Effects of galaxy--satellite interactions on bar formation

Aims. We aim to show how encounters with low-mass satellite galaxies may alter the bar formation in a Milky Way-like disc galaxy. Methods. We use high-resolution N-body simulations of a disc galaxy prone to mild bar instability. For realistic initial conditions of satellites, we take advantage of cosmological simulations of Milky Way-like dark matter haloes. Results. The satellites may have a significant impact on the time of bar formation. Some runs with satellites demonstrate a delay, while others show an advancement in bar formation compared to the isolated run, with such time differences reaching $\sim$ 1 Gyr. Meanwhile, the final bar configuration, including its very appearance and the bar characteristics such as the pattern speed and the exponential growth rate of its amplitude are independent of the number of encounters and their orbits. The contribution of satellites with masses below $10^9 M_{\odot}$ is insignificant, unless their pericentre distances are small. We suggest that the encounters act indirectly via inducing perturbations across the disc that evolve to delayed waves in the central part and interfere with an emerging seed bar. The predicted effect for the present-day host galaxy is expected to be even more significant at redshifts $z \gtrsim 0.5$.Comment: 16 pages, 14 figures and 4 table

Controlling chaos in spatially extended beam-plasma system by the continuous delayed feedback

In present paper we discuss the control of complex spatio-temporal dynamics in a {spatially extended} non-linear system (fluid model of Pierce diode) based on the concepts of controlling chaos in the systems with few degrees of freedom. A presented method is connected with stabilization of unstable homogeneous equilibrium state and the unstable spatio-temporal periodical states analogous to unstable periodic orbits of chaotic dynamics of the systems with few degrees of freedom. We show that this method is effective and allows to achieve desired regular dynamics chosen from a number of possible in the considered system.Comment: 12 pages, 12 figure

Dynamical Friction in a Gaseous Medium

Using time-dependent linear perturbation theory, we evaluate the dynamical friction force on a massive perturber M_p traveling at velocity V through a uniform gaseous medium of density rho_0 and sound speed c_s. This drag force acts in the direction -\hat V, and arises from the gravitational attraction between the perturber and its wake in the ambient medium. For supersonic motion (M=V/c_s>1), the enhanced-density wake is confined to the Mach cone trailing the perturber; for subsonic motion (M<1), the wake is confined to a sphere of radius c_s t centered a distance V t behind the perturber. Inside the wake, surfaces of constant density are hyperboloids or oblate spheroids for supersonic or subsonic perturbers, respectively, with the density maximal nearest the perturber. The dynamical drag force has the form F_df= - I 4\pi (G M_p)^2\rho_0/V^2. We evaluate I analytically; its limits are I\to M^3/3 for M>1. We compare our results to the Chandrasekhar formula for dynamical friction in a collisionless medium, noting that the gaseous drag is generally more efficient when M>1 but less efficient when M<1. To allow simple estimates of orbit evolution in a gaseous protogalaxy or proto-star cluster, we use our formulae to evaluate the decay times of a (supersonic) perturber on a near-circular orbit in an isothermal \rho\propto r^{-2} halo, and of a (subsonic) perturber on a near-circular orbit in a constant-density core. We also mention the relevance of our calculations to protoplanet migration in a circumstellar nebula.Comment: 17 pages, 5 postscript figures, to appear in ApJ 3/1/9

Production System Models of Complex Cognition

There have been a number of production system models which have recently made substantial advances in modeling higher-level cognition. These type of model offers only comprehensive approaches to the modeling of higher level cognition. This symposium will involve presentations by four exemplars of this approach to cognitive modeling (ACT, CAPS, EPIC, and SOAR). The presentations will try to illustrate the range of applications to which such models are appropriate, what the similarities and differences are among the various architectures, and what some of the interesting research questions are within each architecture

Time-delayed feedback control in astrodynamics

In this paper we present time-delayed feedback control (TDFC) for the purpose of autonomously driving trajectories of nonlinear systems into periodic orbits. As the generation of periodic orbits is a major component of many problems in astodynamics we propose this method as a useful tool in such applications. To motivate the use of this method we apply it to a number of well known problems in the astrodynamics literature. Firstly, TDFC is applied to control in the chaotic attitude motion of an asymmetric satellite in an elliptical orbit. Secondly, we apply TDFC to the problem of maintaining a spacecraft in a periodic orbit about a body with large ellipticity (such as an asteroid) and finally, we apply TDFC to eliminate the drift between two satellites in low Earth orbits to ensure their relative motion is bounded

On the Mechanism of Time--Delayed Feedback Control

The Pyragas method for controlling chaos is investigated in detail from the experimental as well as theoretical point of view. We show by an analytical stability analysis that the revolution around an unstable periodic orbit governs the success of the control scheme. Our predictions concerning the transient behaviour of the control signal are confirmed by numerical simulations and an electronic circuit experiment.Comment: 4 pages, REVTeX, 4 eps-figures included Phys. Rev. Lett., in press also available at http://athene.fkp.physik.th-darmstadt.de/public/wolfram.htm

The natural science of cosmology

The network of cosmological tests is tight enough now to show that the relativistic Big Bang cosmology is a good approximation to what happened as the universe expanded and cooled through light element production and evolved to the present. I explain why I reach this conclusion, comment on the varieties of philosophies informing searches for a still better cosmology, and offer an example for further study, the curious tendency of some classes of galaxies to behave as island universes.Comment: Keynote lecture at the seventh International Conference on Gravitation and Cosmology, Goa India, December 201

Global survey of star clusters in the Milky Way VI. Age distribution and cluster formation history

The all-sky Milky Way Star Clusters (MWSC) survey provides uniform and precise ages and other parameters for a variety of clusters in the Solar Neighbourhood. We construct the cluster age distribution, investigate its spatial variations, and discuss constraints on cluster formation scenarios of the Galactic disk during the last 5 Gyrs. Due to the spatial extent of the MWSC, we consider spatial variations of the age distribution along galactocentric radius $R_G$, and along $Z$-axis. For the analysis of the age distribution we use 2242 clusters, which all lie within roughly 2.5 kpc of the Sun. To connect the observed age distribution to the cluster formation history we build an analytical model based on simple assumptions on the cluster initial mass function and on the cluster mass-lifetime relation, fit it to the observations, and determine the parameters of the cluster formation law. Comparison with the literature shows that earlier results strongly underestimated the number of evolved clusters with ages $t\gtrsim 100$ Myr. Recent studies based on all-sky catalogues agree better with our data, but still lack the oldest clusters with ages $t\gtrsim 1$ Gyr. We do not observe a strong variation in the age distribution along $R_G$, though we find an enhanced fraction of older clusters ($t>1$ Gyr) in the inner disk. In contrast, the distribution strongly varies along $Z$. The high altitude distribution practically does not contain clusters with $t<1$ Gyr. With simple assumptions on the cluster formation history, cluster initial mass function and cluster lifetime we can reproduce the observations. Cluster formation rate and cluster lifetime are strongly degenerate, which does not allow us to disentangle different formation scenarios. In all cases the cluster formation rate is strongly declining with time, and the cluster initial mass function is very shallow at the high mass end. (abridged)Comment: 11 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

Safety Analysis Report for Packaging:The ORNL Lithium Hydroxide Fire and Impact Shield

The ORNL Lithium Hydroxide Fire and Impact Shield and its packaging were designed and fabricated at Oak Ridge National Laboratory to permit the transport of Type B quantities of radioactive material and limited quantities of fissionable material. The shield and its packaging were evaluated analytically and experimentally to determine its compliance with the applicable regulations governing containers in which radioactive and fissile materials are transported, and that evaluation is the subject of this report. Computational and test procedures were used to determine the structural integrity and thermal behavior of the shield relative to the general standards for normal conditions of transport and the standards for the hypothetical accident conditions. The results of the evaluation demonstrate that the shield and its packaging are in compliance with the applicable regulations. 16 references, 8 figures, 5 tables