Ejection of high-velocity stars from the Galactic Center by an inspiraling Intermediate-Mass Black Hole

The presence of young stars in the immediate vicinity and strong tidal field of SgrA* remains unexplained. One currently popular idea for their origin posits that the stars were bused in by an Intermediate-Mass Black Hole (IMBH) which has inspiraled into the Galactic Center a few million years ago. Yu and Tremaine (2003) have argued that in this case some of the old stars in the SgrA* cusp would be ejected by hard gravitational collisions with the IMBH. Here we derive a general expression for the phase-space distribution of the ejected high-velocity stars, given the distribution function of the stars in the cusp. We compute it explicitly for the Peebles-Young distribution function of the cusp, and make a detailed model for the time-dependent ejection of stars during the IMBH inspiral. We find that (1) the stars are ejected in a burst lasting a few dynamical friction timescales; if the ejected stars are detected by Gaia they are likely to be produced by a single inspiral event, (2) if the inspiral is circular than in the beginning of the burst the velocity vectors of the ejected stars cluster around the inspiral plane, but rapidly isotropise as the burst proceeds, (3) if the inspiral is eccentric, then the stars are ejected in a broad jet roughly perpendicular to the Runge-Lenz vector of the IMBH orbit. In a typical cusp the orbit will precess with a period of \sim 10^5 years, and the rate of ejection into our part of the Galaxy (as defined by e.g. the Gaia visibility domain) will be modulated periodically. Gaia, together with the ground-based follow-up observations, will be able to clock many high-velocity stars back to their ejection from the Galactic Center, thus measuring some of the above phenomena. This would provide a clear signature of the IMBH inspiral in the past 10--20 Myr.Comment: 12 pages, including 7 figure

Approximating Stellar Orbits: Improving on Epicycle Theory

Already slightly eccentric orbits, such as those occupied by many old stars in the Galactic disk, are not well approximated by Lindblad's epicycle theory. Here, alternative approximations for flat orbits in axisymmetric stellar systems are derived and compared to results from numeric integrations. All of these approximations are more accurate than Lindblad's classical theory. I also present approximate, but canonical, maps from ordinary phase-space coordinates to a set of action-angle variables. Unfortunately, the most accurate orbit approximation leads to non-analytical R(t). However, from this approximation simple and yet very accurate estimates can be derived for the peri- and apo-centers, frequencies, and actions integrals of galactic orbits, even for high eccentricities. Moreover, further approximating this approximation allows for an analytical R(t) and still an accurate approximation to galactic orbits, even with high eccentricities.Comment: accepted for publication in AJ; 12 pages LaTeX, 9 figures (coloured only here, not in AJ) uses aas2pp4.st

Determinants of Compliance with Protectionist Policies for Human Trafficking Victims

The international community has joined together in an effort to combat human trafficking. Utilizing the Protocol to Prevent, Suppress and Punish Trafficking in Persons, Especially Women and Children, commonly known as the Palermo Protocols, the focus of combatant efforts has been tailored to fit three categories: prosecution, prevention, and protection. Because the attention of several countries appears to be primarily devoted to prosecution, protection often falls victim to indifference, leaving victims without the care that they need to readjust to daily life. While the three components are supposedly equal in weight, this inequality is apparent in several countries through their anti-trafficking efforts. In this paper, I examine why some states offer greater levels of commitment to the protection component of the Palermo Protocols on human trafficking than others by examining a possible interactive effect between domestic and international factors. While the interaction was not found to be statistically significant, international factors measured by a state’s level of international involvement as well as the domestic factor considering the presence of strict immigration policies provide a greater understanding of the topic as a whole and guidance for future research endeavors

Self Similar Spherical Collapse Revisited: a Comparison between Gas and Dark Matter Dynamics

We reconsider the collapse of cosmic structures in an Einstein-de Sitter Universe, using the self similar initial conditions of Fillmore & Goldreich (1984). We first derive a new approximation to describe the dark matter dynamics in spherical geometry, that we refer to the "fluid approach". This method enables us to recover the self-similarity solutions of Fillmore & Goldreich for dark matter. We derive also new self-similarity solutions for the gas. We thus compare directly gas and dark matter dynamics, focusing on the differences due to their different dimensionalities in velocity space. This work may have interesting consequences for gas and dark matter distributions in large galaxy clusters, allowing to explain why the total mass profile is always steeper than the X-ray gas profile. We discuss also the shape of the dark matter density profile found in N-body simulations in terms of a change of dimensionality in the dark matter velocity space. The stable clustering hypothesis has been finally considered in the light of this analytical approach.Comment: 14 pages, 2 figures, accepted for publication in The Astrophysical Journa

Irregular Satellites of the Planets: Products of Capture in the Early Solar System

All four giant planets in the Solar system possess irregular satellites, characterized by large, highly eccentric and/or inclined orbits that are distinct from the nearly circular, uninclined orbits of the regular satellites. This difference can be traced directly to different modes of formation. Whereas the regular satellites grew by accretion within circumplanetary disks the irregular satellites were captured from initially heliocentric orbits at an early epoch. Recently, powerful survey observations have greatly increased the number of known irregular satellites, permitting a fresh look at the group properties of these objects and motivating a re-examination of the mechanisms of capture. None of the suggested mechanisms, including gas-drag, pull-down, and three-body capture, convincingly fit the group characteristics of the irregular satellites. The sources of the satellites also remain unidentified.Comment: 51 pages, 17 figures, 5 tables, to appear in ARAA 200

Toussieux – Le Poyat du Chêne

Sur le rebord du plateau de la Côtière d’Ain, le site du Poyat du Chêne à Toussieux est installé sur la très haute terrasse de Villefranche. Il domine ainsi la vallée de la Saône en amont de Trévoux. Au siècle dernier, l’abbé Béroud y effectua de nombreuses prospections, qui lui permirent de récolter du matériel lithique taillé et poli principalement attribuable au Néolithique. Il y a de cela une vingtaine d’années, M. et Mme B. Pallix engagèrent de nouvelles prospections sur le plateau, à l’..

Warm Saturns: On the Nature of Rings around Extrasolar Planets that Reside Inside the Ice Line

We discuss the nature of rings that may exist around extrasolar planets. Taking the general properties of rings around the gas giants in the Solar System, we infer the likely properties of rings around exoplanets that reside inside the ice line. Due to their proximity to their host star, rings around such exoplanets must primarily consist of rocky materials. However, we find that despite the higher densities of rock compared to ice, most of the observed extrasolar planets with reliable radii measurements have sufficiently large Roche radii to support rings. For the currently known transiting extrasolar planets, Poynting-Robertson drag is not effective in significantly altering the dynamics of individual ring particles over a time span of $10^8$ years provided that they exceed about 1 m in size. In addition, we show that significantly smaller ring particles can exist in optically thick rings, for which we find typical ring lifetimes ranging from a few times $10^6$ to a few times $10^9$ years. Most interestingly, we find that many of the rings could have nontrivial Laplacian planes due to the increased effects of the orbital quadrupole caused by the exoplanets' proximity to their host star, allowing a constraint on the $J_2$ of extrasolar planets from ring observations. This is particular exciting, since a planet's $J_2$ reveals information about its interior structure. Furthermore, measurements of an exoplanet's oblateness and of its $J_2$, from warped rings, would together place limits on its spin period. Based on the constraints that we have derived for extrasolar rings, we anticipate that the best candidates for ring detections will come from transit observations by the Kepler spacecraft of extrasolar planets with semi-major axes $\sim 0.1$ AU and larger.Comment: Accepted for publication in Ap

Actions for axisymmetric potentials

We give an algorithm for the economical calculation of angles and actions for stars in axisymmetric potentials. We test the algorithm by integrating orbits in a realistic model of the Galactic potential, and find that, even for orbits characteristic of thick-disc stars, the errors in the actions are typically smaller than 2 percent. We describe a scheme for obtaining actions by interpolation on tabulated values that significantly accelerates the process of calculating observables quantities, such as density and velocity moments, from a distribution function.Comment: 5 pages accepted for publication in MNRA