The velocity-space distribution of the solar neighborhood stars shows complex substructures (moving groups) including the well-known Hercules stream. Recently, the Gaia observation revealed their detailed structures, but their origins are still in debate. We analyzed a high-resolution N-body simulation of a Milky Way (MW)-like galaxy. To find velocity-space distributions similar to that of the solar neighborhood stars, we used Kullback-Leibler divergence (KLD), which is a metric to measure similarities between probability distributions. The KLD analysis shows the time evolution and the spatial variation of the velocity-space distribution. Velocity-space distributions with small KLDs (i.e. high similarities) are frequently but not always detected around in the simulated MW. In the velocity-map with smallest KLD, the velocity-space substructures are made from bar resonances.

Comets in the Oort cloud evolve under the influence of internal and external perturbations from giant planets to stellar passages, the Galactic tides, and the interstellar medium.Using the positions, parallaxes and proper motions from TGAS in Gaia DR1 and combining them with the radial velocities from the RAVE-DR5, Geneva-Copenhagen and Pulkovo catalogues, we calculated the closest encounters the Sun has had with other stars in the recent past and will have in the near future. We find that the stars with high proper motions near to the present time are missing in the Gaia-TGAS, and those to tend to be the closest ones. The quality of the data allows putting better constraints on the encounter parameters, compared to previous surveys.

We simulate the formation and evolution of young star clusters from turbulent molecular clouds using smoothed-particle hydrodynamics and direct N-body methods. We find that the shape of the cluster mass function that originates from an individual molecular cloud is consistent with a Schechter function with power-law slopes of β = −1.73. The superposition of mass functions turn out to have a power-law slope of < −2. The mass of the most massive cluster formed from a single molecular cloud with mass Mg scales with 6.1 M0.51g. The molecular clouds that tend to form massive clusters are much denser than those typical found in the Milky Way. The velocity dispersion of such molecular clouds reaches 20km s−1 and it is consistent with the relative velocity of the molecular clouds observed near NGC 3603 and Westerlund 2, for which a triggered star formation by cloud-cloud collisions is suggested.

In the past decade, observational evidence that Globular Clusters (GCs) harbour multiple stellar populations has grown steadily. These observations are hard to reconcile with the classical picture of star formation in GCs, which approximates them as a single generation of stars. Bastian et al. recently suggested an evolutionary scenario in which a second, chemically distinct, population is formed by the accretion of chemically enriched material onto the protoplanetary disc of low-mass stars in the initial GC population. Using assumptions that represent the (dynamical) conditions in a typical GC, we investigate whether a low-mass star surrounded by a protoplanetary disc can accrete sufficient enriched material to account for the observed abundances in ‘second generation’ stars. We compare the outcome of two different smoothed particle hydrodynamics codes and focus on the lifetime and stability of the disc and on the gas accretion rate onto both the star and the disc.

We perform realistic simulations of the Sun's birth cluster in order to predict the current distribution of solar siblings in the Galaxy. We study the possibility of finding the solar siblings in the Gaia catalogue by using only positional and kinematic information. We find that the number of solar siblings predicted to be observed by Gaia will be around 100 in the most optimistic case, and that a phase space only search in the Gaia catalogue will be extremely difficult. It is therefore mandatory to combine the chemical tagging technique with phase space selection criteria in order to have any hope of finding the solar siblings.

We aim at modeling small groups of young stars such as IRS 13N, 0.1 pc away from Sgr A*, which is suggested to contain a few embedded massive young stellar objects. We perform hydrodynamical simulations to follow the evolution of molecular clumps orbiting around a 4 × 106 M⊙ black hole, to constrain the formation and the physical conditions of such groups.

We find that the strong compression due to the black hole along the orbital radius vector of clumps evolving on highly eccentric orbits causes the clumps densities to increase to higher than the tidal density of Sgr A* and required for star formation. This suggests that the tidal compression from the black hole could support star formation.

Additionally, we speculate that the infrared excess source G2/DSO approaching Sgr A* on a highly eccentric orbit could be associated with a dust enshrouded star that may have been formed recently through the mechanism supported by our models.

We performed numerical simulations of dynamical encounters between hard, massive binaries and a very massive star (VMS; formed through runaway mergers of ordinary stars in the dense core of a young massive star cluster) to explore the hypothesis that this dynamical process could be responsible for the origin of high-velocity (≥ 200 − 400 km s−1) early or late B-type stars. We estimated the typical velocities produced in encounters between very tight massive binaries and VMSs (of mass of ≥ 200 M⊙) and found that about 3 − 4% of all encounters produce velocities ≥ 400 km s−1, while in about 2% of encounters the escapers attain velocities exceeding the Milky Ways's escape velocity. We therefore argue that the origin of high-velocity (≥ 200 − 400 km s−1) runaway stars and at least some so-called hypervelocity stars could be associated with dynamical encounters between the tightest massive binaries and VMSs formed in the cores of star clusters. We also simulated dynamical encounters between tight massive binaries and single ordinary 50 − 100 M⊙ stars. We found that from 1 to ≃ 4% of these encounters can produce runaway stars with velocities of ≥ 300 − 400 km s−1 (typical of the bound population of high-velocity halo B-type stars) and occasionally (in less than 1% of encounters) produce hypervelocity (≥ 700 km s−1) late B-type escapers.

We propose an explanation for the origin of hyperfast neutron stars (e.g. PSR B1508+55, PSR B2224+65, RX J0822–4300) based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star (or its helium core) which attained its peculiar velocity (similar to that of the neutron star) in the course of a strong three- or four-body dynamical encounter in the core of a young massive star cluster. This hypothesis implies that the dense cores of star clusters (located either in the Galactic disk or near the Galactic centre) could also produce the so-called hypervelocity stars – ordinary stars moving with a speed of ~ 1 000 km s−1.

We discuss the model for the Galactic sample of the AM CVn systems with Porb, ≤ 1500 s that can be detected in the optical and/or X-ray bands and may be resolved by the gravitational waves detector LISA. At 3 ≲ P ≲ 10 min all detectable systems are X-ray sources. At P ≳ 10 min most systems are only detectable in the V-band. About 30% of the X-ray sources is also detectable in the V-band. About 10,000 AM CVn systems might be resolved by LISA: this is comparable to the number of detached double white dwarfs that can be resolved. Several hundreds of AM CVn LISA sources might be also detectable in the V- and/or X-ray bands.

A major goal of contemporary astrophysics is understanding the origin of the most massive galaxies in the universe, particularly nearby ellipticals and spirals. Theoretical models of galaxy formation have existed for many decades, although observational evidence at both low and high redshifts is only beginning to put constraints on different ideas. We briefly describe these observations and how they are revealing the methods by which galaxies form by contrasting and comparing fiducial rapid collapse and hierarchical formation model predictions. The available data show that cluster ellipticals must have rapidly formed at z > 2, and that up to 50% of all massive galaxies at z ~ 2.5 are involved in major mergers. While the former is consistent with the monolithic collapse picture, we argue that hierarchical formation is the only model that can reproduce all the available observations.

New GSC-II proper motions of RR Lyrae and Blue Horizontal Branch (BHB) stars near the North Galactic Pole are used to show that the Galactic Halo 5 kpc above the Plane has a significantly retrograde galactic rotation.

We summarize results from the first year of the Lenses Structure and Dynamics Survey (LSD). Using the Echelle Imager and Spectrograph at the Keck-II Telescope, we have measured spatially resolved velocity dispersion profiles of a sample of distant (up to z=1) early-type galaxies that are gravitational lenses. By combining lensing and dynamical analyses we can set firm limits on the dark matter density profile, the controversial inner slope of the dark matter halo, the mass-to-light ratio of the luminous component, the total mass distribution, and on the velocity ellipsoid by breaking the mass anisotropy-relation. The implications of these measurements in terms on the epoch and mechanism of formation of E/S0 galaxies and of the value of the Hubble constant are briefly discussed.

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

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

A sample of nearly 400 stars, selected from the Tycho-2 catalog, has been observed towards the North Galactic Pole with the high resolution echelle spectrograph Elodie. Combining those data with a local sample from the Hipparcos catalog, a detailed study of the disk components is done. A dynamical analysis is also performed, enabling us to put new constraints on the local mass density.

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 Hernquist density profile.

Recent progress on the structure and dynamics of bulges is reviewed. Those aspects that link galaxy bulges either to oblate spheroids akin to elliptical galaxies or to rapidly-rotating, flattened systems more nearly resembling the products of disk internal transformations, are highlighted. The analysis of surface brightness profiles derived from HST data is reviewed to show that unresolved nuclear components detected by HST have biased the determination of surface brightness profiles obtained from the ground; r1/4 profiles are virtually nonexistent in galaxy bulges. Predictions from accretion N-body models on the shape of surface brightness profiles are discussed. The position of bulges on the Fundamental Plane (FP) of elliptical galaxies is examined to infer clues on bulge population ages and bulge dynamical structure. Kinematic diagnostics on the internal dynamics of bulges are examined. Finally, a new approach to the kinematic analysis of galaxies, based on the use of synthetic spectra of single stellar population models instead of the standard stellar templates, is presented.

We use a Fourier analysis method to study the spiral structure of the disc galaxies in the Frei et al. (1996) sample. We show that m=1 components are more often associated with asymmetries between the two arms of an m=2 spiral than with global lopsidedness of the disc. The power in the m=2 components is correlated with the arm class. The majority of galaxies with arm class 9 or 12 show clear signs of modulation in their spiral arms, suggesting the action of an amplification mechanism like the Swing amplification. Finally we report new detections of underlying large-scale bisymmetric components in flocculent spiral galaxies.

A simple model for the evolution of an isothermal disk has been developed. It combines both the chemical evolution (through the Age-Metallicity Relation) 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 the Observatoire de Haute Provence, we derive the underlying Star Formation History and compare it with local determinations using chromospheric activity survey by Rocha-Pinto et al. (2000b).