We have extended the search for luminous (M_{b_J} <= -10.5) compact stellar systems (CSSs) in the Virgo and Fornax galaxy clusters by targeting with the recently commissioned AAOmega spectrograph three cluster environments -- the cluster cores around M87 and NGC 1399, intracluster space, and a major galaxy merger site (NGC 1316). We have significantly increased the number of redshift-confirmed CSSs in the Virgo cluster core and located three Virgo intracluster globular clusters (IGCs) at a large distance from M87 (154-173 arcmin or ~750-850 kpc) -- the first isolated IGCs to be redshift-confirmed. We estimate luminous CSS populations in each cluster environment, and compare their kinematic and photometric properties. We find that (1) the estimated luminous CSS population in the Virgo cluster core is half of that in Fornax, possibly reflecting the more relaxed dynamical status of the latter; (2) in both clusters the luminous CSS velocity dispersions are less than those of the cD galaxy GC system or cluster dE galaxies, suggesting luminous CSSs have less energetic orbits; (3) Fornax has a sub-population of cluster core luminous CSSs that are redder and presumably more metal-rich than those found in Virgo; (4) no luminous CSSs were found in a 10-20 arcmin (60-130 kpc) radial arc east of the 3 Gyr old NGC 1316 galaxy merger remnant or in the adjacent intracluster region, implying that any luminous CSSs created in the galaxy merger have not been widely dispersed.
We present new automatic methods of search for star clusters using the data available in new huge stellar catalogues. Using 2MASS catalogue we have discovered over ten new open clusters in the region of Galaxy anticenter and determined their physical parameters.
Some of the most massive globular clusters of our Milky Way, such as for example omega-Centauri, show a mixture of stellar populations spanning a few Gyr in age and 1.5 dex in metallicities. In contrast, standard formation scenarios predict that globular and open clusters form in one single star-burst event of duration less than about 10 Myr and therefore should exhibit only one age and one metallicity in its stars. Here, we investigate the possibility that a massive stellar super-cluster may trap older galactic field stars during its formation process that are later detectable in the cluster …
Stellar Clusters are identified in images and in the spectra of these star forming dwarf galaxies. These Stellar Clusters have properties similar to those observed in other violent star forming galaxies and may the elementary entities of a starburst.
Observations have revealed that most stars are born in clusters. These systems, containing from tens to thousands of stars and typically significant mass in gas in the youngest systems, evolve due to a combination of stellar and star-gas interactions. Simulations of pure stellar systems are used to investigate possible initial configurations including ellipticity, substructure and mass segregation. Simulations of gas-rich clusters investigate the effects of accretion on the cluster dynamics and on the individual masses that result in a stellar mass spectrum. Further stellar interactions, inclu…
Our VLT (FLAMES) observations near NGC1399 investigate the connection between ultra-compact dwarf galaxies (UCDs), NGC1399 globular clusters and intra-cluster globulars. We have uncovered 30 faint compact stellar systems in the Fornax galaxy cluster, adding to 62 bright UCDs previously reported. The magnitude limit of these stellar systems extends down to the globular cluster domain. We detect a filament of UCDs and globular clusters stretching across NGC1399 and find weak evidence for its rotation. These compact stellar systems not only congregate around several cluster galaxies but are also …
We present a review of some of the current major challenges in stellar cluster research, including young clusters, globular clusters, and galactic nuclei. Topics considered include: primordial mass segregation and runaway mergers, expulsion of gas from clusters, the production of stellar exotica seen in some clusters (eg blue stragglers and extreme horizontal--branch stars), binary populations within clusters, the black--hole population within stellar clusters, the final parsec problem, stellar dynamics around a massive black hole, and stellar collisions. The Modest Questions posed here are th…
The existence of complex stellar populations in some star clusters challenges the understanding of star formation. E.g. the ONC or the sigma Orionis cluster host much older stars than the main bulk of the young stars. Massive star clusters (omega Cen, G1, M54) show metallicity spreads corresponding to different stellar populations with large age gaps. We show that (i) during star cluster formation field stars can be captured and (ii) very massive globular clusters can accrete gas from a long-term embedding inter stellar medium and restart star formation.
A qualitative study is made of the evolutionary tracks of various dense clusters, with allowance for the evaporation of stars, and for head-on collisions, until relativistic collapse occures or the cluster breaks up.
We investigate how the formation and evolution of extrasolar planetary systems can be affected by stellar encounters that occur in the crowded conditions of a stellar cluster. Using plausible estimates of cluster evolution, we show how planet formation may be supressed in globular clusters while planets wider than 0.1 AU that do form in such environments can be ejected from their stellar system. Less crowded systems such as open clusters have a much reduced effect on any planetary system. Planet formation is unaffected in open clusters and only the wider planetary systems will be disrupted dur…
We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5 to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of galaxies. Among the unresolved targets are five objects which are members of the Fornax Cluster. They are extremely compact stellar systems with scale lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known type of stellar system, being more compact and significantly less luminous than other compact dwarf galaxies, yet much brighter than a…
The stellar-dynamical evolution of bound star clusters during the first few Myr is dominated by binary-binary and binary-star interactions, the rapid sinking of the most massive stars to the centre of the clusters and mass loss from evolving stars. The consequences of these processes for the binary and stellar population in clusters, and for the star clusters as a whole, are studied by following the evolution over 150 Myr of a library of compact cluster models containing up to 10^4 stars.
We first summarize work that has been done on the effects of binaries on theoretical population synthesis of stars and stellar phenomena. Next, we highlight the influence of stellar dynamics in young clusters by discussing a few candidate UFOs (unconventionally formed objects) like intermediate mass black holes, Eta Carinae, Zeta Puppis, Gamma Velorum and WR 140.
We present the detection of a 2.0(+1.4,-0.8)x10^4 solar mass black hole (BH) in the stellar cluster G1 (Mayall II), based on data taken with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. G1 is one of the most massive stellar clusters in M31. The central velocity dispersion (25 kms) and the measured BH mass of G1 places it on a linear extrapolation of the correlation between BH mass and bulge velocity dispersion established for nearby galaxies. The detection of a BH in this low-mass stellar system suggests that (1) the most likely candidates for seed massive BHs c…
We review recent work that investigates the formation of stellar clusters, ranging in scale from globular clusters through open clusters to the small scale aggregates of stars observed in T associations. In all cases, recent advances in understanding have been achieved through the use of state of the art stellar dynamical and gas dynamical calculations, combined with the possibility of intercomparison with an increasingly large dataset on young clusters. Among the subjects that are highlighted are the frequency of cluster-mode star formation, the possible relationship between cluster density a…
I report on some recent results in the framework of a complex project aimed to characterize the photometric properties of stellar populations in Galactic Globular Clusters.
We study the rate of escape of stars (``evaporation'') from tidally-limited postcollapse globular clusters having a power-law distribution of stellar masses. We use a multi-mass Fokker-Planck code and assume a steady tidal field. Stellar-dynamical processes cause the inner parts of the cluster to expand, which in turn causes stars to overflow the tidal boundary. Mass loss by stellar evolution is assumed to be unimportant in these later evolutionary stages. The fraction of the cluster mass lost per half-mass relaxation time ($\trh$) is roughly constant, in agreement with simple homologous model…
We demonstrate that the mass of the most massive star in a cluster correlates non-trivially with the cluster mass. A simple algorithm according to which a cluster is filled up with stars that are chosen randomly from the standard IMF but sorted with increasing mass yields an excellent description of the observational data. Algorithms based on random sampling from the IMF without sorted adding are ruled out with a confidence larger than 0.9999. A physical explanation of this would be that a cluster forms by more-massive stars being consecutively added until the resulting feedback energy suffice…
I summarize in terms of evolutionary population synthesis models our current understanding of stellar populations in different environments, from star clusters in nearby galaxies to distant galaxies in clusters or seen through gravitational lenses. Constraints on the age of the stellar populations in clusters and galaxies derived from comparing predicted and observed spectral energy distributions are examined and contrasted with the maximum ages allowed by cosmological models at the corresponding redshift.
A coalescence model using the observed properties of pre-stellar condensations (PSCs) shows how an initially steep IMF that might be characteristic of primordial cloud fragmentation can change into a Salpeter IMF or shallower IMF in a cluster of normal density after one dynamical time, even if the PSCs are collapsing on their own dynamical time. The model suggests that top-heavy IMFs in some starburst clusters originate with PSC coalescence.
In this review, we summarize the observed statistical properties of visual binaries in various young stellar clusters. The binary frequency, orbital period distribution and mass ratio distribution are considered for populations of both low-mass and high-mass stars. These properties are then compared to numerical models of the dynamical evolution of stellar clusters and to the predictions of binary formation mechanisms.
We compiled a catalogue of infrared star clusters in the Galaxy, which are most of them embedded. It condenses the growing literature information. We also include in the sample infrared stellar groups which are less dense than star clusters, such as those embedded in the dark clouds Taurus-Auriga and Chamaleon I. We provide galactic and equatorial coordinates, angular dimensions, different designations and related objects such as nebulae. A total of 189 infrared clusters and 87 embedded stellar groups are included. A fraction of 25 % of the embedded clusters are projected close to each other i…
By means of simple numerical models we discuss whether "collisionless stellar evaporation" from cluster elliptical galaxies could be an effective mechanism for the production of intracluster stellar populations. The effectiveness of this mechanism is due to the fact that, for realistic galaxy and cluster models, the galaxy oscillation periods near equilibrium configurations in the cluster tidal field are of the same order of stellar orbital times in the external parts of the galaxies themselves. With the aid of Monte-Carlo simulations we explore the evolution of stellar orbits in oscillating g…
Three very massive clusters are known to reside in the Galactic Center region, the Arches cluster, the Quintuplet cluster and the Central parsec cluster. We obtained spectroscopic observations of the Quintuplet cluster with the Integral Field Spectrograph SINFONI-SPIFFI at the ESO-VLT. The spectral range comprises the near-IR K-band from 1.94 to 2.45 micrometer. The 3D data cubes of the individual fields were flux-calibrated and combined to one contiguous cube, from which the spectra of all detectable point sources were extracted. We present a catalog of 160 stellar sources in the inner part o…
We investigate the formation of brown dwarfs and very low-mass stars through the gravitational fragmentation of infalling gas into stellar clusters. The gravitational potential of a forming stellar cluster provides the focus that attracts gas from the surrounding molecular cloud. Structures present in the gas grow, forming filaments flowing into the cluster centre. These filaments attain high gas densities due to the combination of the cluster potential and local self-gravity. The resultant Jeans masses are low, allowing the formation of very low-mass fragments. The tidal shear and high veloci…
Blue straggler stars present as secure members in the Galactic open clusters form a major challenge to the conventional picture of evolutionary population synthesis based on the stellar evolution theory of single stars, as illustrated in our previous work. Expansion of our sample in the current work to include younger age clusters provides a larger data base to expose the question raised for the simple stellar population model. The working sample now includes 97 Galactic open clusters of ages ranging from 0.1 to several Gyrs. The contributions of blue straggler stars to the integrated light of…
Photoevaporation driven by the ultraviolet radiation from massive stars severely limits the lifetime of protoplanetary discs around stars formed within stellar clusters. I investigate the resulting influence of clustered environments on the probability of giant planet formation, and show that for clusters as rich, or richer than, Orion, the time available for planet formation is likely to be limited to the length of any delay between low mass and high mass star formation. Under popular models for the formation of massive planets, the fraction of stars with giant planets in rich clusters is exp…
Our ability to determine stellar ages from measurements of stellar rotation, hinges on how well we can measure the dependence of rotation on age for stars of different masses. Rotation periods for stars in open clusters are essential to determine the relations between stellar age, rotation, and mass (color). Until recently, ambiguities in vsini data and lack of cluster membership information, prevented a clear empirical definition of the dependence of rotation on color. Direct measurements of stellar rotation periods for members in young clusters have now revealed a well-defined period-color r…
We present near-infrared J, H, and K_s images and K-band spectroscopy of two newly discovered stellar clusters at different stages of evolution. Our spectra suggest the presence of massive YSOs in the heavily embedded cluster in the star-forming region near radio source G353.4-0.4 and an O5-O6V star in the cluster near radio source G305+00.2. We determine a K-band luminosity function (KLF) for both clusters and an initial mass function (IMF) for the cluster near G305+00.2. The derived IMF slope is -1.5 if the KLF is used to derive the IMF and is -0.98 if the color-magnitude diagram and spectra…
We have made a new compilation of observations of maximum stellar mass versus cluster membership number from the literature, which we analyse for consistency with the predictions of a simple random drawing hypothesis for stellar mass selection in clusters. Previously, Weidner and Kroupa have suggested that the maximum stellar mass is lower, in low mass clusters, than would be expected on the basis of random drawing, and have pointed out that this could have important implications for steepening the integrated initial mass function of the Galaxy (the IGIMF) at high masses. Our compilation demon…
Aims: We investigate the build-up of stellar mass through star formation in field galaxies, galaxy groups, and clusters in order to better understand the physical processes regulating star formation in different haloes. Methods: In order to do so we relate ongoing star formation activity to the stellar mass by studying the integrated specific star formation rate (SSFR), defined as the star-formation rate per unit stellar mass, as a function of integrated stellar mass for samples of field galaxies, groups of galaxies, and galaxy clusters at 0.18 < z < 0.85. The star formation rate (SFR) is deri…
The diffraction effects on gravitational waves propagating through a stellar cluster are analyzed in the relevant approximation of Fresnel diffraction limit. We find that a gravitational wave scintillation effect - similar to the radio source scintillation effect - comes out naturally, implying that the gravitational wave intensity changes in a characteristic way as the observer moves.
We present the results of a spectroscopic investigation of 108 nearby field B-stars. We derive their key stellar parameters, $V \sin i$, $T_{\rm eff}$, $\log g$, and $\log g_{\rm polar}$, using the same methods that we used in our previous cluster B-star survey. By comparing the results of the field and the cluster samples, we find that the main reason for the overall slower rotation of the field sample is that it contains a larger fraction of older stars than found in the (mainly young) cluster sample.
We present detailed luminosity profiles of the young massive clusters M82-F, NGC 1569-A, and NGC 1705-1 which show significant departures from equilibrium (King and EFF) profiles. We compare these profiles with those from N-body simulations of clusters which have undergone the rapid removal of a significant fraction of their mass due to gas expulsion. We show that the observations and simulations agree very well with each other suggesting that these young clusters are undergoing violent relaxation and are also losing a significant fraction of their stellar mass. That these clusters are not in …
This paper explores the quantitative connection between globular clusters and the diffuse stellar population of the galaxies they are associated with. Both NGC 1399 and NGC 4486 (M87) are well suited for this kind of analysis due to their large globular cluster populations. The main assumption of our Monte Carlo based models is that each globular cluster is formed along with a given diffuse stellar mass that shares the same spatial distribution, chemical composition and age. The main globular clusters subpopulations, that determine the observed bimodal colour distribution, are decomposed avoid…
Massive clusters are now seen to form easily in interacting and merging galaxies, making these excellent environments for studying the properties of young clusters. New observations of the Antennae (NGC 4038/39) show that the most luminous young clusters do not have a measurable tidal radius. Most observations suggest that the luminosity function (LF) and mass functions of young clusters are single power laws. However, there are many uncertainties at the faint end of the LF. For example, contamination from massive stars may be important. The shape and evolution of the LF, and more fundamentall…
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).
Globular clusters are gravitationally bound stellar systems containing on the order of 100,000 stars. Due to the high stellar densities in the cores of these clusters, close encounters and even physical collisions between stars are inevitable. These dynamical interactions can produce exotic types of single and binary stars that are extremely rare in the galactic field, but which may be important to the dynamical evolution of their host clusters. A common feature of these dynamically-formed stellar populations is that many of their members are relatively hot, and thus bright in the far-ultravio…
We look for a simple analytic model to distinguish between stellar clusters undergoing a bimodal hydrodynamic solution from those able to drive only a stationary wind. Clusters in the bimodal regime undergo strong radiative cooling within their densest inner regions, which results in the accumulation of the matter injected by supernovae and stellar winds and eventually in the formation of further stellar generations, while their outer regions sustain a stationary wind. The analytic formulae are derived from the basic hydrodynamic equations. Our main assumption, that the density at the star clu…
Structure of a quasi-stationary stellar cluster is modelled assuming that it is embedded in the gravitational field of a super-massive black hole. Gradual orbital decay of stellar trajectories is caused by the dissipative interaction with an accretion disc. Gravitational field of the disc is constructed and its effect on the cluster structure is taken into account as an axially symmetric perturbation. Attention is focused on a circumnuclear region (r<10^4 gravitational radii) where the effects of the central black hole and the disc dominate over the influence of an outer galaxy. It is shown ho…
Small perturbations in spherical and thin disk stellar clusters surrounding massive a black hole are studied. Due to the black hole, stars with sufficiently low angular momentum escape from the system through the loss cone. We show that stability properties of spherical clusters crucially depend on whether the distribution of stars is monotonic or non-monotonic in angular momentum. It turns out that only non-monotonic distributions can be unstable. At the same time the instability in disk clusters is possible for both types of distributions.
Dynamical models of star clusters are maturing in the sense that effects other than simple point particle dynamics are taken into account. We summarize the relevance of and prospects for this new generation of N-body models.
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades and Ursa Major clusters we find that stellar models with updated physical inputs nicely reproduce the location in the color magnitude diagram of main sequence stars of different metallicities. Stars in the helium burning phase are also discussed, showing that the luminosity of giants in the Hyades, Praesepe and Ursa Major clusters appears to be in reasonable agreement with theoretical predictions. A short discussion concerning the current evolutionary scenarios closes the paper.
Recent wide-field photometric surveys (Lee et al.; Pancino et al.) have shown the existence of a previously unknown metal-rich ([Fe/H] -0.6) stellar population in the galactic globular cluster omega Centauri. The discovery of this new component, which comprises only a small percentage (~5%) of the entire cluster population, has added a new piece to the already puzzling picture of the star formation and chemical evolution of this stellar system. In this Letter we show that stars belonging to the newly discovered metal-rich population have a coherent bulk motion with respect to the other cluster…
We are examining mass loss from globular clusters giant stars, focussing on metallicity dependance. We present three sets of observations: TIMMI-2 mid-IR spectra of 47 Tuc, UVES high-resolution optical spectra of several clusters, and an infrared atlas of omega Cen using the Spitzer Space Telescope.
We present distance estimates for eleven peculiar Virgo cluster spiral galaxies based on measurements of the stellar kinematics of their central 2 kpc. Stellar circular velocities were obtained using two-integral dynamical models. Distances were obtained by comparing, at each radius, the stellar circular velocities with synthetic H_alpha rotation curves derived from NIR Tully-Fisher relations.The results show that most of our galaxies are located within 4 Mpc of the core of the cluster. Three of these galaxies, previously classified as "low rotator galaxies" or with "Truncated/Compact" H_alpha…
At which masses does the regime of globular clusters end and the one of dwarf galaxies begin? And what separates these two classes of hot stellar systems? We examine to what extend very massive (>10^7 Mo) young star clusters are similar to their lower mass counter parts and to which degree they resemble other objects in their mass regime (dwarf--globular transition objects (DGTOs), ultra compact dwarf galaxies (UCDs), galaxy nuclei). The comparison is performed by placing the recently observed very massive young clusters onto known scaling relation defined by globular clusters (with typical ma…
We use a high resolution collisionless simulation of a Virgo--like cluster in a $Λ$CDM cosmology to determine the velocity and clustering properties of the diffuse stellar component in the intracluster region at the present epoch. The simulated cluster builds up hierarchically and tidal interactions between member galaxies and the cluster potential produce a diffuse stellar component free-flying in the intracluster medium. Here we adopt an empirical scheme to identify tracers of the stellar component in the simulation and hence study its properties. We find that at $z=0$ the intracluster stell…
Recently, very massive compact stellar systems have been discovered in the intracluster regions of galaxy clusters and in the nuclear regions of late-type disk galaxies. It is unclear how these compact stellar systems -- known as ``ultra-compact dwarf'' (UCD) galaxies or ``nuclear clusters'' (NCs) -- form and evolve. By adopting a formation scenario where these stellar systems are the product of multiple merging of star clusters in the central regions of galaxies, we investigate, numerically, their physical properties. We find that physical correlations between velocity dispersion, luminosity,…
In this brief overview of the stellar populations of galaxies in clusters I highlight some of the most recent results, including the downsizing of the k+a population and the star formation--local density relation. I conclude discussing some open questions regarding the interpretation of the observational results, and speculating on the current meaning of the words ``primordial'' and ``environmental''.
The past few years have seen dramatic improvements in the scope and realism of star cluster simulations. Accurate treatments of stellar evolution, coupled with robust descriptions of all phases of binary evolution, have been incorporated self-consistently into several dynamical codes, allowing for the first time detailed study of the interplay between stellar dynamics and stellar physics. The coupling between evolution, dynamics, and the observational appearance of the cluster is particularly strong in young systems and those containing large numbers of primordial binary systems, and important…
We have begun a survey of the molecular gas surrounding 31 young clusters in order to investigate the link between environment and the resulting cluster. We present here a preliminary comparison of two clusters in our sample: GGD12-15 and Mon R2. Since both clusters are located in the MonR2 molecular cloud at a distance of 830 pc, observational biases due to differing sensitivities and angular resolutions are avoided.
Combining population synthesis models with simple Montecarlo simulations of stochastic effects in the number of stars occupying sparsely populated stellar evolutionary phases in the HRD, I show that the scatter observed in the photometric magnitudes and colors of LMC and NGC 7252 star clusters can be understood in the framework of current stellar evolution theory. The use of a high resolution stellar spectral atlas in population synthesis models improves considerably the quality of the fits to observed galaxy SEDs, making the assignment of a spectroscopic age to stellar populations more reliab…
We have undertaken a long-term project, Planets in Stellar Clusters Extensive Search (PISCES), to search for transiting planets in open clusters. In this paper we present the results for NGC 188, an old, rather populous cluster. We have monitored the cluster for more than 87 hours, spread over 45 nights. We have not detected any good transiting planet candidates. We have discovered 18 new variable stars in the cluster, bringing the total number of identified variables to 46, and present for them high precision light curves, spanning 15 months.
We systematically study the effects of collisions on the overall dynamical evolution of dense star clusters using Monte Carlo simulations over many relaxation times. We derive many observable properties of these clusters, including their core radii and the radial distribution of collision products. We also study different aspects of collisions in a cluster taking into account the shorter lifetimes of more massive stars, which has not been studied in detail before. Depending on the lifetimes of the significantly more massive collision products, observable properties of the cluster can be modifi…
We present evidence for the existence of an old stellar halo in the Blue Compact Dwarf galaxy VII Zw 403. VII Zw 403 is the first Blue Compact Dwarf galaxy for which a clear spatial segregation of the resolved stellar content into a "core-halo" structure is detected. Multicolor HST/WFPC2 observations indicate that active star formation occurs in the central region, but is strikingly absent at large radii. Instead, a globular-cluster-like red giant branch suggests the presence of an old (> 10 Gyr) and metal poor (<[Fe/H]>=-1.92) stellar population in the halo. While the vast majority of Blue Co…
We report on the identification of 54 embedded clusters around 217 massive protostellar candidates of which 34 clusters are new detections. The embedded clusters are identified as stellar surface density enhancements in the 2 $μ$m All Sky Survey (2MASS) data. Because the clusters are all associated with massive stars in their earliest evolutionary stage, the clusters should also be in an early stage of evolution. Thus the properties of these clusters should reflect properties associated with their formation rather than their evolution. For each cluster, we estimate the mass, the morphological …
