Eccentric disc instability in stellar discs formed from inspiraling gas clouds in the Galactic Centre

The inspiral of a turbulent molecular cloud in the Galactic Centre may result in the formation of a small, dense and moderately eccentric gas disc around the supermassive black hole (SMBH). Such a disc is unstable to fragmentation and may lead to the formation of young massive stars in the central parsec of the Galaxy. Here we perform high-accuracy direct summation N-body simulations of a ring of massive stars (with initial semi-major axes 0.1 < a/pc < 0.4 and eccentricities 0.2 < e < 0.4), subject to the potential of the SMBH, a stellar cusp, and the parent gas disc, to study how the orbital elements of the ring evolve in time. The initial conditions for the stellar ring are drawn from the results of previous simulations of molecular cloud infall and disruption in the SMBH potential. While semi-major axes do not evolve significantly, the distribution of eccentricities spreads out very fast (~1 Myr) as a consequence of cusp precession. In particular, stellar orbits with initial eccentricity e>0.3 (e<0.3) tend to become even more (less) eccentric, resulting in a bimodal eccentricity distribution. The distribution is qualitatively consistent with that of the massive stars observed in the Galactic Centre's clockwise disc.Comment: 7 pages, 8 figures, accepted for publication in MNRA

Dynamical evolution of the young stars in the Galactic center

Recent observations of the Galactic center revealed a nuclear disk of young OB stars near the massive black hole (MBH), in addition to many similar outlying stars with higher eccentricities and/or high inclinations relative to the disk (some of them possibly belonging to a second disk). In addition, observations show the existence of young B stars (the 'S-cluster') in an isotropic distribution in the close vicinity of the MBH ($<0.04$ pc). We use extended N-body simulations to probe the dynamical evolution of these two populations. We show that the stellar disk could have evolved to its currently observed state from a thin disk of stars formed in a gaseous disk, and that the dominant component in its evolution is the interaction with stars in the cusp around the MBH. We also show that the currently observed distribution of the S-stars could be consistent with a capture origin through 3-body binary-MBH interactions. In this scenario the stars are captured at highly eccentric orbits, but scattering by stellar black holes could change their eccentricity distribution to be consistent with current observations.Comment: 5 pages, 2 figures. To appear in the proceedings of the Central Kiloparsec conference, 2008, Cret

Experimental joint signal-idler quasi-distributions and photon-number statistics for mesoscopic twin beams

Joint signal-idler photoelectron distributions of twin beams containing several tens of photons per mode have been measured recently. Exploiting a microscopic quantum theory for joint quasi-distributions in parametric down-conversion developed earlier we characterize properties of twin beams in terms of quasi-distributions using experimental data. Negative values as well as oscillating behaviour in quantum region are characteristic for the subsequently determined joint signal-idler quasi-distributions of integrated intensities. Also the conditional and difference photon-number distributions are shown to be sub-Poissonian and sub-shot-noise, respectively.Comment: 7 pages, 6 figure

Outcome of 518 salvage oocyte-cryopreservation cycles performed as a routine procedure in an in vitro fertilization program

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Dynamical evolution of the young stars in the Galactic center: N-body simulations of the S-stars

We use N-body simulations to study the evolution of the orbital eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black hole (MBH), starting from initial conditions motivated by two competing models for their origin: formation in a disk followed by inward migration; and exchange interactions involving a binary star. The first model predicts modest eccentricities, lower than those observed in the S-star cluster, while the second model predicts higher eccentricities than observed. The N-body simulations include a dense cluster of 10 M_sun stellar black holes (SBHs), expected to accumulate near the MBH by mass segregation. Perturbations from the SBHs tend to randomize the stellar orbits, partially erasing the dynamical signatures of their origin. The eccentricities of the initially highly eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that is consistent at the ~95 % level with the observed eccentricity distribution. In contrast, the eccentricities of the initially more circular orbits fail to evolve to the observed values in 20 Myr, arguing against the disk migration scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the MBH over their lifetimes, and that the S-stars are not likely to be ejected as hypervelocity stars outside the central 0.05 pc by close encounters with stellar black holes.Comment: 6 pages, 2 figures. Minor corrections, Sumitted to Ap

Performance Analysis of Direct N-Body Algorithms on Special-Purpose Supercomputers

Direct-summation N-body algorithms compute the gravitational interaction between stars in an exact way and have a computational complexity of O(N^2). Performance can be greatly enhanced via the use of special-purpose accelerator boards like the GRAPE-6A. However the memory of the GRAPE boards is limited. Here, we present a performance analysis of direct N-body codes on two parallel supercomputers that incorporate special-purpose boards, allowing as many as four million particles to be integrated. Both computers employ high-speed, Infiniband interconnects to minimize communication overhead, which can otherwise become significant due to the small number of "active" particles at each time step. We find that the computation time scales well with processor number; for 2*10^6 particles, efficiencies greater than 50% and speeds in excess of 2 TFlops are reached.Comment: 34 pages, 15 figures, submitted to New Astronom