We have studied the validity of the historical Cygnus OB associations and have found that many do not show the kinematic coherence expected for true OB associations. We have revisited these groups by photometrically identifying thousands of OB stars across the region with an SED fitting process which combines photometry, astrometry, spectral and evolutionary models. We applied a flexible clustering method and identified seven kinematically-coherent new OB associations. We observe a distinct correlation between position and velocity for two sets of these associations that suggests an expansion pattern. Tracing the motion of the stars back in the past we find that the sets were at their closest around 7.9 and 8.5 Myr ago. We discuss whether this expansion is a natural by-product of the commonly observed size - velocity dispersion relation of molecular clouds, or requires feedback to initiate the dispersal.

We present results from our ongoing infrared spectroscopic studies of the massive stellar content at the Center of the Milky Way (GC) and across the obscured Galactic disk. Together with the full characterization of these clusters, we seek to obtain a present day metallicity 2-D map of the inner Galaxy and characterize the influence on the bar in the chemical evolution. We will also constrain the clusters IMFs, infer the presence of possible top-heavy recent star formation histories and test massive star formation channels: clusters vs isolation.

We study the evolution of star clusters in the Galactic tidal field starting from their birth in molecular clumps. Our model clusters form according to the local-density-driven cluster formation model in which the stellar density profile is steeper than that of gas. As a result, clusters resist the gas expulsion better than predicted by earlier models.

We vary the impact of the Galactic tidal field λ, considering different Galactocentric distances (3-18 kpc), as well as different cluster sizes. Our model clusters survive the gas expulsion independent of λ.

We investigated the relation between the cluster mass at the onset of secular evolution and their dissolution time. The model clusters formed with a high star-formation efficiency (SFE) follow a tight mass-dependent dissolution relation, in agreement with previous theoretical studies. However, the low-SFE models present a shallower mass-dependent relation than high-SFE clusters, and most dissolve before reaching 1 Gyr (cluster teenage mortality).

The fundamental plane (FP) is the relation between the surface brightness (I), velocity dispersion (σ) and radius (R). The tilt of FP from the virial plane (R = σ2I) not only tells the dynamical states of the system but also its formation and evolution. We motivate to looking for an FP in Galactic open clusters (OCs). To form a sample of OCs, we access the most recent DR 14 data from the SDSS/APOGEE2 and the Gaia-ESO survey. Membership of stars is determined via radial velocity and metallicity, plus star’s location in the color-magnitude diagram. Besides the velocity dispersion (σrv) obtained from SDSS/APOGEE2 & Gaia-ESO, the average surface brightness (IKs), and apparent radii (r2) of OCs are taken from known OC catalog. A weak relation is found: log(r2) ∝ -0.34 * log(σ) - 0.08 * IKs. An implication of this FP needs further investigation.

Open clusters (OCs) are important objects for stellar dynamics studies. The short survival timescale of OCs makes them closely related to the formation of Galactic field stars. We motivate to investigate the dynamical evolution of OCs on the aspect of internal effect and the external influence. Firstly, we make use of the known OC catalog to obtain OCs masses, effective radii. Additionally, we estimate OCs kinematics properties by OC members cross-matched with radial velocity and metallicity from SDSSIV/APOGEE2. We then establish the fundamental plane of OCs based on the radial velocity dispersion, the effective radius, and average surface brightness. The deviation of the fundamental plane from the Virial Plane, so called the tilt, and the r.m.s. dispersion of OCs around the average plane are used to indicate the dynamical status of OCs. Parameters of the fitted plane will vary with cluster age and distance.

Our position in the Milky Way (MW) is both a blessing and a curse. We are nearby to many star clusters, but the dust that is a product of their very existence obscures them. Also, many massive young clusters are expected to be located near, or across the Galactic Center, where the dust extinction is extreme (AV > 15 mag) and can be better penetrated by infrared photons. This paper reviews the discoveries and the study of new MW massive stars and massive clusters made possible by near infrared observations that are part of the VISTA Variables in the Vía Láctea (VVV) survey. It discusses what the studies of their fundamental parameters have taught us.

A robust feature of turbulent fragmentation theories is a universal Salpeter like slope (2.2 -2.4), for the mass spectrum of the fragments, at the high mass end. This is so due to the scale-free nature of turbulence and gravity. There are reports of top heavy / flatter Initial Mass Functions (IMF), inferred for many regions where we expect star formation to take place in gas clouds with comparatively higher gas density. Also, a higher Star Formation Efficiency (SFE) for regions of higher gas density has been proposed, to understand the formation of bound stellar systems in which dark matter is not a significant factor affecting the internal dynamics. In turbulent fragmentation models for star formation, we do not expect the mass of the stellar cluster to influence the maximum stellar mass directly and thereby imply a relation between the maximum stellar mass and the cluster mass. However, such a relation may be expected from statistical considerations. In this context, we explore the density dependence of the IMF, that would arise due to denser clouds producing more massive clusters due to the density dependence of the SFE.

We investigate the formation of stellar clusters from a Galactic scale SPH simulation. The simulation traces star formation over a 5.6 Myr timescale, with local gravitational instabilities resulting in ~ 105 solar masses of star formation in the form of sink particles. We investigate the time evolution of the physical properties of the forming clusters including their half-mass radii, their energies and the depletion time of the gas. Star formation is driven by the large scale flows which compress the gas to higher densities where self gravity takes over and collapse occurs. We show that the more massive clusters (up to ~ 2 × 104 solar masses) gather their material from of order 10 pc due to these large scale motions associated with the spiral arm passage and shock. The bulk of the gas becomes gravitationally bound near 1-2 Myr before sink formation, and in the absence of feedback, significant accretion ongoing on longer timescales. We trace the hierarchical merging process of cluster formation which naturally results in age spreads of order the crossing time of the original region which provides the gas reservoir for the cluster.

Comparative analysis of GMS and MSC is presented based on data collected on these objects in the Galaxy and other galaxies. The model is proposed of gravitational instability of GMC and MSC system. Some implications are discussed.

We perform an all-sky search for classical (type I) Cepheids that are members of Galactic Open Clusters. Our approach is multi-dimensional, using all available spatial and kinematic parameters. The quantification of errors is crucial for this analysis, so care is taken to find adequate and realistic representations of parameter uncertainties. The data employed in the calculation are taken from published catalogues and the literature, supplemented by specific radial-velocity observations. Our work in progress is outlined here, and issues related to the inhomogeneity of cluster radii in the literature are discussed in some detail.

Stars form from molecular clouds, mostly in clusters with tens to tens of thousands of members, and the mass distribution within these clusters, or the Initial Mass Function, seems to be invariable against many parameters and over a wide range of masses. However, masses are a very difficult quantity to assess, and the precision of our determinations of the IMF is systematically lower than usually quoted. I will discuss the process of determining masses from observations and the type of uncertainties associated with this process.

The UKIDSS Galactic Plane Survey (GPS) is surveying the northern and equatorial plane in the J, H and K bands. Here we report initial results from searches for new clusters and star formation regions. 248 clusters have been detected by our Bayesian search, of which 127 are new. A visual inspection of the images is also proving successful. A cross match with Spitzer-GLIMPSE to find clusters of Young Stellar Objects is being attempted. No new globular clusters are detected except for two likely candidates already detected by Mercer et al. in GLIMPSE.

The value of different forms of energy contained in TDC region as well as the structure and physical characteristics of star formation in the Taurus Dark Clouds are analyzed.

The current status of microturbulent velocity and abundance determinations for A and F dwarfs in open clusters is reviewed. A programme to observe several tens of A and F dwarfs in open clusters of various ages was initiated several years ago. We have performed high resolution high signal-to-noise spectroscopy of stars well distributed in mass along the Main Sequence. Microturbulent velocities and abundances of several chemical elements have been derived iteratively by fitting grids of synthetic spectra calculated in LTE to the observed spectra. Curve of growths were used in a few instances as well. The results obtained are reviewed for Coma Berenices, the Pleiades, Alpha Persei and the Ursa Major moving group.

The microturbulent velocities exhibit a broad maximum in the range A5V to about F0V as indicated in Smalley (2004).

About 20% of the high mass O-type stars in the Galaxy are found outside stellar clusters and associations. In the solar neighbourhood this fraction amounts to 43 O-type stars. In the framework of high-mass star formation, we search for the origins of these stars. We aim at separating the O-type field stars from runaway O-type stars, where the former does not betray any indication of past dynamical interactions resulting in their present field location. We specifically search for the presence of stellar clusters, but also evaluate spatial velocities, distance from Galactic plane, locations in the Galaxy with respect to young stellar clusters. Among our sample of 43, we find that 5 stars are in fact observed in the K-band to be members of previously unnoticed small clusters. On the other hand it is plausible that nearly half the sample (22 objects) could have undergone a dynamical ejection from a young cluster. Based on the current available data, ${\sim }4%$ of the Galactic O-type stars cannot be associated with a stellar cluster. Finally by assuming that stars form in clusters with a power law in membership number with index −1.7 down to “clusters” consisting of single stars, we calculate the expected statistics regarding isolated O stars and O stars in OB-associations. We conclude that the results of the calculations are consistent with the observed statistics of O-type stars.