16004057
Spatial diffusion of stars in the inner galactic bulge
Star formation in the inner few hundred parsecs of the Galactic bulge occurs in a flattened molecular layer called the central molecular zone (CMZ). It has been suggested that star formation in the CMZ has been sustained for the lifetime of the Galaxy and that the resulting agglomeration of stars formed in the CMZ has resulted in the prominent r-2 stellar density cusp at the Galactic center having about the same physical extent as the CMZ. This "central cusp" is somewhat less flat than the CMZ; thus, the population of stars formed in the CMZ appears to have diffused out to larger latitudes. We hypothesize that such vertical diffusion is driven by the scattering of stars off the giant molecular clouds (GMCs) in the CMZ and perform numerical simulations of the scattering between stars and GMCs in the presence of the nonaxisymmetric background potential. The simulation results show that the timescale for an initially flattened stellar population to achieve an aspect ratio of the observed OH/IR stars in the inner bulge, 1-2 Gyr, agrees well with the estimated age of those OH/IR stars.
13909300
The Stream-Stream Collision after the Tidal Disruption of a Star around a Massive Black Hole
A star can be tidally disrupted around a massive black hole. It is known that the debris forms a precessing stream, which may collide with itself. The stream collision is a key process in determining the subsequent evolution of the stellar debris: if the orbital energy is efficiently dissipated, the debris will eventually form a circular disk (or torus). In this paper, we have numerically studied such a stream collision resulting from the encounter between a 106 M☉ black hole and a 1 M☉ normal star with a pericenter radius of 100 R☉. A simple treatment for radiative cooling has been adopted for both optically thick and optically thin regions. We have found that approximately 10% to 15% of the initial kinetic energy of the streams is converted into thermal energy during the collision. The spread in angular momentum of the incoming stream is increased by a factor of 2 to 3, and such an increase, together with the decrease in kinetic energy, significantly helps the circularization process. The initial luminosity burst produced by the collision may reach as high as 1041 ergs s-1 in 104 s, after which the luminosity increases again (but slowly this time) to a steady value of a few 1040 ergs s-1 in a few times 105 s. The radiation from the system is expected to be close to Planckian, with an effective temperature of ~105 K.
16066531
Dynamical Friction on Galactic Center Star Clusters with an Intermediate-Mass Black Hole
Numerical simulations of the dynamical friction suffered by a Galactic center star cluster harboring an intermediate-mass black hole (IMBH) have been performed. Gerhard has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in toward the Galactic center, may explain the presence of a cluster of very young stars in the central parsec, where star formation might be prohibitively difficult because of strong tidal forces. However, numerical simulations by Kim & Morris showed that this is possible only if the cluster initially has an extremely dense core. Hansen & Milosavljevic recently suggested that the presence of an IMBH in the cluster core might stabilize the core against tidal disruption during the inspiral through dynamical friction and thus might easily deliver young stars down to the central parsec. We find that the presence of an IMBH does lower the minimum initial core density required to transport young stars down to the central parsec, but this is possible only when the mass of the IMBH is at least ~10% of the total cluster mass. This fraction is significantly higher than that estimated by Portegies Zwart & McMillan with numerical simulations of IMBH formation by successive merging of stars in the cluster core, so it does not appear that a realistic IMBH can help transport young stars into the central parsec.
119519887
Stellar Companions to Stars with Planets
A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High-resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10 m or the Lick 3 m, and direct images or AO images are sensitive to a wider field, extending to 10'' or 38'', depending on the camera. One of the binary system—HD 114762—was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system—τ Boo—was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target—16 Cyg B—was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the τ Boo binary contains an extremely close planet in a tidally circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.
17473739
Stellar collisions and mergers in the galactic center
Stars are most likely to merge or collide in regions of the highest stellar densities, and our own Galactic Center contains many stars packed into a relatively small volume -- even the ambient stellar number density in the central 50 pc is quite high, ~10^3 stars pc^{-3}. More striking, the three compact young clusters in this region have central densities as high as 10^6 stars pc^{-3}. We discuss these extreme environments and the possibility that stellar mergers and collisions have recently occured there. In particular, we predict that at least one massive star in the Arches cluster has already experienced a stellar merger in its short lifetime. Further, the Pistol Star, in the nearby Quintuplet cluster, might owe its apparent relative youth to a rejuvinating stellar merger. Finally, the apparently young stars in the central arcsecond could be products of either collisions, inducing atmospheric stripping, or mergers.
14570068
N-Body Simulations of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young star clusters (CYCs) near the Galactic center using Aarseth's Nbody6 codes. The relatively small number of stars in the cluster (5000-20,000) makes real-number N-body simulations for these clusters feasible on current workstations. Using Fokker-Planck (F-P) models, Kim, Morris, & Lee made a survey of cluster lifetimes for various initial conditions and have found that clusters with a mass 2 × 104 M☉ evaporate in ~10 Myr. These results were, however, to be confirmed by N-body simulations because some extreme cluster conditions, such as strong tidal forces and a large stellar mass range participating in the dynamical evolution, might violate assumptions made in F-P models. Here we find that, in most cases, the CYC lifetimes of previous F-P calculations are 5%-30% shorter than those from the present N-body simulations. The comparison of projected number density profiles and stellar mass functions between N-body simulations and Hubble Space Telescope/NICMOS observations by Figer and colleagues in 1999 suggests that the current tidal radius of the Arches cluster is ~1.0 pc and that the parameters for the initial conditions of that cluster are as follows: total mass of 2 × 104 M☉ and mass function slope for intermediate to massive stars of 1.75 (the Salpeter function has 2.35). We also find that the lower stellar mass limit, the presence of primordial binaries, the amount of initial mass segregation, and the choice of initial density profile (King or Plummer models) do not significantly affect the dynamical evolution of CYCs.
122749310
RENO: Reactor experiment for neutrino oscillation at Yonggwang
The RENO experiment is under construction to measure the value of the smallest and unknown neutrino mixing angle θ13. The experiment will compare the measured fluxes of electron antineutrinos at two detectors located at 290 m and 1.4 km distances from the center of the Yonggwang nuclear reactors in Korea. It is planned to start data-taking in early 2010. An estimated systematic uncertainty associated with the measurement is less than 0.6%. With three years of data, the experiment will search for the mixing angle values of sin22θ13 down to 0.02.
14036761
Dynamics of assisted quintessence
We explore the dynamics of assisted quintessence, where more than one scalar field is present with the same potential. For potentials with tracking solutions, the fields naturally approach the same values — in the context of inflation this leads to the assisted inflation phenomenon where several fields can cooperate to drive a period of inflation though none is able to individually. For exponential potentials, we study the fixed points and their stability confirming results already in the literature, and then carry out a numerical analysis to show how assisted quintessence is realized. For inverse power-law potentials, we find by contrast that there is no assisted behaviour — indeed those are the unique (monotonic) potentials where several fields together behave just as a single field in the same potential. More generally, we provide an algorithm for generating a single-field potential giving equivalent dynamics to multi-field assisted quintessence.
123122242
OBSERVATION OF THE TOP QUARK
Top-quark production is observed in collisions at at the Fermilab Tevatron. The CDF and DO collaborations observe signals consistent with to but inconsistent with the background prediction by 4.8σ (CDF) and 4.6σ (DO). Additional evidence for the top quark is provided by a peak in the reconstructed mass distribution. The kinematic properties of the excess events are consistent with the top-quark decay. The top-quark mass has been measured to be 176±8(stat.) ±10(sys.)GeV/c2 (CDF) and ±22(sys.)GeV/c2 (DO), and the -production cross section to be and 6.4±2.2 pb (DO).
17082200
N-flation: Multifield inflationary dynamics and perturbations
We carry out numerical investigations of the dynamics and perturbations in the Nflation model of Dimopoulos et al. (2005). This model features large numbers of scalar fields with different masses, which can cooperate to drive inflation according to the assisted inflation mechanism. We extend previous work to include random initial conditions for the scalar fields, and explore the predictions for density perturbations and the tensor-to-scalar ratio. The tensor-to-scalar ratio depends only on the number of e-foldings and is independent of the number of fields, their masses, and their initial conditions. It therefore always has the same value as for a single massive field. By contrast, the scalar spectral index has significant dependence on model parameters. While normally multi-field inflation models make predictions for observable quantities which depend also on the unknown field initial conditions, we find evidence of a ‘thermodynamic’ regime whereby the predicted spectral index becomes independent of initial conditions if there are enough fields. Only in parts of parameter space where the mass spectrum of the fields is extremely densely packed is the model capable of satisfying the tight observational constraints from WMAP3 observations.
17211178
N-flation: Non-Gaussianity in the horizon-crossing approximation
We analyze the cosmic nongaussianity produced in inflation models with multiple uncoupled fields with monomial potentials, such as Nflation. Using the horizon-crossing approximation to compute the nongaussianity, we show that when each field has the same form of potential, the prediction is independent the number of fields, their initial conditions, and the spectrum of masses/couplings. It depends only on the number of $e$-foldings after the horizon crossing of observable perturbations. We also provide a further generalization to the case where the fields can have monomial potentials with different powers. Unless the horizon-crossing approximation is substantially violated, the predicted nongaussianity is too small to ever be observed.