the flow of the interstellar matter in the inner cluster regions and find that the density and velocity profiles could help to determine the presence of a central black hole as well as its mass.