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 Physics , 2011, Abstract: A large population of fragile, wide (> 1000 AU) binary systems exists in the Galactic field and halo. These wide binary stars cannot be primordial because of the high stellar density in star forming regions, while formation by capture in the Galactic field is highly improbable. We propose that these binary systems were formed during the dissolution phase of star clusters (see Kouwenhoven et al. 2010, for details). Stars escaping from a dissolving star cluster can have very similar velocities, which can lead to the formation of a wide binary systems. We carry out N-body simulations to test this hypothesis. The results indicate that this mechanism explains the origin of wide binary systems in the Galaxy. The resulting wide binary fraction and semi-major axis distribution depend on the initial conditions of the dissolving star cluster, while the distributions in eccentricity and mass ratio are universal. Finally, since most stars are formed in (relatively tight) primordial binaries, we predict that a large fraction of the wide "binary stars" are in fact higher-order multiple systems.
 Physics , 2000, DOI: 10.1086/312636 Abstract: Motivated by the recently improved knowledge on the kinematic and chemical properties of the Galactic metal-poor stars, we present the numerical simulation for the formation of the Galactic stellar halo to interpret the observational results. As a model for the Galaxy contraction, we adopt the currently standard theory of galaxy formation based on the hierarchical assembly of the cold dark matter fluctuations. We find, for the simulated stars with [Fe/H]$\le-1.0$, that there is no strong correlation between metal abundances and orbital eccentricities, in good agreement with the observations. Moreover, the observed fraction of the low eccentricity stars is reproduced correctly for [Fe/H]$\le-1.6$ and approximately for the intermediate abundance range of $-1.6<$[Fe/H]$\le-1.0$. We show that this successful reproduction of the kinematics of the Galactic halo is a natural consequence of the hierarchical evolution of the subgalactic clumps seeded from the cold dark matter density fluctuations.
 Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19639.x Abstract: We present theoretical calculations for the differential distribution of stellar orbital eccentricity for a sample of solar-neighbour halo stars. Two types of static, spherical gravitational potentials are adopted to define the eccentricity e for given energy E and angular momentum L, such as an isochrone potential and a Navarro-Frenk-White potential that can serve as two extreme ends covering in-between any realistic potential of the Milky Way halo. The solar-neighbour eccentricity distribution \Delta N(e) is then formulated, based on a static distribution function of the form f(E,L) in which the velocity anisotropy parameter \beta monotonically increases in the radial direction away from the galaxy center, such that beta is below unity (near isotropic velocity dispersion) in the central region and asymptotically approaches \sim 1 (radially anisotropic velocity dispersion) in the far distant region of the halo. We find that \Delta N(e) sensitively depends upon the radial profile of \beta, and this sensitivity is used to constrain such profile in comparison with some observational properties of \Delta N_{obs}(e) recently reported by Carollo et al. (2010). Especially, the linear e-distribution and the fraction of higher-e stars for their sample of solar-neighbour inner-halo stars rule out a constant profile of \beta, contrary to the opposite claim by Bond et al. (2010). Our constraint of \beta \lesssim 0.5 at the galaxy center indicates that the violent relaxation that has acted on the inner halo is effective within a scale radius of \sim 10 kpc from the galaxy center. We discuss that our result would help understand the formation and evolution of the Milky Way halo.
 Physics , 2010, DOI: 10.1111/j.1365-2966.2010.16998.x Abstract: We present theoretical calculations for the differential distribution of stellar orbital eccentricity in a galaxy halo, assuming that the stars constitute a spherical, collisionless system in dynamical equilibrium with a dark matter halo. In order to define the eccentricity e of a halo star for given energy E and angular momentum L, we adopt two types of gravitational potential, such as an isochrone potential and a Navarro-Frenk-White potential, that could form two ends covering in-between any realistic potential of dark matter halo. Based on a distribution function of the form f(E,L) that allows constant anisotropy in velocity dispersions characterized by a parameter \beta, we find that the eccentricity distribution is a monotonically increasing function of e for the case of highly radially anisotropic velocity dispersions (\beta > 0.6), while showing a hump-like shape for the cases from radial through tangential velocity anisotropy (\beta < 0.6). We also find that when the velocity anisotropy agrees with that observed for the Milky Way halo stars (\beta = 0.5-0.7), a nearly linear eccentricity distribution of N(e) \alpha e results at e < 0.7, largely independent of the potential adopted. Our theoretical eccentricity distribution would be a vital tool of examining how far out in the halo the dynamical equilibrium has been achieved, through comparison with kinematics of halo stars sampled at greater distances. Given that large surveys of the SEGUE and Gaia projects would be in progress, we discuss how our results would serve as a new guide in exploring the formation and evolution of the Milky Way halo.
 Physics , 2015, Abstract: The origin of high velocity stars observed in the halo of our Galaxy is still unclear. In this work we test the hypothesis, raised by results of recent high precision $N$-body simulations, of strong acceleration of stars belonging to a massive globular cluster orbitally decayed in the central region of the host galaxy where it suffers of a close interaction with a super massive black hole, which, for these test cases, we assumed $10^8$ M$_\odot$ in mass.
 Physics , 2000, DOI: 10.1046/j.1365-8711.2001.04346.x Abstract: We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars likely to be found within our Galaxy today. By PopIII we shall be referring to stars formed at large redshift ($z>4$), with low metallicities ($[Z/Z_{\odot}]<-2.5$) and in small systems (total mass $\simlt$ $2\times 10^{8} M_{\odot}$) that are extremely sensitive to stellar feedback, and which through a prescribed merging history (Lacey & Cole 1993) end up becoming part of the Milky Way today. An analytic, extended Press-Schechter formalism is used to get the mass functions of halos which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modeled as a mini galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disk instabilities, all of which determine the fraction of the baryons which are subject to star formation. Use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, give accurate estimates of their expected numbers today, and their location in $L/L_{\odot}$ vs. $T/K$ HR diagrams. A first comparison with observational data suggests that the IMF of the first stars was increasingly high mass weighted towards high redshifts, levelling off at $z\simgt 9$ at a characteristic stellar mass scale $m_s=10-15 M_\odot$.
 Physics , 2009, DOI: 10.1111/j.1365-2966.2009.14809.x Abstract: We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary 50-100 Msun star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of $\ga$ 200-400 km/s (typical of pulsars), while 3-4 Msun stars can attain velocities of $\ga$ 300-400 km/s (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.
 Physics , 2003, DOI: 10.1086/374306 Abstract: The present day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between satellite accretion'' and monolithic collapse''; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers (2000,AJ,119,2843) for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity- and integrals of motion-phase space projections of these stars, coupled with a series of N-body/Smoothed Particle Hydrodynamic (SPH) chemodynamical simulations, we suggest an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.
 Physics , 2012, DOI: 10.1088/0004-637X/757/2/164 Abstract: Oxygen abundances of 67 dwarf stars in the metallicity range -1.6<[Fe/H]<-0.4 are derived from a non-LTE analysis of the 777 nm O I triplet lines. These stars have precise atmospheric parameters measured by Nissen and Schuster, who find that they separate into three groups based on their kinematics and alpha-element (Mg, Si, Ca, Ti) abundances: thick-disk, high-alpha halo, and low-alpha halo. We find the oxygen abundance trends of thick-disk and high-alpha halo stars very similar. The low-alpha stars show a larger star-to-star scatter in [O/Fe] at a given [Fe/H] and have systematically lower oxygen abundances compared to the other two groups. Thus, we find the behavior of oxygen abundances in these groups of stars similar to that of the alpha elements. We use previously published oxygen abundance data of disk and very metal-poor halo stars to present an overall view (-2.3<[Fe/H]<+0.3) of oxygen abundance trends of stars in the solar neighborhood. Two field halo dwarf stars stand out in their O and Na abundances. Both G53-41 and G150-40 have very low oxygen and very high sodium abundances, which are key signatures of the abundance anomalies observed in globular cluster (GC) stars. Therefore, they are likely field halo stars born in GCs. If true, we estimate that at least 3+/-2% of the local field metal-poor star population was born in GCs.
 Ortwin Gerhard Physics , 2003, Abstract: Recent observational and theoretical work suggests that the formation of the Galactic stellar halo involved both dissipative processes and the accretion of subfragments. With present data, the fraction of the halo for which an accretion origin can be substantiated is small, of order 10 percent. The kinematics of the best halo field star samples show evidence for both dissipative and dissipationless formation processes. Models of star-forming dissipative collapse, in a cosmological context and including feedback from star formation, do not confirm the simple relations between metallicity, rotation velocity, and orbital eccentricity for halo stars as originally predicted. The new model predictions are much closer to the observed distributions, which have generally been interpreted as evidence for an accretion origin. These results are broadly consistent with a hierarchical galaxy formation model, but the details remain to be worked out.
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