We study the survival of sub-structures (clumps) within larger self-gravitating galaxies or dark matter halos. Building on scaling relations obtained from N-body calculations of violent relaxation, we arguethat the tidal field of galaxies and halos canonly destroy sub-structures if spherical symmetry is imposed at formation.Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenise the halo on a dynamical timescale.Phase-mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disc.

We present the first determination of fundamental parameters (positions, reddenings, distances, ages and kinematics) for a sample of 22 open clusters in the solar vicinity. A systematic search for unknown open clusters using the astrometric and photometric data included in the Tycho2 catalogue has resulted in the identification of several candidates. In this work, we introduce 11 of the most promising open cluster candidates. The reality of the stellar aggregates is supported by the analysis of proper motions, colour-magnitude diagrams, stellar density distributions, and by the visual inspection of the Digitised Sky Survey (DSS) plates. The proper motion data included in the Tycho2 catalogue has also allowed us to identify members in 11 open clusters for which no information, besides their positions and estimates of their apparent diameters, was previously available. Reddenings, distances and ages were determined for most objects using colour-magnitude diagrams for cluster members, built using the photometry included in Tycho2, complemented by spectral types found in the literature. We find that the majority of the aggregates are nearby (closer than ~500 pc) open clusters.

The ages of 203 open clusters from the list of Dambis(1999) are computed in terms of Cambridge evolutionarytracks with and without the allowance for convective overshooting(Pols et al. 1998). The vertical scaleheight of thecluster layer for 123 objects at Galactocentric distances$ R_{\rm g} =R_0 \pm1$ kpc is found to varynon-monotonically with age exhibiting a wavelike pattern similarto the one earlier found for the Cepheid population (Joeveer1974). The period of these variations is equal to $P_{Z}= 74 \pm 2$ Myr and $P_{Z} = 92 \pm 2$ Myr if cluster ages arecomputed in terms of evolutionary models of Pols et al. (1998) without and with overshooting, respectively. Ifinterpreted as a manifestation of vertical virial oscillations,the implications of the pattern found are threefold: (1) theperiod of vertical oscillations can be reconciled with the knownlocal density of visible matter only if cluster ages are computedwith no or mild overshooting ($P_{Z} = 74 \pm 2$ Myr), which impliesa maximum local density of $\rho = 0.118 \pm 0.006~M_{\odot}$ pc-3 compared to $\rho = 0.102~M_{\odot}$ pc-3recently inferred from Hipparcos data (Holmberg & Flynn2000), whereas the period derived from ages computed usingmodels with overshooting ($P_{Z} = 92 \pm 2$ Myr) implies amaximum local density of only $\rho = 0.075 \pm 0.003~M_{\odot}$ pc-3 and is thus totally incompatible with recentestimates; (2) there is not much room left for the dark matter($\rho_{\rm DM} \le 0.027~M_{\odot}$ pc-3) in the Galactic disknear the solar Galactocentric distance, and (3) at the time oftheir formation open clusters have, on the average, excesskinetic energy (in the vertical direction) and as a populationare not in virial equilibrium; moreover, the initial verticalcoordinates of open clusters (at the time of their birth) arestrongly and positively correlated with initial verticalvelocities (the correlation coefficient is $r(Z_0,V_{Z(0)}) =0.81 \pm 0.08$), thus favoring a scenario where star formation inthe disk is triggered by some massive objects falling to theGalactic plane.

The main sequence mass functions (MF) of a large sample of Galacticclusters (young and old) can be well reproduced with a tapered powerlaw distribution function with an exponential truncation of the formdN/dM ∝ m-α[1 - exp(-m/mp)β]. Theaverage value of the power-law index α is very close to Salpeter(~2.3), whereas the peak mass mp is in the range0.1-0.6 M⊙ and does not seem to vary in a systematic way withthe present cluster parameters such as metal abundance and centralconcentration. A remarkable correlation with age, however, is seen inthat older disc clusters have higher mp, although this trend doesnot extend to globular clusters, whose value of mp is lower thanthat of old open clusters. This trend most likely results from theonset of mass segregation following early dynamical interactions in theloose cluster cores. Differences between globular and younger clustersmay depend on the initial environment of star formation, which in turnaffects their total mass. Mass functions of field populations such asthe solar neighbourhood and bulge are consistent with the hypothesisthat they were built up over time by contributions from many functionsof this type with different peak masses.

UBVRI photometry of the open cluster NGC2422 (age~108 yr) down to alimiting magnitude V ≃ 19 is used to study the clusterspatial distribution and the luminosity and mass functions. From the Color-MagnitudeDiagram (CMD), we obtaineda list of candidate cluster members based on a photometric criterion.Using a comparison field region and an iterative procedure, a correction for contaminating field stars has been inferred in order to obtain the luminosity and themass functions in theM=0.4-3.5 M⊙ range.By analyzing thespatial distribution, we infer that a non-negligible number of cluster stars lie outside our investigated region. We estimate a correction to the mass function of the cluster in order to take into account the complete stellar population. The Present Day MassFunction of NGC2422 can berepresented by a power-law of index α = 3.07 ± 0.08 (rms) in the mass range 0.9 ≤ M/M⊙ ≤ 2.5. The index α and the total mass of thecluster are very similar to those of the Pleiades.

We obtain ages and metallicities of four open clusters fromcharacteristics of SB2 eclipsing binary members of these clusters.In addition, we present similar estimates obtained with a modificationof the color-magnitude diagram method for six open and globular clusters.

Massive stars are found, and, because of their age, are probably formed,in cluster cores. When the stars begin to shine, strong stellarwinds of ≳ 103 km s-1 exert a tremendous force on any remaining gas,which has at the same time been heated and expands at the sound speed(~10 km s-1). The gas will be rapidly expelled fromcluster core in much less than a core crossing time. This reduces thepotential of the core, dramatically affecting the dynamics of the corestars. This process is simulated using NBODY2 to follow thedistributions of massive stars over a few crossing times for variousinitial gas fractions. The core of massive stars is found to remainintact for a core gas fraction of 4 (or equivalently, a star formationeffeciency of 20%). The space and velocity distributions are alsoinvestigated.

We study the merging of star clusters out of cluster aggregates similar to Knot S in the Antennae on orbits close to the one of ω Cen by carrying out high resolution numerical N-body simulations. We want to constrain the parameter space which is able to produce merger objects with similar properties as ω Cen.

The concept of central super-massive stars (${\cal M} \ge 5 \times 10^4 M_{\odot}$,where ${\cal M}$ is the mass of the super-massive star)embedded in dense stellar systems was suggested as a possible explanation for high-energy emissions phenomena occurring in active galactic nuclei and quasars (Vilkoviski 1976; Hara 1978), such as X-ray emissions (Bahcall & Ostriker 1975).SMSs and super-massive black holes are two possibilities to explain the nature of super-massive central objects, and super-massive stars may be an intermediate step towards the formation of super-massive black holes (Rees 1984).Therefore it is important to study such a dense gas-star system in detail. We addresshere the implementation of radiative transfer in a model which was presented informer work (Amaro-Seoane & Spurzem 2001; Amaro-Seoane et al. 2002). In this sense,we extend here and improve the work done by Langbein et al. (1990) by describing theradiative transfer in super-massive stars using previous work on this subject (Castor 1972).

As the first step in our study of the spiralstructure of disk galaxies in the Frei sample, we determine the valuesof their deprojection angles. We introduce two methods based on the analysis of the spiral structure. Thesetwo methods behave as well as the rest of the methods used normally in the literature.

We have compiled a new catalogue of open clusters in the Galaxy which updates the previous catalogues of Lyngå (1987) and of Mermilliod (1995) (included in the WEBDA database). New objects and new data, in particular, data on kinematics (proper motions, radial velocities) that were not present in the old catalogues, have been included. Virtually all the clusters (1622) presently known were included, which represents an increment of about 450 objects relative to the Lyngå (1987) catalogue. The catalogue is presented in a single table containing all the important data (positions, reddenings, distances, ages, kinematics) which makes it easy to use. The catalogue includes mean absolute proper motions for hundreds of open clusters (Dias et al. 2001, 2002a) obtained using the Tycho2 proper motions. Unpublished mean radial velocities of several dozens of clusters are also presented in this work. The catalogue is being constantly updated and the latest version can be accessed on-line at http://www.astro.iag.usp.br/~wilton/ were more data such as membership probabilities for individual stars can also be accessed.

We investigate the nature of the clumpy structures seen at high redshift.For this we use a 3D chemodynamical simulationincluding dark matter, stars and a multiphase interstellar medium aswell as feedback processes from the stars. The infall of baryonicmatter leads to the formation of a disk which later fragments anddevelops starforming sub-galactic clumps. Calculating U, B, V, Kcolors of the model including absorption by interstellar material wefind similar morphology and colors as for multiple clump and chaingalaxies in the HDF.

We have discovered 22 GHz water vapor maser toward a narrow-line type 1 (NLS1) Seyfert galaxy, NGC4051. The detection of water masers insuch galaxies is very rare. The Doppler-shifted maser componentssymmetrically straddle a component lying at about 712 km s-1. Thelocation of each maser component is of great interest for studyinggalactic dynamical structures around an active nucleus. We report onpreliminary results of VLA observations.

Since the first HI observations of the Milky Way warp 50 years agonumerous hypotheses have been proposed to explain this common galactic phenomenon.While actually most of the works on warps are based on the assumption of a disk embedded in a dark matter halo, the possibility of a heavy self-gravitating disk is often neglected.The aim of this paper is to better understand the constraints imposed bythe shape of the dark component on the dynamics of the warp. Two set of simulations have been performed. The first one is based on a traditional dark matter halo, while the second one uses a heavy flaring self-gravitating disk.

We have examined the spectroscopic properties of a sample of 19 optically bright, low luminosity core-dominated radio sources; this sample wasexpected to contain a high fraction of objects observed at small angles to the line-of-sight (blazar-like objects). Our study focuses on the properties - stellar populations and optical emission-line spectra - of such nuclei.Stellar population synthesis shows that their nuclear populations are composed, in a general way, of old stars of solar metallicity or lower; the dust content is weak. After subtraction of the stellar contribution, we are left with a set of nuclear emission-line spectra; their analysis shows that most of the objects harbour a Low Ionisation Nuclear EmissionRegion (or LINER), whose contribution was highly diluted by thehost galaxy starlight. Such a low ionisation spectrum is in agreementwith the black hole mass values and sub-Eddington accretion ratespublished for some BL Lacs; also, the stellar populations derivedin this study are in agreement with those usually found in LINERs.Our findings give strength to a unifying picture involving lowluminosity blazar-like objects and FR I radio-galaxies (which usuallydisplay LINER-type spectra).

A simple model for the evolution of an isothermal disk has been developed.It combines both the chemical evolution (through the Age-MetallicityRelation) and dynamical evolution (through the age-dispersion relation). Applying this model to a sample of Red Clump Stars from the Tycho-2 catalog,observed with the high resolution echelle spectrograph Elodie at theObservatoire de Haute Provence, we derive the underlying Star FormationHistory and compare it with local determinations using chromosphericactivity survey by Rocha-Pinto et al. (2000b).

A capture scenario of the origin of ω Centauri is studied numerically. We show that the most serious problem of this scenario - how to move a progenitor of ω Cen from a typical dwarf orbit to the current position of ω Cen - can be solve for a nucleated dwarf galaxy with Hernquistdensity profile.

We use the method of statistical parallaxes to construct the rotationcurve of the Milky Way spanning the Galactocentric distance intervalfrom 2 to 14 kpc based on observational data for young disk populationsand refine the distance scales of the objects belonging to young diskpopulations (classical Cepheids, young open clusters, blue supergiants,HII regions, and HI).