We have performed a study of the spatial distribution of BOUND open clusters, YOUNGER THAN 100 MYR, in the solar neighborhood. We discriminate between Gould Belt (GB) and local Galactic disk (LGD) members, using a previous estimate of the structure of both systems obtained from a sample of O-B6 Hipparcos stars. Single membership probabilities of these clusters are also calculated in the separation process. Using this classified sample we analyze the spatial structure and the kinematic behavior of the BOUND cluster system in the GB. We find that the vertex deviation of the GB, derived from the cluster distribution, is similar to that of the LGD; this result is clearly different from that found for isolated or loosely assembled OB stars. Thus, unbound OB star groupings show a different behavior in space distribution and kinematic structures than the bound open clusters belonging to the GB. Furthermore, while single stars show a bimodal distribution in UV space, the proportion of clusters associated to one of these peaks is almost null, displaying a completely different pattern in the space of velocities. In the light of these results, we discuss the possible nature of the GB as a chance superposition of at least two stellar subsystems with different physical properties and star formation modes and histories.

Stars form in the cold dense cores of interstellar molecular clouds and the detailed knowledge of the spectrum of masses of such cores is clearly a key for the understanding of the origin of the IMF. Employing a robust method that uses extinction of background starlight to measure core masses we determine the mass spectrum of the largest ensemble of dense cores in one molecular cloud complex. We find the first robust evidence for a departure from a single power-law form in the mass function of cores and find that this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor o about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stage of core evolution. We will discuss the implication of these results on the general framework of molecular gas conversion into a stellar cluster.

We performed a careful re-analysis of the WFPC2 observations of the young star cluster system in the Antennae galaxies, and especially their luminosity function. This nearest pair of merging spiral galaxies currently experiences a major burst of star and star cluster formation. We put particular emphasis on a detailed completeness analysis (hampered due to strong variability in the galaxy's background and a large amount of bright single stars) and on a thorough statistical fitting procedure to analyse the observed luminosity functions with both Gaussian and power-law distributions and compare the fitting accuracy for these to, most widely used, cases. We find that, contrary to previous claims, the Gaussian model fits statistically significantly better than the power law. This has severe implications to our understanding of star cluster formation from Giant Molecular Clouds.

Neutral hydrogen (HI) supershells are among the most spectacular phenomena that can be witnessed in the interstellar medium of spiral galaxies. Based on the high sensitivity and stray radiation corrected HI survey carried out at the Instituto Argentino de Radioastronomia, we have discovered a new HI supershell. This object, designated GS62-02+45, is located at a distance of 5 kpc and has a major diameter of 650 pc.Besides determining its physical characteristics (mass, expansion velocity, dimensions, kinematical age,etc.) we explore the possibility that the formation of the OB-association Bochum 7 (2-7 x 10**6 years old) could have been triggered by the HI supershell. Evidence for the interaction of the stellar association with the walls of HI defining GS262-02+45 has been found.

We present our recent results on NTT and VLT diffraction limited NIR imaging of young massive galactic clusters Trumpler 14 and Westerlund 2. In this talk we propose to address the observational effects often attributed to mass segregation from a numerical point of view. Starting from synthetic clusters constructed via Monte Carlo simulations of non-segregated collections of stars that follow a given density profile and a 'normal' IMF, we conclude that the slope of the mass function close to the cluster core is, on average, flatter than Salpeter merely because of statistical fluctuations, and without having to evoke a different distribution of the massive (or of the low mass) stars. The large scatter in this result allows also, although with a smaller probability, for clusters with steeper IMFs in their centre. This result has a severe impact in a range of young cluster issues, e.g., in the significance of the upper mass cutoff found for massive clusters.

NGC 604 is, together with 30 Doradus, the extragalactic prototype giant H II region. Its stellar content and mass {~200 O+WR stars, ~10^5 Msol}, age {~3.5 Myr}, proximity (840 kpc), and low foreground extinction, E(B-V) = 0.1, make it an ideal object to study the massive stellar population of a scaled OB association, its interaction with the surrounding medium, and the effects caused by the strong differential extinction. We observed NGC 604 using HST/ACS and NICMOS instruments. The NICMOS observations presented here are part of a multi-wavelength study of NGC 604 from the FUV to the NIR. These observations obtained in broadband filters similar to J, H and Ks allow us to determine the NIR properties of the central scaled OB association and to identify embedded very-young stellar populations hidden inside dust clouds to the southen part of nebula. Our final goal is to analyze in detail a well-resolved giant H II region in order to understand the properties of similar but unresolved objects located at cosmological distances.

The distinction between 'bound' and 'unbound' clusters in the literature is often used. Here we argue that such a distinction is not needed when discussing cluster formation by presenting three lines of arguments. The first is that star-formation in galaxies is hierarchical, with dense clusters being simply one of the lower levels, but being fundamentally the same as looser associations. This will be shown by comparison of thousands of galactic and extra-galactic star forming regions which span 4 and 7 orders of magnitude in size and mass respectively. The second is that the vast majority of clusters appear to be disrupted within the first few Myr after their formation. Whether or not a cluster survives this phase has been shown to be critically influenced by the star-formation efficiency of the GMC core of which it formed, and not due to any specific property of the cluster (size, mass, metallicity, etc). Finally, the lifetime of a cluster is also heavily dependent on the potential in which it finds itself, so the same cluster will be defined as either 'bound' or 'unbound' depending on its location.

We present a study of the luminosity profile (LP) of massive clusters. The driving questions of the study are: 1) what LP are clusters born with (i.e. EFF or King or unknown) and does it vary? 2) how does it evolve with time (i.e. can you turn an EFF into a King profile) 3) what can we learn about the evolution of a cluster from its LP? and 4) in particular for the ultra-massive clusters (M >10^7 Msun), can we use the LP to constrain formation models? A large sample of LPs will be presented in order to address these questions.

Using NIR integral-field observations of the circumnuclear starburst rings in five nearby spiral galaxies, I will discuss the stellar populations and relative ages of the numerous "hot spots" along the rings. I will also explain how the results constrain competing theories for the mechanism(s) that govern the star formation in and morphological evolution of star formation rings around the nuclei of disk galaxies.

The mass-to-light factor $\eta$ is a strong function of time in dense, young clusters. I will present some results of this evolution over the first 500 Myr of evolution and argue that King model fits to young clusters can be severely biased regardless of mass. I also will present results of a study of another type of bias induced by the fraction of binary stars in clusters. These results are then set in the context of a merger galaxy, with special application to the Antennae galaxy. The profiling of the cluster mass function may not be scale-free.

I will present an overview of the cases for and against a universal IMF including the latest evidence from high resolution spectroscopy in the optical and infrared obtained using HIRES and NIRSPEC on the Keck telescopes. These data also yield valuable insights into the cluster formation process via detailed abundance analyses.

Initial mass cluster distribution (IMCD) inferred by synthesis models makes the implicit assumption of a fixed value on the mass-luminosity ratio, independent in on the cluster mass. This assumption, although would be valid in average, is certainly erroneous when applied to individual cluster, where a given initial mass produce a distribution of possible values of the luminosity for clusters with similar physical conditions (age and metalicity). In this work we study the impact of a correct treatment and interpretation of synthesis models results in the determination of the IMCD.

I will present results on the properties (e.g., ages, masses, and sizes) of young star clusters in hosts as different as the Antennae galaxies (a galaxy merger), the SMC (a typical star forming irregular galaxy), and M101 (a typical spiral galaxy) among others. A comparison reveals interesting patterns in these very different galaxy environments, which provide important clues to the dominant formation and destruction mechanisms of young cluster systems.

The lack of tightly bound and easily identifiable young open clusters leads to the quest for OB associations as tracers of spiral arms. Massive stars are usually found in groups, and the presence of a Wolf-Rayet star, WR12 in the catalogue of galactic WR stars (van der Hucht 2001), at alpha = 8:44:47.2, delta = - 45:58:55.5 (J2000.0), triggered the search for an surrounding OB association, now known as Bochum7 (Moffat & Vogt 1975). From UBV imaging of a 30 arcmin field surrounding WR12, obtained at CTIO, Chile, aperture photometry was obtained to select candidates and optical spectroscopic observations were obtained in CASLEO, Argentina, for over one hundred stars. 63 stars were classified as OB type, including the stars LS1131, 1132, 1135, 1137, 1140, 1144, 1146, 1147 already present in the LSS catalogue. The OB stars were used to test the relation between the diffuse interstellar band at 4428A which grows linearly with E(B-V) line of sight until 1 mag. Combining our photometric data with 2MASS IR photometry, and with the aid of CHORIZOS code (Maiz-Apellaniz, 2004) we were able to test the stars temperature and probe changes in the interstellar medium behaviour. This UBVJHKs analysis were made in more than 800 stars in the line of sight. Combining the astrometric data of these O and B stars present in different catalogues, we calculated their proper motions. With this data and the spectrophotometric data we find one OB Association named Bochum 7 at a distance of 4 and 5 kpc to the Sun. Bochum 7 is divided in two groups with stars of different evolutionary states, one of these harbouring an ultracompact HII region. By this, we infer that Bochum 7 can be a region showing a two-stage sequential stellar formation.

I review the long-term survival chances of young massive star clusters (YMCs), hallmarks of intense starburst episodes often associated with violent galaxy interactions. I address the key question as to whether at least some of these YMCs can be considered proto-globular clusters (GCs), in which case these would be expected to evolve into counterparts of the ubiquitous old GCs believed to be among the oldest galactic building blocks. In the absence of significant external perturbations, the key factor determining a cluster's long-term survival chances is the shape of its stellar initial mass function (IMF). It is, however, not straightforward to assess the IMF shape in unresolved extragalactic YMCs. I discuss in detail the promise of using high-resolution spectroscopy to make progress towards this goal, as well as the numerous pitfalls associated with this approach. I also discuss the latest progress in worldwide efforts to better understand the evolution of and disruption processes affecting entire cluster populations, and whether we can use recently gained insights to determine the nature of at least some of the YMCs observed in extragalactic starbursts as proto-GCs. I conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.

We present spectrophotometric data of moderate resolution ( 3000 to 11000) for circumnuclear star-forming region in three early type spiral galaxies, from which the physical properties of the ionised gas have been derived. The metal content of these HII regions, as estimated by different semi-empirical methods, turn out to be slightly over-solar. The properties of these CNSFR as derived from observations are compared to those of disc HII regions of comparable metal abundance.

The fraction of stars that end up in bound clusters appears to vary from region to region and among different galaxies. Infant mortality rates vary, and the fraction of stars that form in clusters versus those scattered throughout the clouds may vary too. We suggest that all clusters are the dynamically mixed cores of a turbulent, self-gravitating hierarchy of cloud structures, and in this sense, the clustered state is defined by the stars' own actions (mixing) rather than by some initial cloud boundary. The youngest stars in embedded clusters still show the hierarchies and filaments of the gas structures in which they form, and the largest and oldest regions of star formation, the star complexes, are hierarchical too, with OB associations and OB subgroups as different sublevels. In both cases, the hierarchical regions are younger than the mixing times. When viewed in this way, the differences between bound and unbound clusters and between clustered and dispersed star formation amount to differences in the star formation efficiency at the density of a cluster. In high pressure regions, this efficiency is high, leading to a higher fraction of cluster boundedness, while in low pressure regions, this efficiency is low, leading to a higher proportion of unbound associations. Because the pressure and star formation rate per unit area scale together, the boundedness fraction, along with infant mortality, should depend on the star formation rate.

This talk will present a new way to understand the formation and evolution of young massive star clusters.

Analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to find out whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star formation rate, burst strength, star formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few gigayears later.

Compact young star clusters with high masses are common in starburst galaxies. The resulting concentrations of OB stars and their SNe II produce extreme over pressures in the surrounding interstellar medium that lead to large scale gas outflows from galaxies, the galactic winds. Using nearby starbursts such as M82 as examples, we can study the sources, structure, and evolution of galactic winds, even reaching to the scales of individual star clusters. The results of this exercise lead to a better understanding of the galactic wind phenomenon in youthful galaxies, galactic chemical enrichment processes, and the cycling of baryonic matter between galaxies and diffuse intergalactic gas.

We present results about the star formation process in the giant HII region NGC 5471 in the outskirts of M101. From resolved HST/WPFC2 photometry we find that star formation has been going for the last 70 Ma. We further compare previous results from integrated infrared-optical photometry with the stellar resolved CMD and we discuss the star formation properties of this region and its individual knots, as well as characterizing the different stellar content. This result has very important consequences in our understanding of the burst versus continuous star formation activity in spiral galaxies.

In recent years several studies on the luminosity function (LF) of star clusters have become available. Most of these LFs can be approximated well by a power-law distribution with an index close to -2. The luminosity of the brightest cluster in a galaxy scales almost linearly with the total number of clusters in that galaxy. These scaling follows naturally from a power-law distribution with index -2 where the luminosity of the most luminous object is determined by the size of the sample. Does this scaling also hold for the cluster mass? To answer this question we look at the mass of the massive cluster as a function of increasing age intervals. For a constant formation rate of clusters and a power-law cluster initial mass function (CIMF) with index -2 the same linear scaling is expected as found for the luminosity. For the SMC and LMC clusters this scaling is indeed found, suggesting that the top end of the CIMF results naturally from the total number of clusters. For the clusters in M51 and the Antennae galaxies this relation between most massive cluster and increasing age interval is nearly flat around 10^6 M_sun, suggesting that that is a physical maximum in these galaxies and that the CIMF is sampled up to that mass at all ages. Recently, a cluster disruption model was introduced in which roughly 90% of all clusters is removed each age dex, independent of cluster mass. In this scenario the mass of the most massive cluster is also expected to be flat with increasing age interval. The LF can be used as an independent check. When the CIMF is truncated, then the LF shows a distinct bend which does not appear in the mass independent disruption scenario. The LF of clusters in M51 and the Antennae galaxies both show this bend and this type of LF can be well reproduced by cluster population models.

We present a multiwavelength study of the formation of massive stellar clusters, their emergence from cocoons of gas and dust, and their feedback on surrounding matter. Using data that span from radio to optical wavelengths, including Spitzer and ACS observations, we examine the population of SSCs in the dusty central starburst region of the irregular galaxy IC 4662. We model the radio and mid-IR emission of embedded young SSCs to determine the properties of their HII regions and dust cocoons, and use near-IR and optical data with mid-IR spectroscopy to constrain the properties (e.g. mass, age, massive stellar content) of the embedded clusters themselves and their feedback on the surrounding environment. We also investigate the ages, stellar populations, and extinctions of more evolved and less embedded SSCs, the distribution and excitation properties of the nebular gas, and ultimately the recent star-formation and cluster-evolution history of the galaxy.

Nuclear stellar clusters are a common phenomenon in spirals and starburst galaxies. They have been detected in 50-70% of these galaxies (Carrollo et al 2002; Boeker et al. 2004). Compact nuclear sources, that maybe stellar clusters, have been detected in elliptical galaxies of the Virgo Cluster (Ferrarese et al. 2006). Therefore stellar clusters are a natural consequence of the star formation processes in the central region of galaxies. On the other hand, evidence has been accumulating during the past few years about the ubiquity of BH in the nuclei of galaxies. Furthermore, the tight correlation of the BH mass and stellar velocity dispersion implies that the creation and evolution of a BH is intimately connected to that of the galaxy bulge. So, a natural consequence of the physical processes that formed present-day galaxies should be the co-existence of a BH and powerful stellar clusters in the nuclear region of galaxies. In this talk, we present the recent results that we have obtained analyzing HST images (mainly with the ACS, and WFPC2) of several samples of galaxies with nuclear activity (Seyferts and low-luminosity AGNs, LLAGNs) to find out the frequency of compact nuclear sources (presumably stellar clusters), their properties and their role on AGNs. We also present the results that we have obtained for NGC 4303, a LLAGN object. HST+STIS optical and UV spectra are used to derive the properties of the nuclear stellar cluster. Finally, the implications for the L(bulge) vs. BH mass correlation are discussed.

It appears that *all* stars form in binary and multiple systems. This implies that early in the dynamical evolution of star clusters many (especially low-mass) binary systems are destroyed or disrupted. I will discuss the possible dynamical and observational consequences of this process.

Using resolved wide-field multi-color photometry, we have measured the ages of almost 2000 young clusters in the Large and Small Magellanic Clouds. The age determinations were carried out by fitting Geneva and Padua isochrones to the color-magnitude diagrams of the clusters, which cover the entire area of both Clouds. These data permit us to look at the cluster number distribution as a function of age, at the half-light radii and inferred disruption time scales as a function of age and position, and to compare ages based on resolved stellar populations to those of integrated simple stellar population models. The clusters trace the recent star formation history of the Clouds and are contrasted with results for the field star formation history. Peaks in the cluster formation efficiency appear to be correlated with times of close encounters between the Magellanic Clouds and the Milky Way. Finally, we comment on similarities and differences between the system of young massive clusters in the Milky Way and in the Clouds.

We present high spectral resolution ( R ~ 20000) observations in the blue and the far red of cicumnuclear star-forming regions in several early type spirals which have allowed the study of the kinematics of stars and ionized gas in these structures and, for the first time, the derivation of their dynamical masses. In some cases these regions, about 100 to 150 pc in size, are seen to be composed of several individual star clusters with much smaller sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 10E07 solar masses for the whole CNSFRs.

Super Star Clusters are one of the most extreme star-forming environments in the universe, and they hold clues star formation during the time of galaxy assembly and the formation of globular clusters. Because of their exceptional masses and stellar densities, these clusters provide access to a region in parameter space that is important for testing our theories about star formation. One issue of debate is whether super star clusters are just the statistical tail of a power-law distribution, or whether they require special conditions to form. In this talk I will overview our efforts to search for natal clusters and probe their environments. By studying these objects at the time of their birth, we can gain insight into their initial mass spectrum, and whether super star clusters are actually super.

Stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in Galactic molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Collapse sets in to build up stars and star clusters. Turbulence plays a dual role. Or global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamical behavior of parental gas cloud, which governs when and where protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material to build up stars. Slow, inefficient, isolated star formation is a hallmark of turbulent support, whereas fast, efficient, clustered star formation occurs in its absence. The equation of state (EOS) plays a pivotal role in the fragmentation process. Under typical cloud conditions, massive stars form as part of dense clusters. I focus on the clustered mode of star formation and discuss results from numerical calculations of gravoturbulent cloud fragmentation in different cloud environments. Special emphasis lies on the complex dynamical evolution of nascent star clusters and on the resulting mass spectrum of stars, the IMF.

The post-starburst region B in M82 has been the focus of multiple studies, in particular its massive star cluster component. Previous studies have reported that there is a large population of coeval clusters of age ~1 Gyr, which were created with a Gaussian initial mass distribution. This is in stark contrast with other studies of young star clusters, which invariably find a featureless power-law mass distribution. Here, we present optical spectra obtained with Gemini-North of 8 star clusters in M82B and show that their ages are all between 100 and 300 Myr (a factor of 3-10 younger than previous results based on photometry) and that their extinctions range from A_V = 0.5-4 mag. Using new ACS-HRC U-band observations we age date an additional ~30 clusters whose ages/extinctions agree well with those determined from spectroscopy. Completeness tests show that the reported 'turn-over' in the luminosity/mass distributions is most likely caused by an under-estimation of the effect of the resolved nature of the clustes (i.e. the detection limit changes by ~1.5 mag between point sources and sources with FWHM~5pc). We also show that the velocities of the clusters are inconsistent with the clusters belonging to a bound region.

Young massive star clusters are hallmarks of intense starbursts. In fact, star cluster formation is likely the dominant mode of star formation in the extreme environments of merging and interacting galaxies. Yet only a fraction of the young star clusters survive long enough to evolve into old (>10 Gyr) globular clusters. We use detailed N-body simulations to explore the evolution of star clusters as a function of initial conditions: the density distribution, the IMF, the virial ratio, and the properties of the binary population. As our N-body simulations automatically take into account the evolution of all these parameters, these are also studied as a function of time. We apply selection criteria to our simulations as one would encounter when dealing with real observations. This allows us to constrain the boundary conditions for the long-term evolution of massive star clusters.

Star clusters are born in a very compact configuration and expand as a result of residual gas expulsion. The stellar IMF is the same in all currently observed clusters, but the maximum stellar mass depends on the star cluster mass. Stars are observed to form mostly as binaries, but in the Galactic field the binary fraction is about 50 per cent, while globular clusters appear to have an even lower fraction. I will discuss these issues and the physical processes probably responsible for explaining the observations. I may also touch upon the likely implication of star-cluster birth on galactic morphology.

Propose to discuss ground-based near-infrared and Spitzer Space Telescope Observations of embedded cluster structure and how knowledge of their structure may yield insights into their origin and early evolution.

Embedded clusters are the fundamental units of star formation in our Galaxy therefore studying their properties is critical for understanding how star formation proceeds on both the local and Galactic scale. I will discuss recent results from our FLAMINGOS Star Formation Survey. FLAMINGOS, the FLoridA Multi-Object Imaging Near-Infrared Grism Observatonal Spectrometer, is a wide field NIR imager and the world’s first fully cryogenic NIR multi-object spectrometer, offering an unparalleled opportunity to study young embedded clusters. We have surveyed embedded clusters in local molecular clouds with FLAMINGOS and are investigating the star forming histories, IMF, structure and evolution of these young clusters. In this presentation, I will focus on our results for the clusters in the Rosette and Orion star forming complexes where we find evidence for the evolution of cluster structure, sequential star formation and variations in the low mass IMF.

The empirically derived survival time of star clusters differs from Galaxy to galaxy. This demonstrates that the destruction of star clusters depends strongly on their environments. I will discuss the observations and the derived cluster dissolution times in several galaxies. A study of the star clusters in the solar neighbourhood shows that especially encounters with giant molecular clouds are life-threatening for star clusters. Therefore we may expect that the survival time of clusters in star-burst galaxies will be much shorter than in more quiescent galaxies. This is qualitatively in agreement with observations.

Red supergiants (RSG) provide most of the near-IR emission of many star forming galaxies. In these dusty environments, near-IR wavelengths sometimes provide the only direct access to photospheric light. We will summarize efforts made recently to improve the modelling of the spectra of young stellar populations at these wavelengths. Topics discussed will include: (i) a brief survey of current difficulties in this field and implications for issues like the IMF in young star clusters, (ii) the determination of stellar parameters from model fits to the near-IR spectra, using solar and RSG-specific surface abundances, (iii) predictions from population synthesis models using recent stellar spectra that extend through the near-IR range, (iv) applications to the spectra of clusters in M82.

Our current understanding of star formation in "clusters" versus "the field" faces somewhat of a crisis. On the one hand, it is often claimed that the majority of all stars form in clusters. On the other hand, huge variations in the relative numbers of surviving star clusters relative to the corresponding number of field stars are observed, not all of which are easily explained as being simply due to differences in the long-term survival times in different environments. This is illustrated most strikingly by the large variations in the "specific frequency" variations among old globular cluster systems, but similar effects are observed for younger stellar populations. I will discuss several possible ways out of this dilemma, including 1) variations in the cluster initial mass function (CIMF), 2) differences in the "infant mortality rates", and 3) genuine differences in the cluster formation "efficiency". In the near future, important clues to this problem are expected from direct observations of field stars and star clusters in several nearby star-forming galaxies.

I discuss two of the possible sources of biases in the determination of the IMF: binning and the existence of unresolved components. The first source is important for clusters with a small number of stars detected in a given bin while the second one is relevant for clusters located at the distances of the Magellanic Clouds and beyond. For both cases I will present results of numerical simulations and I will discuss strategies to correct for their effects.

Very young clusters such as Arches and Quintuplet exist close to the galactic center (30 pcs or so). The dynamical evolution of these clusters is difficult to study numerically, because the timescale of orbital evolution of the cluster due to dynamical friction, timescale of stellar evolution and mass loss from the cluster, timescale of two-body relaxation within the cluster are all similar. In particular, orbital evolution is difficult to study since it is affected by the mass lost from the cluster. We have developed a new N-body method in which we can model both the cluster itself and the parent galaxy by N-body models. The parent galaxy and interaction between the galaxy and cluster are handled by a fast tree algorithm, and internal dynamics within the cluster is handled by direct integration. We have calculated the evolution of young cluster born close to the galactic center using this new method. We found that the orbital evolution of the cluster is significantly faster than that estimated by Chandrasekhar's formula, and that if the initial orbit of the cluster is highly eccentric, the star stripped from the cluster can form disk-like structure which is observed at the galactic center.

It is generally accepted that most stars are born in clusters from the densest gas clumps in giant molecular clouds. Understanding the way in which clusters and stars in clusters form and evolve requires observations of the highly obscured regions in clouds where the formation takes place. These regions are even hidden in the mid-IR making very difficult to study the early phases of cluster formation at wavelengths shorter than 60 microns. Direct measurements of the physical properties of the massive protostars and protoclusters are possible by measuring the rotational lines of molecules in vibrationally excited levels. Rotational transitions from vibration states of HC3N have been used successfully to study the physical properties of massive protostars in the Galaxy. I will present new high resolution images of molecular emission in highly excited transitions of HC3N towards the star forming regions of Cep A , Orion A and Sgr B2 with luminosities ranging from few 104 Lo to 107 Lo . These data allow to study the formation of massive star in clusters with very different luminosities. The properties of the protoclusters will be described and the processes of massive star formation taking place in the different environments will be discussed. Finally I will also report on recent results of searches of protoclusters of massive stars in nearby starburst galaxies and in Ultraluminous Infrared Galaxies.

We use laser guide star adaptive optics (LGS/AO) on the 10-m Keck II telescope to obtain high spatial resolution images of super star clusters in NGC 1569 and M82. These data probe SSC structure and the relation of the SSCs to the ambient field star population. The higher resolution of Keck LGS/AO relative to HST/NICMOS in the near-IR enables us to examine whether young SSCs are monolithic or heirarchical assemblies. In SSCs with modest foreground extinction, comparison of our LGS/AO images with archival HST/ACS data can reveal the nature of mass segregation.

Observations of young massive star clusters (YMSCs) present us with a puzzle regarding infant mortality: Whereas around 90% of all clusters in a population seem to disrupt within each decade, a major fraction of all YMSCs studied individually apparently has the potential to survive for several Gyrs. Westerlund 1, as the only Galactic cluster with a mass in the 10^5 Msun league, was targeted for a spectroscopic analysis in order to decide whether the assumption used in all studies of individual star clusters, that they are in virial equilibrium, is justified. The analysis of our medium-resolution near-infrared ISAAC/VLT spectra reveals no sign for rapid expansion or collapse.

I present virial mass estimates of Young Massive Clusters (YMCs) in M83 and NGC 1140, determined from high spectral resolution VLT echelle spectroscopy and high spatial resolution HST imaging. The survivability of such clusters is important in testing the scenario that YMCs are proto-globular clusters. As young clusters, they lie in the domain in which dynamical masses appear to overestimate true cluster masses, most likely due to the clusters not being virialised. I discuss the long-term survivability of the clusters, by considering the degree to which the clusters appear to be out of virial equlibrium, and thus the likelihood that they will evolve into globular clusters in a Hubble time.

Extragalactic star clusters can be used as tracers of star formation in their host galaxies. Using the resolution of the Advanced Camera for Surveys on board HST, it is possible to disentangle the star clusters out of the stellar population. This in combination with broad-band photometry and simple stellar population models, allow us to derive cluster masses and ages. I will discuss the derived star cluster properties in 5 nearby spiral galaxies, using them as star formation tracers in the past of each galaxy yielding differential star cluster formation comparisons, star cluster properties comparison, and host galaxy environment differences.

The presence of circumnuclear starbursts in Seyfert 2 (Sy2) nuclei is now well established. Powerfull circumnuclear starbursts have been unambiguously identified in 40\% of Sy2 galaxies (Gonz\'alez Delgado et al. 2001; Cid Fernandes et al. 2001, 2004). It has been shown by HST observations that young massive star clusters are the building blocks of these starbursts. Whether this is also true for Sy1 galaxies or Low Luminosity AGN (LLAGN) is still a matter of debate. The determination of the properties of the nuclear and circumnuclear star cluster population is critical in order to understand the past and present evolution of the bulge and the environment of the Super Massive Black Hole. In order to do so, we have constructed two big samples of Sy galaxies (75 objects) and LLAGN (40), imaged with the ACS camera of HST in its high resolution configuration and near UV wavelength (F330W). This wavelength is optimal to trace unobscured star formation and disentangle the star clusters light from the bulge contribution. We complement our observations with other HST archival images at comparable resolution, at this and other bands (WFPC2 optical and near-IR with NICMOS).

What fraction of field massive stars were dynamically ejected from clusters as runaway stars, and what fraction were formed in situ? How many field massive stars exist, and formed, in true isolation, with no lower-mass companions? We examine these questions with HST imaging and kinematic data for field OB stars in the Small Magellanic Cloud.

The massive and compact stellar clusters show a bimodal hydrodynamic solution, where the internal parts of the cluster are thermally unstable, while the outer layers produce an outflowing wind. I like to discuss the relevance of this solution to the SSC formation and show the possible consequences for galaxies at the times of SSC formation.

I investigate how the cluster initial mass function differs with respect to the mass spectrum of the cluster gaseous progenitors. I show that a bell-shaped mass function may be the imprint of expulsion from the protocluster of the leftover star forming gas due to supernova activity, provided that the cloud mass spectrum shows a lower mass limit.. Owing to the weakening of its gravitational potential, the protocluster retains a fraction only of its newly formed stars. The mass fraction of bound stars extends from zero to unity depending on the star formation efficiency $\epsilon$ achieved by the protoglobular cloud. We investigate how such wide variations affect the mapping of the protoglobular cloud mass function to the GC IMF. A power-law cloud mass function truncated at low mass generates bell-shaped cluster IMFs, the turnover location being mostly driven by the lower limit of the cloud mass range.

The physical processes leading to the dissolution of star clusters is a topic barely studied and still not understood. I am currently working on a project to directly detect compact and less compact star clusters in nearby galaxies through resolved stars using HST/ACS data. The first results on two galaxies revealed a great potential for the new approach. This method will provide quantitative information on age, compactness, shape, and stellar mass of all star clusters. This is the very first extensive study for many fundamental aspects of star cluster evolution.

Infrared Dark Clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-IR Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveal many compact (less than 0.5 pc) and massive (10-2100 Msun) cores within them. These pre-stellar cores hold the key to understanding IRDCs and their role in star formation. Here we present high-angular resolution spectral line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-millimeter Array toward six high-mass IRDC cores. In total, we detect sixteen bright, compact (less than 0.024 pc) protostellar condensations. Four of the cores are resolved into multiple protostellar condensations. The remaining two cores contain single, compact protostellar condensations with a very rich molecular spectrum, indicating that these are hot molecular cores. The spectral line maps reveal evidence for circumstellar disks around two of the condensations. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar and four cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.

We have carried out a detailed study of the Young stellar populations (YSPs) in the diffuse light for a sample of local Ultraluminous Infrared Galaxies. For some of the objects it is possible to compare the spectroscopic results with those obtained for the YSPs located in clusters, with the aim of tracing the star formation history of such objects. This is the case for the ULIRG/Radio galaxy PKS1345+12, for which we were able to carry out both a photometric and spectroscopic study based in HST images and WHT/ISIS spectra. The results emphasize the necessity to combine both techniques in order to obtain an adequate general impression of the star formation activity in the host galaxies of ULIRGs. Intriguingly, the spectra show line splitting coinciding with the location of super star clusters (SSC). Given their kinematics it is possible that the SSCs have been formed either in fast moving gas streams/tidal tails that are falling back into the nuclear regions as part of the merger process or as a consequence of jet-induced star formation linked to the extended diffuse radio emission detected in the halo of the galaxy.

The question of how the combination of low metallicity and dust content can affect the star formation processes has implications for our understanding of how stars formed in the early universe. NGC 346, the most massive and active star-forming region in the Small Magellanic Cloud (SMC), is an ideal benchmark for this research, due to its sub-solar metallicity and its 1/5 solar dust content. In addition, its close proximity allows us to perform a detailed and accurate census of its stellar population. Using deep ACS/HST observations, we identified a rich population of pre-Main Sequence stars in the mass range between 3.0 and 0.6 solar masses, indicating that star formation in the region is recent and, possibly, still ongoing. We found that star formation occurred in many, likely coeval, sub-clusters. We find that the mass function of the cluster is slightly steeper than the value derived by Salpeter for the solar neighborhood, in agreement with results obtained for other young star clusters in the Magellanic Clouds, and we find evidence of primordial mass segregation. We used synthetic color-magnitude diagrams to infer that star formation likely stared in NGC 346 ~7 Myr ago, it reached its maximum about ~3 Myr ago and it is still ongoing. We will discuss in detail the star formation history of the region.

Evidence for a link between intense star formation and AGNs has grown steadily in recent years. Here we investigate the interplay between circumnuclear star clusters, accretion onto the central super massive black hole, and gaseous outflows that result from the thermalization of the matter injected by supernovae and stellar winds inside the cluster. We propose a self consistent spherically symmetric hydrodynamic solution which allows to search how the accretion rate onto the central BH depends on the star cluster parameters and also to study how the presence of the super massive BH affects the outflow of the hot gas from the cluster. The expected appearance of the massive star clusters with the central super massive BH in the X-ray regime is discussed.

TDB

Young resolved clusters in the SMC provide us with the opportunity to study the efficiency of feedback mechanisms at low metallicity. HST/ACS H alpha images reveal that the young clusters NGCs 346 and 602 are surrounded by complex gas structures, indicating that they are clearly interacting with their environments. To probe the dynamics of the ionized gas in these two clusters, we have obtained high resolution optical spectroscopy for a number of slit positions across the nebulae. Surprisingly, we find little evidence of gas motions in the ionized gas associated with either cluster. This suggests that, at the low SMC metallicity, the cluster O star winds are not powerful enough to sweep away the residual gas. Instead, we find that stellar radiation is the dominant process shaping the interstellar environments of these two clusters. We discuss the implications of these findings for feedback at low metallicity.

Combining Keck LGS-AO and VLT NAOS/CONICA observations, we have measured the proper motion of the Galactic center Arches cluster with respect to the surrounding field population to $212 \pm 20 km/s$. The proper motion measurement, when combined with the known radial velocity of the cluster, yields a first estimate of the 3D space motion of the Arches in the Galactic center potential. I will discuss the implications of the large velocity of the cluster in view of possible formation scenarios of young, massive clusters such as the Arches in the immediate environment of the Galactic center.

Studies of globular cluster systems play a critical role in our understanding of galaxy formation. Imaging with the Hubble Space Telescope has revealed that young star clusters are formed copiously in galaxy mergers, strengthening theories in which giant elliptical galaxies are formed by the merger of spirals (e.g. Whitmore et al. 1993; Miller et al. 1997; Zepf et al. 1999; Ashman and Zepf 1992). However, the formation and evolution of globular cluster systems is still not well understood. Ages and metallicities of the clusters are uncertain either because of degeneracy in the broad-band colors or due to variable reddening. Spectroscopy of these clusters are needed to improve the metallicity and age measurements and to study the kinematics of young cluster systems. In this paper we present the results (Ages, metallicities, kinematics, masses and their environment implications) of a large spectroscopic survey of star clusters in galactic mergers (NGC3256,NGC4038).

Large clusters, potential protoglobular clusters, have been detected at very early stages (embedded compact HII regions) in nearby galaxies. We present radio, millimeter, and infrared imaging and spectroscopy of gas near these young super star clusters. CO and dust emission indicate that star formation efficiencies can be very high, 75% or more on hundred-pc sizescales, in these systems, consistent with the formation of bound clusters. Radio and IR recombination lines in super star cluster nebulae have narrow cores, similar in width to those of Galactic HII regions around single stars. However, super star clusters are so massive that gravity is important in the evolution of these HII regions, and they are not simply freely expanding, as Galactic HII regions are. Super star cluster HII regions may dynamically resemble photospheres, with the possibility of a cluster wind developing. These observations allow us to begin to construct a picture of the formation and early evolution of super star clusters.

The early evolution of dense stellar systems is governed by massive single star and binary evolution. Core collapse of dense massive star clusters is unavoidable and this leads to the formation of very massive objects, with a mass up to 1000 Mo and even larger. When these objects become stars, stellar wind mass loss determines their evolution and final fate, and decides upon whether they form black holes (with normal mass or with intermediate mass) or explode as a pair instability supernova. I will discuss current ideas about the evolution of massive stars and binaries, with special emphasis on the evolution of very massive stars. A convenient evolutionary recipe for the very massive stars is presented that can readily be implemented in an N-body code. Finally, using a dynamical N-body code where the massive and very massive star evolutionary updates are iincluded, we discuss a) the effects of dynamics on the early evolution of the massive binary frequency in dense stellar systems and b) the formation of intermediate mass and stellar mass black holes.

I will present the results of a survey of N-body simulations showing a possible dynamical origin for early mass segregation observed in young clusters and the implications of mass segregation on the early and long term-evolution of star clusters.

The exact nature of the interaction between hot, fast-flowing star cluster winds and the surrounding clumpy ISM in starburst galaxies has very few observational constraints. Besides furthering our knowledge of ISM dynamics, detailed observations of ionized gas at the very roots of large-scale outflows are required to place limits on the current generation of high-resolution galactic wind models. To this end, we conduct a detailed investigation of the ionized gas environment surrounding the young star clusters in the starburst galaxy NGC 1569. Using high spatial- and spectral-resolution Gemini/GMOS IFU observations, we accurately characterise the line-profile shapes of the optical nebular emission lines and find a ubiquitous broad (~300 km/s) component underlying a bright narrower component. By mapping the properties of the individual line components, we find correlations that suggest that the broad component results from powerful cluster wind--gas clump interactions. We propose a model to explain the properties of the line components and the general turbulent state of the ISM.

I will present a general framework for understanding the demographics of star cluster systems, and a toy model which incorporates a universal initial power law mass function, selected formation histories, selected disruption laws, and a convolution with common artifacts and selection effects found in observational data.

It has been shown that the winds of very massive and compact clusters evolve in a special bimodal regime in which the material inserted by stars into the inner cluster region becomes thermally unstable, forms cold dense clumps, and eventually feeds next episodes of star formation, while the material inserted into the outer region creates a quasi-stationary wind. We perform 2D numerical simulations of such winds using the finite difference hydrodynamic code ZEUS for which the cooling routine has been modified to make it suitable for modeling of extremely fast cooling regions. We estimate the fraction of the deposited mass which is accumulated inside the cluster depending on cluster parameters.