Collaborative research of open star clusters

Preliminary results on observations of open clusters are presented. The project has been initiated in the framework of the Uzbek-Taiwan and Taiwan-Baltic collaboration, mainly to upgrade and make use of facilities at Maidanak Observatory. We present detailed, multiwavelength studies of the young cluster NGC 6823 and the associated complex nebulosity, to diagnose the young stellar population and star formation history in the region. In addition, 7 compact open clusters have been monitored for stellar variability. We show how observations like these could feasibly be used to look for exoplanet transit events. We also expect to join the Whole-Earth Telescope effort in future campaigns for asteroseismology.     

Kinematical distances to open star clusters

Kinematical distances are estimated for six open star clusters. They agree fairly well with the photometric distances. The kinematical distances cannot, at present, be estimated better than the photometric distances. When more accurate proper motion measurements become available the kinematical distances will improve considerably and may then be used to calibrate the cosmic distance scale.     

Two populations of open star clusters in the Galaxy

Based on our compiled catalogue of fundamental astrophysical parameters for 593 open clusters, we analyze the relations between the chemical composition, spatial positions, Galactic orbital elements, age, and other physical parameters of open star clusters. We show that the population of open clusters is heterogeneous and is divided into two groups differing by their mean parameters, properties, and origin. One group includes the Galactic clusters formed mainly from the interstellar matter of the thin disk with nearly solarmetallicities ([Fe/H] > ?0.2) and having almost circular orbits a short distance away from the Galactic plane, i.e., typical of the field stars of the Galactic thin disk. The second group includes the peculiar clusters formed through the interaction of extragalactic objects (such as high-velocity clouds, globular clusters, or dwarf galaxies) with the interstellar matter of the thin disk, which, as a result, derived abnormally low (for field thin-disk stars) metallicities and/or Galactic orbits typical of objects of the older Galactic subsystems. About 70% of the clusters older than 1Gyr have been found to be peculiar, suggesting a slower disruption of clusters with noncircular high orbits. Analysis of orbital elements has shown that the bulk of the clusters from both groups were formed within a Galactocentric radius of ??10.5 kpc and closer than ??180 pc from the Galactic plane, but owing to their high initial velocities, the peculiar clusters gradually took up the volumes occupied by the objects of the thick disk, the halo, and even the accreted halo of the Galaxy. Analysis of the relative abundances of magnesium (a representative of the ??-elements) in clusters that, according to their kinematical parameters, belong to different Galactic subsystems has shown that all clusters are composed of matter incorporating the interstellar matter of a single protogalactic cloud in different proportions, i.e., reprocessed in genetically related stars of the Galaxy. The [Mg/Fe] ratios for the clusters with thick-disk kinematics are, on average, overestimated, just as for the field stars of the socalled ??metal-rich wing?? of the thick disk. For the clusters with halo kinematics, these ratios exhibit a very large spread, suggesting that they were formed mainly from matter that experienced a history of chemical evolution different from the Galactic one. We point out that a large fraction of the open clusters with thindisk kinematics have also been formed from matter of an extragalactic nature within the last ??30 Myr.     

Dynamical evolution of two-component star clusters

The dynamical evolution of two-component star clusters, each of which is enclosed within a perfectly reflecting sphere, is investigated by numerically solving moment equations derived from the Boltzmann equation. One of the two adopted model clusters evolves, starting from a state of no mass segregation, toward an equilibrium state at a quite slow rate. The other one evolves away from an equilibrium state and its central density increases without limit. The different evolutionary behaviors of the two model clusters are explained by the fact that there exists no equilibrium state for such clusters if the total energy is less than a certain critical value. The critical value increases with increasing total mass fraction of the heavier stars. This is qualitatively the same as Spitzer's theorem (1969) expressed in another way.     

Heterogeneity of the population of open star clusters in the Galaxy

Based on published data, we have compiled a catalogue of fundamental astrophysical parameters for 593 open clusters of the Galaxy. In particular, the catalogue provides the Galactic orbital elements for 500 clusters, the masses, central concentrations, and ellipticities for 424 clusters, the metallicities for 264 clusters, and the relative magnesium abundances for 56 clusters. We describe the sources of initial data and estimate the errors in the investigated parameters. The selection effects are discussed. The chemical and kinematical properties of the open clusters and field thin-disk stars are shown to differ. We provide evidence for the heterogeneity of the population of open star clusters.     

The evolution of star clusters: the resolved-star approach

We present the first results of a new technique to detect, locate, and characterize young dissolving star clusters. Using Hubble Space Telescope/Advanced Camera for Surveys archival images of the nearby galaxy IC2574, we performed stellar PSF photometry and selected the most massive stars as our first test sample. We used a group-finding algorithm on the selected massive stars to find cluster candidates. We then plot the color-magnitude diagrams for each group, and use stellar evolutionary models to estimate their age. So far, we found 79 groups with ages of up to about 100 Myr, displaying various sizes and densities.     

Test computations on the dynamical evolution of star clusters

Test calculations have been carried out on the evolution of star clusters using the fluiddynamical method devised by Larson (1970a). Large systems of stars have been considered with specific concern with globular clusters. With reference to the analogous standard model by Larson, the influence of varying in turn the various free parameters (cluster mass, star mass, tidal radius, mass concentration of the initial model) has been studied for the results. Furthermore, the partial release of some simplifying assumptions with regard to the relaxation time and distribution of the target stars has been considered. The change of the structural properties is discussed, and the variation of the evolutionary time scale is outlined. An indicative agreement of the results obtained here with structural properties of globular clusters as deduced from previous theoretical models is pointed out.     

Analysis of the mass functions for three open star clusters

Based on our own observational data, we have constructed the mass functions for three young-and intermediate-age Galactic star clusters, Trumpler 2, NGC 7243, and NGC 1513. Their mean slopes Γ fall within the Scalo range: Γ = ?1.7 ± 0.5 for 1M _⊙ < M < 10M _⊙. For each of the clusters, we also analyze the mass distribution of their members as a function of the distance to the center. Noticeable mass segregation in the sense of massive-star concentration toward the cluster center has been detected in Trumpler 2 and NGC 7243. Deviations from rectilinearity in the same direction are observed at log M ≈ 0.41 in each of the three zones of Trumpler 2, which can be interpreted as a small dip in the mass function. We have found peculiarities in NGC 1513 that are difficult to explain by observational selection alone. Comparison of our results with similar studies reveals a similarity in radial Γ distribution for NGC 7243 with h Per and for Trumpler 2 with IC 1805.     

Roche volume filling and the dissolution of open star clusters

From direct N‐body simulations we find that the dynamical evolution of star clusters is strongly influenced by the Roche volume filling factor. We present a parameter study of the dissolution of open star clusters with different Roche volume filling factors and different particle numbers. We study both Roche volume underfilling and overfilling models and compare with the Roche volume filling case. We find that in the Roche volume overfilling limit of our simulations two‐body relaxation is no longer the dominant dissolution mechanism but the changing cluster potential. We call this mechanism “mass‐loss driven dissolution” in contrast to “two‐body relaxation driven dissolution” which occurs in the Roche volume underfilling regime. We have measured scaling exponents of the dissolution time with the two‐body relaxation time. In this experimental study we find a decreasing scaling exponent with increasing Roche volume filling factor. The evolution of the escaper number in the Roche volume overfilling limit can be described by a log‐logistic differential equation. We report the finding of a resonance condition which may play a role for the evolution of star clusters and may be calibrated by the main periodic orbit in the large island of retrograde quasiperiodic orbits in the Poincaré surfaces of section. We also report on the existence of a stability curve which may be of relevance with respect to the structure of star clusters. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relative content of Be stars in young open star clusters

Based on high- and medium-resolution spectra, we analyze the population of Be stars in eight young open star clusters. We have found a clear dependence of the relative content of early-type (B0-B3) Be stars on the cluster age. The relative concentration of Be stars of spectral types B0?CB3 gradually increases with cluster age, reaching its maximum value of 0.46 in clusters with ages of 12?C20 Myr. The almost complete absence of Be stars in older clusters can be easily explained by the fact that B stars leave the main sequence. The few emission objects in clusters with ages of 1?C7 Myr are most likely Herbig Be stars. Such a distribution of Be stars in clusters unequivocally points to the evolutionary status of the Be phenomenon. We also briefly consider the causes of this pattern.     

An investigation of 11 previously unstudied open star clusters

The main astrophysical properties of 11 previously unstudied open star clusters are probed with JHK Near-IR (2MASS) photometry of Cutri et al. [Cutri, R., et al., 2003. The IRSA 2MASS All-sky Point Source Catolog, NASA/IPAC Infrared Science Archive] and proper motions (NOMAD) astrometry of Zacharias et al. [Zacharias, N., Monet, D., Levine, S., Urban, S., Gaume, R., Wycoff, G., 2004. American Astro. Soc. Meeting 36, 1418]. The fundamental parameters have been derived for IC (1434, 2156); King (17, 18, 20, 23, 26); and Dias (2, 3, 4, 7, 8), for which no prior parameters are available in the literature. The clusters’ centers coordinates and angular diameters are re-determined, while ages, distances, and color excesses for these clusters are estimated here for the first time.